Updated 21 MAY 08
|Past articles, writing & tech tips
|This was in response to a post that asked why two different amps from the same maker of the same model sounded
different. This was my response.
It is much less likely today than in the past. Today, 5% resistors and precision caps are much more common and not as expensive as in the 50s
and 60s. The amps of those eras had wide variations in parts. Look at the B+ voltage of 5 Marhall 100 watters of the 60s, and the voltage can
range anywhere from the low 400 to 600 volts.
Today there is more consistancy, and whether or not you are a fan of PCBs (printed circuit boards), the fact remains that PCB's are more
consistant, have trace runs that have been optimized for being quiet on CAD equipment, and they do not absorb moisture like tag board used in the
50s and 60s is prone to do.
A good circuit design implemented with proper mechanical design and assembly, will sound good and be reliable. Try to listen, for the most part,
and don't get too caught up in dialogue.
Marhall and Fender both made amps of wide variation. That is why the same amps of the same model and year sound different. They did the best
they could at the time, and stayed in business, which was their main concern (and there is no problem with that).
Today you have amps such as Carr Amps ... hand wired, point to point, with all the finest components available. They are fantastic amplifiers,
frankly ... magic. Then you have amps such as Rivera and Bogner ... with mil grade epoxy PCBs .... great design and construction, and also
wonderful amps. The comes along another thought ... Matchless ... a update of a timeless classic which exceeded the orignal in many ways, and
you do not need to disassemble the entire amp to change tubes either. The Ashdown Pacemaker series of heads ... well, if you ever saw an
original Hiwatt with its mil-spec constuction, Ashdown takes this even further, right to full British mil-spec for aircraft. Lets not forget THD ... with
circuit boards made out of the best glass layup I have ever seen (and I used to build sailboats out of composite materials) ... his boards are four
times thicker than need be ... his etch work is so thick that a grinder is needed to cut a trace, its all teflon wire and silver solder.
Those are some of the classics of today, but unlike their brothers of the past, they will sound the same when you pass them down to your
grandkids, unlike the Fender and Marshall amps of my generation.
This was an article on tube manufacturers and how the "Groove Tubes" 1-10 scale could be used for tubes that
were desired for a Mesa Boogie amp which has a fixed bias.
Tech Tip 021402 - Groove Tubes vs. Mesa Boogie Tubes
Many people do not realize that there are not all that many vacuum tube factories in the world.
Mesa Boogie does not make tubes.
Groove Tubes does not make tubes (with the exception of their new 6L6GE which will be coming out in the next few months, which is in BETA test
now in a few cases.
Ruby does not make tubes.
The major factories are:
Reflector in Moscow .... makers of Sovtek and Electro Harmonix tubes.
Svetlana in St. Petersburg ... makers of Svetlana
JJ / Tesla in Prague .... makers of Tesla, and they were the OEM in the past for Siemens, Mullard, and Telefunken in many cases.
Sino in China ... which is a group of a number of tube factories in China.
There has always been talk that the Chinese tubes were bad, but in the last year, the money that they have put into new tooling has paid off. They
are on their 3rd newest generation of the 12AX7, and it looks as though it will be found to be a pretty good tube.
In any case, Mesa and Groove tubes both sell all these tubes. In the case of Groove Tubes, they use a rating scale of 1-10. Mesa Boogie uses a
color code system. Here is the cross reference to use the same "grade" of tube if you switch from one vendor to the other:
Mesa Scale crossed to Groove Tubes scale
The reason for Mesa's narrow range, is that their amplifiers do not have an adjustable bias. This way if you have one of their amps, you can use
any of their tubes safely.
For amplifiers without an adjustable bias, if you want more headroom and/or power as an example ... and you amp had a #5 or Mesa "Green" tube
in it .... and had an idle dissapation of about 50%, you would just go to a "White", "Blue" or a Groove Tubes #6. You will see your idle dissapation
go up to maybe 55%. This is a way to fine tune your amp.
Technical article on the importance of a matched output section and matched phase inverter / driver tube
MATCH YOUR OUTPUT SECTION!
Why do two Marshalls or two Fenders, of the same model, and even year, sound different?
For one thing, there was a +/- 20% or more variance in components used, but the more common reason is below.
I originally wrote this with a subject of dead spots in your sound or tone that people felt were due to dead spots in their guitar (or bass) neck.
In the last few months while blueprinting amplifiers, I have had to explain over and over about a mis-matched output section and its impact to many.
I thought I'd write a little here ....
In any class A/B amplifier, because of the NFB (negitive feedback loop ... usually labeled as the PRESENCE control), any disparity between the
upper part of the sine wave (produced by half of the output tubes), and the lower part of the sine wave (produced by the other half of the output
tubes), is cancelled out by the NFB circuit by design. This is the reason some notes "sing" when your amp is pushed in the output section (rather
than pushing the input in a master volume amp), and other notes do not have the same magic.
The reason some of the great blues players have that tone, is that their amps are taken care of people that know how to adjust or deal with some
of the issues that cause this lack of luster.
Since no tubes are even close to identical, this cancellation is always going on. The object is to limit this as much as possible.
The most common way people match an output section, is to use good quality matched tubes. The industrial spec for a match can be as high as
+/- 20%. A good match by a lot of tube vendors is +/- 10%. I believe that even the untrained ear can hear the difference when a output section
matched within +/- 5% is used. In the amps I set up for the folks that retain me, my spec is less than 2.5%.
The most overlooked and misunderstood part of the output section is the 12AX7/ECC83 (Marshall style) or 12AT7 (Fender style in vintage cases)
Phase Inverter tube. This is the tube that drives the output tubes. A lot of folks that specialize in making amps sound great don't understand this,
but fix this accidentally. They tend to use very good tubes, such as JAN spec 5751's etc., where the match is closer, and closer matched tubes in
the output section. They also use tubes that sound good in the first gain stage positions, rather than the common Sovtek WA tubes which most
manufacturers use (because they are sturdy, not as expensive, and ship well without developing microphonics).
When I scope an amp in the lower frequency region, the vast majority of the time, the upper and lower parts of the sinewave are not even close to
equal. This is more disparent than just a slightly mismatched set of output tubes. At this point, I install a matched phase inverter / driver.
The problem with phase inverters, is finding a matched tube. You have to remember that a 12AX7 / 12AT7 etc., is NOT a single function tube as an
output tube. It is TWO tubes (two triodes), sharing a single bottle.
VERY FEW TUBE COMPANIES MATCH THE A AND B SIDES OF PREAMP TUBES. They warrant the tubes to work, and warrant them not to be
microphonic, but do not say they are matched. This is not any bad commentary on tube suppliers .... to do this matching is time consuming and
requires specialized equipment. If you can find somebody that has a Tektronix tube curve tracer, and bring them a bunch of tubes, maybe you will
be lucky and find a match. There are a few tube vendors on my feeble little website that do offer matched preamp tubes, mostly those dealing in
high end audio applications.
Matched phase inverters and output tubes are one of the reasons some amps "sing" and others are pedestrian compared to their brothers and
If you seem to have a lot of dead spots, try a new phase inverter tube. This is usually the preamp tube that is the closest to your output tubes. It is
a trial and error process, but you may get lucky.
By the way ... THIS IS EXTREMELY CRITICAL WITH DROPPED TUNINGS, 7 STRING GUITARS WHEN IN THE LOWER RANGES, AND EVEN MORE
CRITICAL WITH BASS GUITARS WHEN USING TUBE AMPLIFICATION.
This was in response to a forum question where somebody was asking which of two amps was "better"
Hmmmmm.... Bogner or Soldano? Which is better?
Can I ask a question to help answer the question ....
You are walking down the street on a nice day. You are a single male, open to a night out with female company if the situation presents itself. As
you walk down the street, coming right toward you are two girls.
One is 5'8", blonde hair, green eyes, and drop dead gorgeous.
The other is 5'4" brunette, blue eyes, and equally as much of a knockout.
As they both approach and get close enough to speak, one of them gives you a big smile and starts the opening of conversation, and starts to flirt
with you in a nice way.
Do you really care at this point which one of them it was?
Just like girls .... or guys (if you're female) ... or actually girls and girls or guys and guys or .... Its a matter of what you are open to at the time
and what appeals to you. Both the girls in this case are terrific, and I think both your amp examples are terrific too... its a matter of taste and timing
.... how much cash do you have in your pocket when you walk into the store and one of those amps is on the floor.
Preamp Tubes - The most critical, least expensive, most overlooked tubes in your amp.
The tonal signature of "your sound" and interchangeable without adjustment or the need of an amp tech.
Unlike power/output tubes, which are routinely matched when they are sold (in different ways, some much better than others), preamp tubes are
tested at best to: (a) make sure they work ? (b) they are not microphonic. In testing, we have found that some suppliers don?t seem to test their
preamp tubes at all, as we have found one side of the triode that is dead at times. Since most warranty preamp tubes for up to 6 months and
longer, they possibly figure that is cheaper to just send them out as they get them in, and if there is a problem, it is cheaper to just give the
customer another tube. This is of little comfort to somebody that either has to make another trip to their music store, or worse, box up the bad tube
and ship it back to the supplier, and then wait for its replacement. This is one reason to consider a proven supplier when you buy preamp tubes.
Today?s amplifiers, whether modern high gain types or boutique amplifiers, have one thing in common; the preamp tube in the first gain stage
(usually V1 and / or V2) sets the tone and initial gain structure of the amplifier.
Amp design -
Today?s modern amps get just about all of their characteristics in the preamp section. How the gain stages are set up, how the EQ is set up, gain
structure, and tone stacks, all are the main aspect of the sound character of the amplifier.
Amps such as Mesa Boogie, Fender, Marshall, Bogner, Peavey, and others, all use the same Sovtek, Svetlana, JJ/Tesla, Electro Harmonix, and
other power tubes from the same factories. In spite of the same output sections, and in many cases the same range of B+ voltages on the plates of
the output tubes, these amps sound different. This is all because of different designs, primarily in the front end, or initial gain section of the
Today?s newly made preamp tubes are very inconsistent compared to the tubes of the 1940?s to 1960?s. There is little need in the medical sector
or the military sector for tubes. They are primarily used today in audio applications. For the high end audiophiles, their needs are more easily met,
as their tubes are not subjected to the same stresses as those on a guitar amplifier, they use less of them, and they last much longer. There are
high end audio suppliers that will match tubes and hand select them, at much higher costs (check out a Western Electric 300B matched pair for
example). They pop their tubes in, and ten years later, all is still just fine.
Tubes for the guitar and bass player for use in the preamp section, are a different story. The tubes today are very inconsistent. You contact your
local tube supplier, plunk down your money, and the roulette wheel is now set into motion.
To show the inconsistency, we went through a batch of over 100 tubes that were from the Electro Harmonix 12AX7EH, ECC83, 7025, Sovtek
12AX7WA, LP, LPS, Chinese 12AX7C (old tooling and new tooling), and a few others.
Basically, the standard 12AX7 spec that applies to 12AX7 / ECC83 / 7025 tubes, has a reference of 1.2 mA at 250 volts with a -2 volt bias.
Some people like to use those little references that say if you want less gain than a 12AX7, use a 12AT7, as it has only 70% of the gain of a 12AX7
etc. These little tips are cute, but with the wide range of inconsistency out there, they are not all that useful, as it is still a matter of chance. The
12AT7 has a different current capacity than a 12AX7, so if you are just looking for less gain, then you may, or may not get it with just a different
12AX7, even from the same brand, same date code, and same batch ? just by swapping tubes around already in your amplifier. With today?s
inconsistent offerings, the old tables of gain cannot be used with much accuracy.
In the tubes we went through, keeping in mind our 1.2 mA / 1600 transconductance industry standard spec, we found our samples ranged from 0.7
mA to 1.6 mA. When you take into account, that the amplification factor of a 12AX7 is 100, there is a dramatic difference in these tubes.
Looking at a 1.6 mA tube, we see a factor of increase over the standard of 33%. This is a LARGE number. A 1.0 tube versus a 1.2 tube will turn
the gain you loved in your 5150 into something less than what you used to know what you liked there. You sit dumbfounded ? how can this be? I
just put in new tubes, the same as what I had before?
You want even MORE GAIN from your Triple Rectifier or Bogner? Look at those first gain stage preamp tubes, and get some tube vendor to
measure them for you. If you have a 1.1 in there, and put in a 1.3, you will hear the difference in gain IMMEDIATELY. This is not a subtle change
that only the "experts" can hear. Leave the settings on the guitar and amp the same, swap the tube, and listen again.
When we see a transconductance of 900=1200 versus the 1600, the way the tube reacts is different too, in this case, its rise time is about 25%
slower. This might be just the ticket for a blues player, looking for some nice initial compression on the pick attack, but it may not be the sound for a
metal or speed player.
Transconductance in the testing, ranged from 1060 to 1790. 1600 is the industry standard.
There is one other aspect of preamp tubes. Unlike power tubes, where one tube is one tube, a preamp tube is two tubes in one bottle. There is an
A side and a B side. The are independent units sharing only the heater. In a Marshall amp as an example, the NORMAL channel input 1 uses one
side of the tube, and input 2 (lower gain input) the other side. The BRIGHT channel uses the other side of the preamp tube. BUT, anytime we use
that tube in the phase inverter position or driver position of the amp (which is the driver for the power tubes), then having the two sides matched is
important. This matching subject has been covered before, so I won?t elaborate on this again here.
New versus Vintage amp needs -
NOS tubes are sought after by folks that have original amps like Fender Tweeds and the like. If you want the original sound, feel, and character of
these amps, then NOS is about the only way to go. Getting NOS tubes for your amp to be correct is much easier in some ways than getting decent
new tubes for an amp. There are folks that deal with NOS tubes. Some of them are on my website, but I will say here, that I can recommend KCA
and Tube World very highly. Either of these folks have the tubes and the equipment that will let you know that when you purchase a Mullard or
RCA 12AX7 or whatever, its characteristics will be noted for your information. If you are in Europe, check with Watford Valves, as they also have
NOS offerings. Eurotubes in the USA, a JJ importer, also may have some NOS offerings.
When it comes to new tubes for you modern or class amp, or new boutique amp, there is the problem. Preamp tube suppliers guarantee the tubes
to work, and not be microphonic. That is about all they can do. Going though tubes that retail for less than $20 in most cases, one at a time to
measure them, is beyond reason economically. Watford Valves will do this for folks. Other folks offer these services, and in most cases, they are
an additional cost. In my mind, the money is well spent. When you want a nice high gain tube for your Rivera or Demeter, putting in a tube that is
30% down from spec., is not the ticket! At that point, what can you do with that tube? Take it back? Why? It works. The store or vendor never
stated it would do anything more than "work". Perhaps they will exchange it, and now you start the process over? And over. And over.
Recently I was working with a fellow named Tom Dunn. He plays a 5150 II. We performed a blueprinting session of his amp, and it was found after
going through maybe 2 dozen of his own tube stock, that he had picked out his tubes by ear, and placed them in the most advantageous position in
his amp. You can do this by ear, if you have the ears of this guy, and also the time (he did this over many months), and the tube stock.
Your first gain stage in your amp is its soul, sound, and character. We talked here about gain, and a little about rise time, which is a subject in
itself. We did not get much into "sound", such as the articulation and definition that comes from NOS tubes like the Mullards and Telefunkens. If
you have an older amp with a more moderate gain structure, and want it to sound closer to magic, than this is the way to go. In a modern amp, a lot
of the articulation from the output section is not the target of these designs. Today?s designs look for two or three or more stages of gain, channel
switching which we did not have on the older amps of yesterday, and flexibility. The only flexibility we had when I was the age of most of you, was a
high gain input and low gain input ? or tuning the reverb on or off J
All I can suggest, is try to find a tube vendor that can supply you with the tubes you need with some degree of classification. This way, if you have a
1.3/1670 tube in there now, and you want to tone it down a bit, then maybe go for a 1.1 ? it will make a difference. If you want tonal changes in
color, rather than gain and compression, then you want to go with a little stash of tubes, depending on your use for the day or evening. Most of my
clients keep the following:
JAN 12AX7A - Most often general use.
12AX7C - Chinese 12AX7 - take off some amp edge or brightness
12AX7EH - Electro Harmonix - general use
ECC83 - for the Marshall sort of sound
7025 - for the Fender 60?s and 70?s sound
5751 - for blues and less aggressive attack (and perhaps less gain as compared to an in spec 12AX7)
(this was written in May 02 or earlier. Since that time, there have been many new developments, so be sure to look around in other areas here for
ongoing test reports on all the latest 12AX7 / ECC83 / 7025 family.
Here is a post to me that came up on one of the forums on 9/11/02. I thought it was interesting enough in
a personal way to put here....
Originally posted by Boggs:
Yeah, Myles, you do rock, but sometimes I get the feeling that you need to get out more... Boggs
When I am here at work, I may want to get out more, but not all that often. When I am at home (with a 7 and 11 year old), I KNOW I need to get out
more when they start going at each other!
Actually, over here at the "day job at GT", it's really one of those gigs where you get to fool around with classic guitars and amps, and get paid to
do just that. Just earlier today, my "task" in the morning was to check some voltages on one of Aspen's black faced Fender Super reverbs, then
play it with his pre-serial # PRS, a vintage strat, and a dual P-90 LP junior. Then Aspen came in and I would tweek while he played .... tough gig???
During the day, we usually have somebody that most folks may have heard of, that we support as part of our "friends and relations". if you look at
the list over at: www.groovetubes.com/f-n-r.cfm ... there is probably a name or ten that you might recognize. So... then you get to work with these
folks and make their rigs sound better, they get all fired up and happy, and you go home with a smile on your face. It's great to have a problem
and then be able to solve it in a day, rather than corporate issues that take years. It's also great to come up with an idea for an amp, effect, or
studio piece of gear, go to "the boss", and have him say "go ahead and try it". You have a warehouse full of parts, a full machine shop with a
lathe, sheet metal fabricating equipment, Bridgeport mill, hydrogen ovens, vintage tube making equipment (grid machines, sealing machines,
vacuum pump down stations), and all the resources you could ever need in regard to vendors for custom transformers, and little parts, and right
down to vintage bakelite RCA knobs. The entire crew here are all great to work with ... it's more of a family than group of "employees". Meetings,
unlike in the "corporate world" last a few minutes .... not hours (and without the bickering and grandstanding that is done by all the folks on the VP
or Director track for their personal goals.
When all of that is not going on, we are designing new high end amps and studio stuff, preamps, mics, etc. Like Aspen says .... "maybe someday I
will get a real job". I can see where that came from (its the quote out of the Tube Amp Book on one of the color photos).
At times it is hard to get away from the business. There are evenings and weekends where support before a show or tour is necessary. Part of
the job is also from my Blueprinting business. I guess the advantages for most here, would be free access to see a show, but typically, I bail out
after the first few songs once I know all is well.
But ... on the practical side .... there is a lot of free backstage food, and some of it is great. The best catering in the business is Vince Gill, Kenny
Wayne Sheppherd, and Bruce Srpingsteen. The best venue in the Los Angeles area for food in Gotham Hall on the Santa Monica Mall. They
also have one of the nicest "rooms", where you can be up close, or in a number of different areas away from the crowds. Their food is great even
when you just want dinner on the "west side", even when there is no entertainment. Carl over there is the guy that oversees the place (and the
kitchen, and the menu), and he is great. If Carl and Tom (Dunn) are playing there, don't miss it! Think of them as the voice of Eric Clapton (15
years ago ... that's Carl ... who can play a Strat with the best of them), and Van Halen with a twist (Tom Dunn). By the way, both of them have
blueprinted amps, but to be honest, Tom Dunn's amp was pretty darn terrific long before I ever laid a jand on it. His Peavey 5150 II has been
described as "the best sounding 5150" around, by the designer and Peavey. His CAE 100 is equally impressive. Tom is an amazing player, just
as versitile in hard rock as he is in an acoustic ballad. By the way, if you do get to Gotham Hall, say hello to Carl (who is also a master chef as well
as a great player and song writer). If you are lucky, maybe Carl will take you to the bar and you may meet Rachel, one of the bartenders. Leave
her a BIG tip, she is a sweetheart. I have so much PULL over there, that if you leave a big tip and drop my name, you may get a few extra ice
cubes in your glass :)
But I digress here ... sorry. You would think I weigh 250 pounds from my talk here on the "eats".
The other plus ... even though I am not exactly known as a fashion trendy sort of guy, is free clothes!.... I have PRS stuff, Norm's Rare Guitar stuff,
Fender stuff, etc. The more shirts, the less often laundry happens! That is a big plus.
The smaller groups are more fun to work with in most cases, than the large ones. Maybe I should say they are more of a challenge. They play all
sorts of venues and in many cases, different types of music. Tehy are on a limited budget, so making the most out of each expense is really
important. These are the folks I use to test new stuff for me. Big groups have the same sets all the time, use the same settings, and play in large
venues with great electrical power and sound systems. That tells me a lot less, than a band ithat plays in a "dive" bar with 95 volts feeding their
amps off an extension cord run from the kitchen. Sometimes they play hard rock one gig, cover tunes the next, then a wedding on the weekend.
They need to be versitile and have a wide range of tones available from a single amp many times.
I guess I do get out a lot in one sense, but it is always centered around the same subject matter ... amps, guitars, and music. Even now at 54
years old, is still a blast.
NOS TUBES? The hype, myth, and reality.
Are NOS tubes any better or different than the new tubes made today? Many folks cannot seem to tell the difference. After a recent batch of new
tube testing, where NOS tubes are used as a standard for some areas of the test, I thought some explanation of this subject was warranted.
Why can?t some folks hear and feel the difference? This is actually pretty easy to explain. A lot of today?s players, especially the younger ones,
have never played an amp from the Fender Tweed era. These amps had simple circuits and lower gain. Playing one of these amps yields a feel
that cannot be approached in higher front end gain amps. The higher the gain, the less articulation, harmonic content, and frequency response
in many cases. These vintage design amps also would sustain at low volume levels, or most any volume level.
You don?t want to accept this? Try this simple test. Take your high gain amp, turn the master way down and the gain pretty far up. Hit a nice
power chord, and while the chord is ringing, hit a note or two ? in a lot of amps one will barely hear the added notes. Crank the gain all the way up
now. In a lot of amps, the sound has become so indistinct, that even chord changes are difficult to discern.
Vintage amps and vintage circuits have a different tone and the amp plays differently. Most of the younger generation have not had a chance to
play some of these amps, if any of them. It is like making comments on how fast a Ferrari BB512 is as you watch from the curb. It is different than
making the comments after sitting in the drivers seat.
So, the hype of NOS tubes? The hype is generally the comments from folks that plug a nice NOS tube in their very high gain amp and crank it
up. These folks need to spend a bit of time and also learn what to listen for such as harmonic content, life in a note that is held, and body and
depth in the notes.
The reality of NOS.
There are many factors, but economics and law suits and liability, and the EPA are two of the today.
Liability? You bet. The money goes to safety. Cars ? airplanes, etc. This boils down to STEEL. Yup,
That stuff your car body is made of, turbine blades in jet engines are made of, etc.
Steel was really coming into it?s own around World War 1. Submarines needed metals that could be compressed at depth over and over.
Airplanes need crankshafts that could stand up to their job. Metallurgy continued to improve. There were great steel companies such as US
Steel in the USA, and the fine Swedish and Finnish steels. Today, most of the US steel companies are gone. The great controlled steel recipes
are reserved for more critical applications such as aerospace or the automotive and aircraft industries, where a lawsuit is just around the corner.
If a tube fails in an amp, who cares?
The EPA makes production of cathode coatings and like items more costly and at times almost impossible.
Then, there is economics from a geographic point of view. The Russians are not cranking out defense equipment any more. They have no
money. One batch of steel is completely different than the next batch. In Eastern Europe, it is the same when it comes to the quality control of
metal alloys. There is little economic reason to have high quality control standards in a cheap vacuum tube.
When tubes were used in scientific, medical, and defense equipment, quality was an issue. Today it is not an issue.
So, where does this get us?
Today?s new tubes are very inconsistent. Their specs run plus to minus 50%. They are not linear (those plate alloys react differently as the
frequency changes and heat changes for one thing), and they do not meet specification that were established in 1957. They may meet one
spec., but only if you are lucky.
Recently I tested some new Sovtek 12AX7LPS tubes. These brought this subject home. Using a myriad of test equipment, it was found that in
one area, these tubes did beautifully. This area was simple gain. Using the Vacuum Tube Valley small tube characterizer, these tubes produced
the ?gain? of 100 or over a bit in some cases, just as a 12X7 should. BUT ? a tube has more than one factor.
Gain. Think of gain as horsepower. Just like cars today, some have a lot of horsepower via small displacement and turbo charging. It took a few
decades to get back horsepower into cars on the road after the gas crisis of the seventies. There are big dollars in cars, and consumers wanted
horsepower and performance. Over the years, even with CART standards, we got back horsepower. We got little else initially, but with all those
consumer dollars at stake, by the late 1990?s we also got back overall performance.
Today's tubes have gain. They may not have linearity (driveability), but they have gain, in varying degrees.
Output. Think of this as torque. This is one thing today?s tubes are lacking. Output is what pushes your signal though an ever expanding load of
resistors, signal processors, effects, and tons of additional circuitry in your signal chain. Look at these three and four channel amps, where V1
(your first gain stage and the mother of your tone), is expected to drive the rest of that mess of a signal path.
You can look at that Sovtek 12AX7LPS that had a gain of 100 or 105 in some cases, out with an output of 0.5 ? 0.6 millamps, it had half the
expected output of a good 12AX7.
Today's tubes do not have output. No torque.
Transconductance. Think of this as acceleration. The ability to react to a signal (or green light) and get off the line fast, slow down, speed up,
and take the corners. This also relates to how linear an tubes response curves perform. With a nice linear response, note changes have the
same levels. If this factor is lacking, you are fighting your instrument trying to compensate for it?s non linear character. Again, the Sovtek
12AX7LPS did not perform even close to 1957 or NOS spec.
By the way, the Sovtek 12AX7LPS is a great tube by today?s standards and performs better than a lot of them out there.
So, this third factor, TC or acceleration, is also missing.
Where does that leave us?
NOS for one thing is a short term option. Short term? Sure, this is limited stock. The reason the 50?s tubes fare better than the 60?s tubes in
some cases, and the 60?s better than the 70?s, and so on, was that in the 50?s, more tubes were used for critical applications. They had better
metals and QA.
There are some points of light on the horizon too.
Well, thankfully, the Chinese have money and are putting it into consistency. They see the economics of the music business as a strong market
with MTV, VH1, CMT, and more amp makers than ever before in music history. Each Chinese batch of 12AX7?s is better than the next. Their
?torque and acceleration? are currently better and closer than any of the Europe tubes generally. Their consistency is also the highest.
The last hope lies in the hands of folks like Aspen Pittman (Groove Tubes). GT spent a LOT of money to remake a tube called the 6L6GE as an
example. This was not an easy task, taking years. Using the original tooling was not all that hard (Aspen just bought it from a factory when it shut
down), but setting it up properly was another costly story. Then the daunting tasks of those materials in the recipe was the issue. Well, he pulled
it off, after many years and many dollars. We now have a great NVM ( I coined that NVM by the way ? new vintage manufacture) output tube. It is
not cheap, but it can be produced over and over now. I am not too worried personally on how long this tube can be produced, as I am 53, and I
saw enough tonnage of original plate material metal to make enough of these to last at least my lifetime!
There are other folks that work directly with some of the major tube factories. Some do this for a love of tone and amps, such as Aspen, some do
it for economics. The folks that do this for economic reasons seem to take shortcuts which show up in the end result product, as is the case in
some of today?s preamp tubes.
When it comes to new preamp tubes, keep in mind that today you have gain ? although all over the scale and inconsistent. You rarely have the
other factors, output and TC. If you know your preamp tube vendor, your chances with new tubes are much better. GT, as an example, tosses
about 50% of their tubes, as they have to meet tests for output, low noise, and a lack of microphonics. That is part of the reason for their cost that
some folks feel is higher than some untested tubes.
The bottom line here, is that NOS tubes were superior to the products made today. If you cannot hear this and FEEL this, turn down your gain,
raise your master, maybe learn what to listen for, and perhaps have your ears cleaned.
If after all that, you still feel there is not a difference, then you are fortunate to have standards which are easily met. BUT ?. You still cannot
escape the basic math and seeing that the data on today?s tubes do not meet those of yesterday unless you have a vendor that will go through
1000 of them to find a few great ones.
Myles S. Rose
Amplifier Power - Modeling Amps - and the misunderstanding of amplifier power
The vast majority of the time when I first walk into a venue where I will be listening to music for the night, I can generally tell if the performance will
be a memorable one. I can generally tell from the equipment setup, and not to brag, I have about a 90% track record.
The 10% of the time I am mistaken, it is generally easy to explain. I did not know the performer or group, and what to expect. I was invited as a
guest, and the music was already known to me as to not be of my particular taste, or a few other reasons.
The big tip off, is amplifier compliment ? amplifier power that is.
There are folks that have blinding fast technique. Speed metal players, fast articulate players, folks with speed as their underlying strong point.
Frankly, this is not my personal taste. I am generally impressed for ten minutes, but then my attention is lost. I generally ask myself, are these
folks practicing, or just looking for the right note? One note played with feeling and that has tone, is worth 100 64th note triplets from my point of
The biggest problem is amplifier power. When I see a 100 watt amp on the stage of a 150 seat venue, I know that I am in for trouble ? most of the
time. If it is a jazz player looking for a clean sound, then I am safe. If it is a speed player, well, then it is what I expect. If it is a rock or blues band,
then I know I am in for a very one dimensional performance most of the time. I know with good prospects, a few other things. The player does not
understand amps or tone, perhaps his main rig is broken and this 100+ watt amp was borrowed, or they are into a hi-fi sort of sound with little or no
A 100 watt amp, or even a 50 watt amp, will not distort in it?s output section at rational volume levels. Folks that are known for great tone and to be
great players, even in the largest venues, generally stay at around 50 watts or less. They are looking for a particular sound, tone and feel. They
let the stadium sound systems do the rest. If you cannot turn your amp, and most amps, to at least 6 or so on the volume, you will never tap the
soul of most tube amps.
Folks also do not understand ?loudness?. Many think a 100 watt amp is TWICE as loud as a 50 watt amp. This is not all the case. Double your
wattage, and all you gain is 3db. Sure, folks talk about ?headroom?, and think this is a huge requirement. Folks that actually need headroom are
clean players ? rhythm players, jazz players for some styles, and pedal steel players to name a few. A amp with a lot of headroom is a hi-fi amp. It
will be clean, and not have the dimension of touch dynamics of a lower powered amp. If you want more loudness out of a 50 watt amp, double your
speaker area, or go with a more efficient speaker. Going from a 83db speaker to an 88db speaker, is almost the same gain in volume as going
from 50 watts to 200 watts in amp power.
Then there are those folks that have the great idea of pulling two tubes out of their 100 watt amp to turn it into a 50 watt amp. To put it bluntly and
get a lot of argument, this is a stupid idea. A great amp is made up of many components. Power transformers, output transformers, capacitors,
and other parts, make up the design. If one takes a Marshall 100 watt Super Lead, and pulls two of the tubes, and properly sets the impedance
selector, turning the amp into a 50 watt amp, what actually happens? Well, we have a 100 watt power supply, that is now even less taxed than
before. The "50 watt" Marshall will now have less dynamics, less feel, less touch sensitivity. It will be a nice, clean, hi-fi, 50 watt amp. It?s 100 watt
power supply will never reach saturation. It?s output transformer will never be pushed. It will actually be cleaner than it was as a 100 watt amp.
The only distortion you will get is when the output tubes are at their limit, and this will be an unbalanced sound, although some might think this is
just to their own tastes.
Modeling amps? In the past, I have written a lot on solid state amps versus tube amps, and modeling amps versus the amp originals they are
modeling. Some folks think that modeling amps have some strong points such as a lot of sounds for the dollar or in a given space, or for
recording. Some think in a live venue, modeling amps can have limitations. To my way of thinking, this live aspect is sort of a ?yes and no? to me.
I see many folks with 50 watt amps in small clubs, where the soul of the amp is never tapped. Put an amp like a Line 6 Vetta, Fender Cyber Deluxe
or Cyber Twin, or Vox modeling amp out there, and you may be surprised. I may still prefer the PROPER wattage tube amp as first choice, but I will
take the modeling amp every time over the wrong tube amp. Why? Modeling amps allow a degree of touch dynamics and tonal ranges to be
captured at most any level. You have all sorts of controls for this ability. A Fender Tweed Bassman in a small venue will never be able to be
cranked to it?s level of tone potential for some music styles. A Line 6 Vetta may pull off the ?tweed sound? of the virtual Bassman in a much more
convincing and pleasing manner, at least to my tastes.
I think to end this, all I can suggest is, listen to amps, and play them. See how they react to your touch. If this is not a part of your music and style,
such as many folks that start the song at 110db and end it at 100db, then most any amp will work pretty well. As you develop an ear for different
tone aspects, and fingers and touch that can give you at least two more playing dimensions, then you will move to the next step of being a better
player, and also have a more heightened ability as a listener.
Point to Point amps - some myths and why some "poor" techs love them
Point to Point (PTP) and Printed Circuit Board (PCB) amps - an ongoing debate that rages on.
Why do many of the most "non-technical" techs prefer these? I know ... a lot of flak coming my way for my thinking on this, but it is my personal
thinking, whether you agree or not.
1. PTP is used, generally, on the most simple amp circuits. For a tech to find a problem, it is a much more simple matter of lifting one side of a
component and metering it, than dealing with a circuit board mounted board.
2. The simple circuit design makes it easier to "shotgun" and find the bad component.
3. Many PCB amps bring in a lot of business. It is easy to justify high charges .... "man, this is a pretty darn old amp, it's bound to need a lot of
work". Many of these amps make all sorts of "wierd noises" too, as their components have no support other than the ends of the component leads.
Think of them as big guitar strings with a weight in the middle.
Properly designed PTP amps are wonderful. Many are based on classic designs with simple toneful circuits. On the best of them, they are costly,
due to the attention to design and very high component costs. As an example, a 15 cent metal film resistor will be less costly than a 65 cent carbon
comp resistor. When these amps are built and wired properly, with high grade parts, the result is magic.
Many amps are called PTP, but not true PTP amps. Fender amps such as most of the tweed era, are really more of a basic form of circuit board
amp, a piece of tag board type of material is used to support the the components. This was one factor that made the amps much more sturdy and
road worthy than some of the other amps of the day. This construction was also used in the later tolex amps.
There are amps that use other methods of "PTP" such as Hi Watt, Ashdown, and Roccaforte. These amps use turret boards or standoffs. Here,
the components are phusically supported with shorter leads, and one component does not drag another and another to it's next attachment point.
These amps are some of the highest caliber and strength in construction. This was the sort of construction used in military gear in the past.
PTP wired amps, true PTP, are very inconsistant. With a modern PCB amp, there are curcuit simulators, that do the board layout with the trace
runs optimized for the least noise and best performance. In a true PTP, two amps can sound very different. Also keep in mind, that in the past, true
PTP amps were built with some of the least expensive labor they could find, which often reflected in the end product. Checking these old PTP
amps though, was easier, as they were of very simple design.
There are some great PTP amps, such as Steve Carr's amps, but you won't find these cheaply. High grade components and components
anchored and wire strain relieved, is all part of the cost of these amps. On the "hybrid" PTP amps, such as the Fender Tweed style newly made
amps, you have folks like Victoria. Again, attention to the highest grades of construction and parts help here. On the turret board front, you have
folks like Roccaforte. None of these amps are cheap, and each of them are worth every penny of their retail cost.
PCB amps .... why some techs don't like them. Again, complex design more often due to multi channels, swithing effects loops, and other features.
It is hard for the "technically challenged" to find a problem. They are less expensive at the lower or entry end of the amp scale, and offer a lot of
features for the dollar, but at times user lower cost components and faster construction methods. There are hybrid folks, that use the best of PCB
and PTP such as Andy Marshall (THD) as just one example. Chassis mounted tube sockets wired to a PCB substrate that is many times thicker
than most F-16 PCBs, and traces on the PCB that are so thick, that a moto tool is needed if you ever want to cut one of them.
A well built amp, well designed, and using great components, is a great amp, whether PTP, Hybrid, or PCB.
My most "supported" amps, those in the shop the most often, are the true PTP amps that are out on tour on the road, that were done by
"boutique" designers that jumped on the vintage bandwagon. They have parts that are true PTP, not tagboard (as the Fender Tweed and Tolex
era amps), and not turret board, but true PTP. These amps have those bigger caps, just hanging out in space, swaying around as the equipment
truck or bus bounces hundreds of miles down the road, from city to the next city. These amps end up with cracked solder joints at best in many
many cases, and most of them play their own symphony physically, depending on the note coming out of the speaker, as the components
So, before you consider an amp with PTP vs. PCB vs. Hybrid, do a little more thinking. If it is a multi channel amp with switching ability, the last
thing I'd personally ever buy would be a NON-PCB amp in this arena.
If your tech thinks that replacing a component on a PCB board is hard, then frankly, find a new tech. Components have been replaced for decades
on computer and military PCB's where folks lives rely on the unit .... not just a simple guitar amp. BUT, there is a bit of proper soldering knowledge
attention, and the proper tools, which are pretty darn easy to come by. If you tech complains about burnt etches and problems in PCB's, this may
be from first hand experience, and you may want to consider a different tech in the future. If they can't do this right, they are probably making
other mistakes much bigger in other areas.
Just as a close, some of what I think are the finest examples of various types of amp construction today in some examples:
Pure PTP - Steve Carr Amplifiers.
Turret Board Mil Spec - Roccaforte.
PTP/PCB - Groove Tubes Soul-O series amps, and THD Electronics - The best of both worlds.
PCB channel switching high grade - Rivera Amplifiers
PTP Tagboard (Fender tweed/tolex era) - Victoria Amplifier
PCB general - Fender amplifier in the reissue series amps.
I know there are many others, but to me, these are the best of the best. In the case of each of the above, there are some points where they excel
in all areas. Rivera amps (look at the cabinet costruction ... no self tapping wood screws as an example, but machine screws). Victoria amps (look
at the components, the wiring, and overall detail). THD (look at the boards, the traces, and overall detail). Carr (componets, properly supported
runs and parts, and overall detail). Roccaforte (overall detail, parts, and the amps are literally bomb proof). Fender new amps (very nice PCB work
in the target price range. Give me a "reissue" twin, super, or deluxe, and a few hours, and then you tell me which is the original after playing).
I know this will "piss off" a lot of folks, expecially some amp techs, and folks that may have spent way too much for amps that jumped on the vintage
bandwagon perhaps. There are many great amps that I did not mention, as this was long enough already, and the amps above I mentioned, have
had no downtime due to failures over the time I have worked on them for general upkeep.
I am now ready for folks to pick one or two details of the above, and write scathing retorts, rather than seeing the overall picture here.
Reissue versus the original amps - part II of Point to Point amps - some myths and why some "poor" techs love them
In the above "article", there may be the misconception (or from some of my two forum's comments), that I was saying that the reissue amps can "be"
every bit the same as the originals. This is not the case in many ways. It had also been pointed out that true PTP does not have minute
intercapacitance issues that would be the same as a tag board amp as an example.
On this second point, the tag board issue, one needs to remember, that guitar amps are a different breed than audio amps. Audio amps attempt in
design, to be "flat" and not color the sound over the intended frequency range. In guitar amps, they have coloration. This is why we love them, and
prefer one over another. The construction and components are a part of this sonic area.
When comparing an original amp like a Fender Super Reverb with it's original, it is easy to make an observation if we compare to cars as one
example. The original and reissue of the amps can be made to sound very much the same when recording. This is different than actual playing.
Like a car, if a BMW M5 is shown in a movie, it is still a car we recognize, whether filmed in 35mm, or on video tape. Same car, same color, and
same driving direction. If we sit in the back seat of the actual car, it is also different than the experience of actually driving the car. It is much the
same with guitar amps and playing.
If one plugs something like a POD, and we plug into a mixer with a CD of a know artist where we can play along, we can dial up the "same" amp as
was used, play the same riff, and it sounds the same or at least very close to another typical listener (maybe not a player themself). The reissue
amp can be made to sound close, or even "better" than a lot of the original amps. Part of this is because the original amps in many cases, have not
If we take an original, and set it up properly, now we can do a more accurate test. The amps will generally have the same general sound, but the
player reaction and touch dynamics will be different. There are many reasons for this. Some of these reasons are things like transformers in the
originals (their material, interleaving, voltages), capacitors, and resistors. In the case of even a simple item like a resistor, the original amps used
carbon composition resistors rather than the metal film resistors of today. The carbon comps are not as stable, and as they get warmer, their value
changes. This makes them more musical to many folks tastes. The top makers of some amps like the Fender Tweed style of people like Mark Baier
of Victoria, only use items such as CC resistors, the proper caps, transformers, and construction methods and materials. Most reproductions or
reissues from the original maker though, use metal film resistors, different caps, and in many cases, circuit boards. The amps may sound similar,
but will "drive" very differently.
Part of the joy in music that is not in the realm of the "non-player", is the vast dimension of the playing of the instrument and the interaction of the
instrument (guitar-amp) with the player in touch dynamics and countless additional areas. If you want to just lay down a track that sounds like the
original, then many amps or virtual amps are available. In a live application, the differences become more of a factor, and if a player, an even
stronger set of issues. You don't have to be some "monster" player either. In fact, at times, the "better" or more articulate the amp, the better one
may sound. Hit an "A" note on one of the upper strings while sitting right next to an amp. It should be at a lower level. Try to observe, where the
note or sound is sitting on the face of the speaker cone in your mind. As the note decays, does it sit in the same spot, or "swirl" around the speaker
cone to various areas around and more and less toward center?
If it moves, you have an articulate amp, or pehaps a good speaker, a matched output section and great tubes perhaps. This is just a part of the
difference that you will see happen more often in the original amps or the amps from some of the great makers.
Written by Andy Marshall, the President of THD Electronics (www.thdelectronics.com) in response to a forum article I
wrote in a response to one of the comments by a member of the forum.
Doug, you've got a pretty good handle on most things on the whole, though I find you a bit bold in some of your sweeping statements. You clearly
do not understand PCB layout or assembly techniques and sound like someone terribly afraid of going out of business due to competition.
Not all manufacturers choose to use PC boards just to save money. We use them for consistency more than for price, but making a somewhat
affordable amplifier is a nice benefit. I don't think that someone should have to be a lawyer or Microsoft Millionaire to be able to afford a new
amplifier that is hand-built, reliable and sounds and feels good to play.
If a PC board is designed correctly and the correct components are used, the amplifier production should be absolutely consistent from one unit to
the next. No re-routing of traces should ever be necessary to make an amp function or sound right. If you find it necessary to change and re-rout
wires in your amps, then you are not in production, but are just making a series of unstable prototypes. Treble reduction to the point where it
reduces the clarity of the amplifier is not an acceptable stabilizing technique for either a PTP or PCB amplifier.
Recently, we got a call from a tech complimenting us on our old Plexi model amplifier (that we built between 1990 and 1995), but he said that it was
just a little bit "stiff in the high-end" compared to a real Marshall Plexi. To back up his point, he told us that he had a real Marshall Plexi on the
bench next to ours and was comparing the two side by side. What he did not seem to realize was that no two Marshall Plexis sound the same. They
were terribly inconsistent with their component sources and values, not to mention the inconsistencies in wire routing.
Taking a point to point or a turret-board amplifier, if one moves the wires around, the entire sound and character of the amplifier can change, often
dramatically. This is a well-recognized phenomenon.
If you understand these interactions well, you can design a PC board to sound and feel any way you want it to. Furthermore, every one will sound
the same. How many times have you plugged into an old Marshall-50 watt head, only to be terribly disappointed by the sound and feel of the
amplifier? While this may be caused by poor tubes, at least in part, inconsistencies in the internal layout of the amplifier often play a significant role.
If you understand how one component affects the component next to it and how one trace affects the trace next to it, then you should be able lay
out a circuit board correctly the first time, not by building 10 and picking the best one. Mind you, it takes many years of experience to develop the
sort of understanding of the capacitive and inductive interrelations involved. In the old days, I did this for a living for other companies, designing
circuit boards for the audio sections of amplifiers, mixing consoles, signal processing equipment, etc... While I am under confidentiality agreements
with almost all of my former clients, I can tell you that there is hardly a professional recording studio in the US or Europe that does not have some
audio circuit board with my layout in some piece of equipment.
After a few hundred such projects, one develops an intricate understanding of how traces and components interact
A number of years ago, Guitar Player magazine did a review of one of our amplifiers. They stated that they, as a general rule, do not care for circuit
board amplifiers, but also said that I had addressed every one of their concerns, and that they had nothing bad to say regarding our use of circuit
boards. It felt good to see someone start to understand what it is that we do and why.
Certain components throw a rather large field. Others do not. Some components are very susceptible to the fields from other components, while
some are not. Components can affect the signal passing through traces, and traces can affect the signal passing through components. It ends up
being an enormous network of positive and negative feedback between components within each other's sway. This is why the distance between
specific components on the board and the physical orientation of the components relative to one another (rotational orientation, as well as lateral
placement) cannot be ignored. Furthermore, which traces are parallel to one another and at what distance, which traces are perpendicular to one
another and that what distance, and the amount of ground plane in-between them can seriously affect the overall sound and feel of the finished
Most people design circuit boards either haphazardly or for the greatest parts density/easiest and least expensive manufacture. Neither of these
methods belongs in a high-end amplifier, and such approaches give PC Board designs a bad name.
If you know what you are doing, a thicker board is better than a thinner board (ours are .093" or 3/32", most are .062? or 1/16?) and that thick
copper is a good idea (ours is 4 oz, most use 1/2 oz or 1 oz). One of the greatest problems facing most circuit board amplifiers is board flex. Board
flex creates metal fatigue in the copper. As the copper cannot really "break", it just crystallizes and makes tons of noise. This is much worse in
combo amps, of course. We go to the trouble to support our boards ever few inches. Our design standard is that 100 pounds of force on a 1/4"
diameter probe should not be able to flex the board more than 20 thousandths of an inch at any point on the board. All of our amps designs must
pass this test. For comparison, most Marshall and Fender circuit boards would break under such force, and would flex more than 3/8 of an inch just
Through-plated holes are an absolute must, with solder pads on both sides. This makes it much harder for a repairman to inadvertently lift a pad or
a trace by overheating or from poor technique. The way that we have addressed this is to start with boards that are clad with 2 oz copper, and in
the through-hole plating process we add another 2 ounces. This leaves us with traces and ground planes of 4 ounces, and through plated holes
with 2 oz copper in the holes themselves. I have seen some other people start with 3 oz copper, plating on an additional 1 oz, and I have not like the
results I have seen. The through-holes pull out too easily.
Contrary to popular belief, ?Orange Drop? film capacitors are far from great. They are OK for certain position in certain circuits, but their
consistency from one to the next is atrocious. Maybe this is part of why so many people who use them in PTP amps find the need to make wire
adjustments. This is a big part of what I mean by using the right components.
As for PCB solder joints becoming problematic with time, this is no more a problem than on PTP. A good solder joint with absolute minimum stress
on it (using the right component with the right lead length and the right mounting technique) will yield the longest and most consistent life. Assuming
that the flow-solder machine is correctly set up, the right solder, right flux, right solder temperature, right flux temperature, right pre-heat, right
cooling, etc? are done, a flow-soldered board will last longer and have higher quality solder joints than a hand-soldered board. If you doubt this,
ask yourself the following questions: How do you decide what solder to use? Do you choose SN60, SN63, SN96, Savebit or some other? How do
you decide what flux to use in your solder and how much? How do you decide what temperature to set your iron at? It all makes a HUGE difference
in the quality and consistency of your solder joints. If you cannot answer all of these questions, then you cannot even have a clue about the
long-term consistency and life expectancy of your products. This, along with countless other points, is part of what separates the hobbyist from the
In a PTP amp, the entire surface of the solder joint is exposed to air, and thus, to corrosion. In a through-plated PCB amp, only the top and bottom
surfaces of the solder joint are exposed to corrosion, not the majority of the joint, which is within the through-hole, which is where most of the
contact is made.
We use only FAA-approved aircraft assemblers in every stage of our manufacturing. They have to understand all of these points completely. The
FAA is even more stringent than the military. Also, the aircraft industry is just about the only industry left that uses PCBs for the electronic
components wired to chassis-mounted electro-mechanical components like the controls and connectors. They do this because countless FAA tests
have shown that devices built this way last longer and are more reliable and consistent than any other method, even taking cost out of the picture
entirely. This is, of course, why we use the exact same methods.
In closing, I absolutely believe that circuit boards, when they are well-designed and laid out, are in all ways superior to other manufacturing
techniques when one is building amplifiers in quantities. If I did not believe this firmly, I would not be doing it. This said, I think it is a terribly
expensive and cumbersome method for hobbyists to attempt. If you don't have a great deal of experience under your belt designing circuit boards,
you won't like the results. Point to point and turret-board techniques offer the hobbyist and the small-scale amp shop the opportunity to easily tweak
their designs, as is so often necessary. So, unless you're going to be building 50 amps a month or more, it is probably best to stay away from circuit
Preamp Tubes - Gain / Output and Matching - Part II
I have written other papers on each of these subjects, and questions and comments come up where additional explanation has been helpful for a
lot of folks.
The first basic point to remember here, is that unlike a power tube such as a 6L6, EL-84, EL-34, 6550, and others, a preamp tube is a dual triode
in most cases. This includes tubes such as a 12AX7, ECC83, 7025, 12AT7 and others.
In regard to matching of two sides of a preamp, many folks feel that in a balanced circuit which in a way, sums the two sides of the tube, that
balancing is not necessary. The high end audio industry recognizes the need for a balanced phase inverter or drivers, unlike some in the musical
This is necessary (the balance of two sides of a triode). I will use an example to try to illustrate. Taking a twin engine airplane, let's say we have a
typical light twin with two 300 HP engines designed to cruise a 250 knots. We have each engine at 1800 rpm. One engine is at 24" of manifold
pressure, while the other is at 22" of MP. The airplane structure itself, is the "balance", much like the balanced circuit that some amp builders feel
will negate the need for a balanced phase inverter. The plane may fly just fine, but we need to add a bit of rudder trim, and out fuel use will be
higher, and overall performance and balance of the airplane (or amp) will suffer.
Now in the "art" of balancing a dual triode, there are many folks that will perform ?matching? for a few dollars of additional cost. In almost all cases,
this match is a gain match, or a current match, or a transconductance match. Due to very high inconsistencies in today's preamp tubes, matching
for any of these factors, is an improvement over an unknown tube. This sort of matching at least makes sure that we do not have a 200 HP engine
on one side of our airplane and a 400 HP engine on the other side. These numbers seem like wide examples? Not in the least. A typical new
preamp tube these days at a given bias and voltage, in the 12AX7 family, will run from ½ a milliamp to three times that. They typical average is
about 70% of what is expected as standard spec by the way.
Now, in our airplane example, our horsepower may be way off with two different HP engines in the stock amp, and with the easiest form of balance
as above, at least our HP is the same, which is of help. But, we still have the factor that one of the engines is running at a different RPM or
Manifold Pressure. This is the ?time? component that is missed in most matching. You can balance the circuit all you want, from an amp designers
standpoint, to compensate for voltages or current, but you cannot balance the time component, all one can do is average in a manner.
A true balanced dual triode, is just like two output tubes. We want their characteristics balanced in many aspects. This is why we do not plug in
one EL-34 and one 6L6, even though this would work, and make some sound come out of the amp. In fact, in this example, these two totally
different tubes would be closer in characteristics than the typical new dual triode of today.
What is required here is to select for output, and TIME. This can only be done on very sophisticated equipment such as a vacuum tube curve
tracer. The two curves, as all voltages of operation, and with a signal applied to the tube, are compared and sought to be as close to identical as
can be achieved. This is very costly as it is very time consuming. At times, only 1 in 50 tubes will make the grade. Remember, it is THIS little tube
that dives your final output stage.
The folks in the high end audio industry know the difference in balanced or matched inverters, and many amp makers do also. There are still a lot
of amp folks out there that want to fight the points here, mostly because they have not taken the time component into their thinking. If they have
tried ?matched? phase inverters from simple sources with simple current match methods, this may be one reason that there was not as much
difference as they had expected.
The Special Applications Group (SAG) at GT was formed specifically to address issues such as vacuum tube development, testing of factory
samples and production, and unique products. The SAG-AX7-MPI and SAG-AT7-MPI are just two of their products for the audio industry.
Part 2 - Gain Versus Output in preamp tubes.
Think of your days back in science class, where you built or saw one of those big ball devices, that created a half a million volts, made a great light
show, and you could touch it, and have the classes hair stand on end! Gain in a guitar amp is much the same. You can have a 20 watt amp with
high gain, and shred all day long, just as ?gainy? as a 100 watt monster. It is the wattage, or output, that differ.
Today's 12AX7s as an example, all have about the same gain, which in this case should be about 100. Most today fall below this, in the range of
maybe the mid 80?s, with some samples going up to maybe 110. The big difference in tubes though is current output. A typical 12AX7 is expected
to put out 1.2 milliamps at a given test voltage. Today?s tubes in general, put out as little as ½ of that in 80% of the cases. A tube at 0.8 milliamps
has a full 30% less output than what is expected. This is like a 50 watt amp putting out 30 or so watts. Some tubes are better than others from
various manufacturers. Some examples of this are tubes like the widely used, and perhaps the most popular are the Sovtek 12AX7WA. This is a
sturdy tube, generally free of microphonics, with acceptable gain. Part of the reason they are quiet, is they tend to be lower in gain than many
other 12AX7?s, but are also much lower on output current. It is much the same as with power tubes; where in the same amp, one duet of output
tubes will put out 50 watts, while another set only 45 watts. Thus, the Sovtek 12AX7WA, is quiet, due to lack of output and gain in many amp
circuits. These tubes are sturdy and inexpensive, and help a lot of amps make it through the warranty period. On the other side of the coin are
the JJ ECC83 tube. This is a part of the 12AX7 family, but different construction, plate materials, cathode coatings, and other factors give this tube
a bit more gain than most others. This can be 100-110 Mu, rather than the 80-85 or so of a 12AX7WA Sovtek. The big difference in the ECC83 in
general though, is it's current output, as times over 1.5 milliamps, or in some cases, three times MORE than a Sovtek12AX7WA.
Like I have said in the past, preamp tubes are a crapshoot. You buy your tubes and take your chances. Some folks like Groove Tubes, screen
and test for microphonics, noise, AND low output. The ones that do not pass all tests are rejected, in some cases, over 50% of the factory run.
There is still a range of specs even in these more rigidly tested tubes, but the spread is much tighter. In any of these cases, the end user still does
not know what the tube is actually doing. The SAG area at GT takes tubes, and runs them through another process, where all the specs are
recorded and traces are performed. These traces show rise time, and other factors. Like the SAG-MPI?s, there are other kits such as the MHG
(Marshall High Gain) kits, which can be used in any 12AX7 based amp, not just Marshalls. I guess I should have called this a High Power kit, which
would have been more accurate actually. There are Fender High Gain Kits, and Fender Soft Touch Kits, and the SAG generally tailors preamp
tubes for specific uses and tastes.
In any case, don't confuse gain and output. They are very different qualities in a tube.
Myles S. Rose
Guitar Amplifier Blueprinting
Gain and Current in preamp tubes
Over the weekend of 11/8/03 - 11/9/03, I ran into numerous amplifier problems. ALL of these could have been avoided if some basic information
was known or considered by the amp owner. I have said a lot on this subject, but it is worth repeating, maybe in a different way, as some folks out
there are just not getting the message.
Gain and Output are NOT the same thing.
Today's off the shelf preamp tubes are very inconsistent. You better get them from a vendor that actually tests them and has standards.
Today's 12AX7 tubes do NOT have a gain of 100 generally, it is closer to 85. This is not that big of a deal for the most part, as most amp
designers have designed around this in many cases.
THE BIG PROBLEM IS CURRENT OUTPUT. A 12AX7 tube at a reference test voltage and bias should be able to supply 1.2 milliamps of output
current. 80%-90% of 12AX7s today fall far below this. It is common to see tubes in the 0.5-0.8 milliamp range. This is the cause of ALL my
problems with folks this weekend.
Current .... think of this like there are two football players. They are the same height, and can run the 50 yard dash in the same amount of time
(think of this as gain), but one guy weighs 190 and the other weighs 285 (think of this as output).
Now, in a simple front end amp, such as a Fender Tweed era amp, or some of the high end classis design amps, this is not as much of a factor.
This is like the guys running down the field, and only encoutering one of two folks they are hit by. There will be an impact, but not as severe as if
you are hit by six guys.
Channel switching complex front end amps (Bogner, Diesel, Rivera, Mesa, Marshall DSL/TSL, etc.) have lots of trace routing, lots of components,
lots of pots and tone controls. Every single component in the signal path is one more guy on the field, trying to stop you from getting to your goal.
Even if you do manage to get to the goal, you are going to be pretty beat up and weak.
VERY FEW tube sellers can check for current. If they check anything at all, it might be gain (and not even real gain, but sort of a version of actual
gain), or transconductance. To check this properly (output or gain) takes time and expensive equipment. Your preamp tubes would cost $25.00
each easily, in labor alone, to check these factors accurately and properly. There are a few folks out there that do have output current minimums,
and this is a big help and much better than tubes that are checked for little more than if some noise comes out of them.
In any case, if you get a chance to try a proper output tube in your complex front end amp in the first gain stage (which feeds the rest of the stages
too by the way), you may be surprised.
The phase inverter - The most important tube in your amp? CLICK HERE
Biasing todays current production tubes
In the past output tubes and preamp tubes were much more consistent than they are today. One could use the older method to
bias output tubes with the scope / crossover method which had one adjust bias until the crossover notch distortion seen on a
scope was nearly absent from the waveform. This was a good method that worked fairly well as long as the tubes were somewhat
close to design spec. Today this is typically not the case. Tubes with low emissions, weak tubes will require the bias to be
adjusted to the point where the plates will glow orange or red and you may still not reduce the crossover notch to the desired
target. The tubes are just too weak and cannot come close to putting out their design wattage.
Another fault of the scope / crossover method is that it is not repeatable. It is the techs view or opinion on how they perceive the
waveform. Today, every major amplifier manufacturer uses the plate current method of adjustment using one of the many fine
tools available on the market today. Some of these are the Weber Bias Rite, GT bias probe, and there are others as well.
Most grid biased class A/B amps are happy running between 50% and 70% of what is called maximum plate dissipation. Cathode
biased amps (most EL84 amps but NOT a Fender Blues Jr as something of a departure) are self biasing. But, these amps were
designed to run at around 100% dissipation and were designed around a mid range / design spec tube. A weak tube will not
operate properly and a tube that is too strong will run hot, sound harsh and have short life.
Back to conventional grid biased class A/B amplifiers. Bias depending on plate voltage at 50%-70% of plate dissipation as a
starting point. You can find the maximum design plate specs for tubes in their respective data sheets. As a very general
guideline, an EL84 is a 12 watt tube, a 6V6 is a 14 watt tube, an EL34 is a 25 watt tube (except the GT E34LS is a 30 watter and
not to be confused with the JJ E34L which is a 25 watter), a 6L6 of modern design is a 30 watter (they ranged from 19-30 watts if
NOS types), a 6550 is about 42 watts and a KT88S or SV is a 50 watter. Some KT66s like to be treated as 25 watters and some
30 watters so here use your ears and inspect the tube for red plating. I have a lot of tube data sheets that you can download by