Be sure to scroll down and see the review on the new
Groove Tubes "NVM" (new vintage manufacture)
6L6GE which testing started on 3/6/02
- Now shipping
as of May 2002!
Groove Tubes also designs and builds their own
line of amplifiers from their custom shop, the
Soul-O Series.  For more information including
downloadable owner's manuals

For GT power amps CLICK HERE

For the GT Trio Preamp CLICK HERE

For Groove Tubes speaker cabinets CLICK HERE

For Groove Tube studio microphones CLICK
HERE for GT info or CLICK HERE to head to the
distributor M-Audio

For Groove Tubes high end audio equipment
such as the DITTO Box, the ViPRE (award
winning mic preamp used now by many major
studios, and the Speaker Emulator II,

For the Groove Tubes SFX systems, CLICK HERE
Preliminary first report on the Groove Tubes 6L6GE tube

3/6/02 - Guitar Amplifier Blueprinting - West Hills CA.

We hear the term NOS (new old stock).  After seeing some of the initial curve traces of this tube, I had to wonder if a
new "moniker" such as NVM (new vintage manufacture) or NMOS (newly manufactured old stock) seemed more
appropriate.  When talking to the folks at Groove Tubes, the term "reissue" was said by one of them, but "reissue" as
in amplifiers, can mean the same circuit, but with changes.  Items such as printed circuit boards or other changes are
a bit different than the originals.

I looked at this tube very closely.  To my way of thinking, it is anything but a reissue.  It is an original GE tube made
today.  It is made on machines from decades ago.  The parts inside are of the same metals, dimensions, and design
as the GE.  The coatings are of the same chemical content, formulations, and thickness.  Even the glass case is the
same glass formula, thickness, and dimensions.  The getters are side getters, and the mica spacers are identical to
the originals, made of the same material on the same machines as the originals.

Well, sometimes a copy of something can be a copy visually and physically, but in practice it can be a different
matter.  So ? it was off to the vacuum curve tracer and scopes, meters, and loads.

First, a standard was needed.  What better standard than the original GE data sheets.  These sheets show what was
expected from an "average" tube.  As we changed voltages, one thing became more clear as time passed, this tube
(which is still not released) did not have a Groove Tubes rating number, but probably would have been a #6.  The
curves (as some of the charts show), were just a touch stronger than the average GE tube.  This was well within the
range of a number of the originals.  These curves are as linear as anything we have tested.  The usable range of
this tube as various voltages was wider than the Svetlana, Electro Harmonix, or JJ Tesla tubes.  The wider range is
not a plus or minus, just an indication of a very well engineered piece of equipment.

Listening tests done earlier showed this tube to be a winner to the folks tastes that were present.  This tube was
tested in the new Groove Tubes Solo Class A amp, in both the combo and head version.  It was also tested in Mikey
Wright?s Fender Custom Shop Vibroverb.  Mikey can probably give the ability to have a listen if you are in the area.  
If you are interested, drop me an email.  I normally refrain from listening tests as these are so subjective, but I will say
that my old favorite KT-66 tubes will possibly be replaced by these 6L6GE?s when they are commercially available in
quantity.  For now, this tube is in something of BETA test, with about 30 or so musicians out there.

This is the first phase of out testing, and I will post more as I have more to tell you all.  I will say that so far, this is the
best "data wise" 6L6 available.  As far as sound quality, that is subjective.  If you like the original Fender sound, than
this tube will be work investigation.

Also down the road, Groove Tubes is developing a 6CA7 (EL-34).  The USA 6CA7 had a higher vacuum than their
Europe counterparts, and were of a heavier construction also.  This may be the ticket for the folks out there that want
a strong EL-34 type tube with a high degree of reliability.

Now, I did not see any spare equipment lying around to make another tube type at Groove Tubes ? but ? maybe if I
can get a lot of people pushing hard enough, we can get them to make a "NVM" versions of a 6V6 !!!  Expected
availability of this tube is expected by early summer.
This is a reprint of a Harmony Central article that was posted on April 29th

Groove Tubes Brings Back the GT6L6GE Power Tube

April 29, 2002 -- Groove Tubes announces the long awaited USA made reissue of the GT6L6GE, a faithful
reproduction of the famous General Electric 6L6 "clear top" power tube. The new Groove Tubes
offering has a 90% USA made content, and is largely produced with the original GE machines located at
their San Fernando, California factory.

Historical Background:

The original GE 6L6 was perhaps the music world's most beloved power tube. It was used by scores of
USA manufacturers during the Golden Era of US made tube amplifiers. Both the legendary McIntosh Hi
Fi amps and Fender guitar amps commonly used this tube from the early 1950's through to the mid
1970's productions. This was likely the tube used by Jimi Hendrix while recording thru the Fender Twin
Reverb amp he commonly used in studio, as well as the tube used in Carl Perkins' Fender amps while
recording for Elvis. General Electric discontinued production of receiving tubes in the late 1980's, and
sold their entire remaining production line to Richardson Electronics of Illinois.

The new USA tube adventure:

In 1998, Groove Tubes purchased from Richardson the original critical materials and machines GE used
to produce both the GE6L6 and GE6CA7 (aka:EL34), along with the original production processing
formulas. In other words, Groove Tubes was able to acquire the the recipe, the original ingredients,
and the kitchen too! The adventure had begun.

Next, several original GE vendors were enlisted to faithfully reproduce many of the internal
components. For example, the company who made the original mica insulators has provided the
identical part, on new tooling from original GE drawings. Also, the special plate designs are made on
the same vintage 4 slide machines and are identical to the original parts, using the original GE plate
material that is no longer in production. GT was able to purchased enough original GE plate material for
many years of production. This special multi layered alloy formula appears a soft dull gray after heat
processing and give this tube it's soft, warm well as contributing to it's superior power
performance. While currently, some of the processing is located "off site" and will continue there until
all processing clearances can be arranged for the San Fernando factory, eventually plans are to do all
processing under the GT roof.

However, the internal content is now more than 90% USA origin (Incidentally, this content ratio is similar
to to the original content of the GE tube as GE also used off site processing). It has been almost 4
years of hard work and significant financial investment in tooling and setup to be able to release the
final product, but the wait was worth it.

Technical Details:

The GT6L6GE tube is a beam tetrode, although it is also commonly referred to as a Beam pentode. It
has a maximum anode dissipation of 25 watts when used in a common Class A/B circuit design. It is
highly recommended for replacement for all 6L6/KT66 /5881 type audio amplifiers for superior fidelity
and improved power output performance, especially in musical instrument amps but as well for High
Fidelity reproduction amplifiers.

This tube is being made in limited production runs in order to control quality and to achieve the
original high performance specifications. Therefore, it may be some time before production "ramps up"
to allow for general distribution into the worldwide tube market. Meanwhile, is currently only available
from authorized Groove Tubes dealers located in the USA and 34 foreign countries in performance
tested and matched Duets @ $90 and Quartets @ $180 (Singles, Sextets and Octets available upon

As of February 2003, this tube has also been reviewed by Legendary Tones, Guitar Player Magazine (
Feb 03) and other pulications.

For more information, visit their web site at
The above are the Groove Tubes 6L6GE points plotted using the original GE average tube data
chart.  The plots were done from 150 volts to 500 volts and in both triode and pentode
configurations.  The pentode charts versus an original GE sample were too close to identical to
graph.  A scope camera is being procured to show those curves.
Hopefully this chart below will help some folks understand the Groove Tubes rating
system of 1-10.  This is the lower portion of the chart, the "comparative study".  The
numbers here are WATTS at various frequencies.  The numerically lower set of
numbers is the clean wattage that was generated with the #1 and #10 set of the
same tube.  The "max" numbers below the OS numbers are then the maximum
wattage developed with the same tubes.  What this is showing, is that it is less a
matter of power output and more a matter of distortion charicteristics that their 1-10
numbers indicate.  In the last example above, we see see than an OS #1 at max in
one frequency range produces 113.3 watts in the amp (a Groove Tubes Solo 75
amp), and a OS # 10 produces 118.6 watts.  Remember, it takes TWICE the power to
give you 3db more of power.  So the amount of power change here is very small
across their 1-10 range, it is more of the point and manner in which the tube distorts.

In the upper part of the chart, you again see wattage at various frequencies with a
number of the tubes that Groove Tubes offers.  This is a nice study also.

The bottom line here is;

Lower numbers will start to distort at lower volume settings on your amp, and have
more dynamic touch and feel.  These are the GT 1-3 range, and the Fender "blue"
painted tubes.  Many Jazz or Blues players like this range, and they work well for
recording where levels are going to be lower, or in smaller venues.

Medium numbers are the most versitile, will drop into most amps and be very close to
most factories bias settings, and are the best general choice for most players wanting
a versitile well rounded amp.  These are the GT 4-7 range, and the Fender "white"
painted tubes.  The range of 4,5,6 are able to be used in Mesa Boogie fixed bias
amps, and on the Mesa scale convert roughly to:

Mesa scale        Groove Tubes scale   
Red                         4
Yellow                      4
Green                       5
Gray                        5
Blue                        6
White                       6

High number will have to be driven at higher volume levels to begin to reach output
stage distortion.  These are liked by some Jazz players that want a very clean
sound.  They are also preferred by some heavy metal folks, who want very clean high
headroom.  These folks like to get most of their distortion from front end effects,
pedals, or by running their preamp levels very high.  These tubes have the least
dynamic touch and will give the most clean headroom.  These are the GT 8-10 range,
and the Fender "red" painted tubes.
Groove Tubes "Hype"?  Cost too much? Comments in regard to
tube matching in general and distortion rating scale
Basically, a #1 will distort sooner, and a #10 later.  If, for example, with a mid range tube, say a #5, makes your amp
start to break in the output section at a volume setting on the amp of "4", then with a lower number tube, like a #2,
your amp would have a same sort of break into output distortion at say a volume setting of "3".  With a higher tube,
such as an #8, then you amp would stay clean to about perhaps "6" on  the volume.

High rating numbers are not more or less powerful, they just distort later.  These are preferred by heavy rocker that
want maximum clean output, as they get their distortion and tone from effects or pedals. These are not as touch

Low number tubes are very touch dynamic, and more suited for a lot of folks, for smaller venues and recording.

Most folks prefer the 4-7 range tubes, as they are the closest in character and touch to what the amplifier designer
had in mind.  They are also the most versatile.
I have been using Groove Tubes products since about 1985.  I have been working with amplifiers and musicians

Over time I have heard comments in regard to the higher cost of Groove Tubes own tube offerings, and comments
that they sell the same tubes as everybody else and just stick their name on them.  In some cases, the do sell the
same tubes, but the GT logo does not go on the tube until the tube goes through their own unique  testing process.

On the subject of the
same tubes, there are differences worth noting.  Many tubes are made by the world's tube
factories to specific GT designs and specifications.  Some of these are the KT-66HP (which at first glance looks
much like the Chinese variant, but is not made in China), the GT-E34LS which comes from the JJ factory but is not
the same tube as the JJ E34L (look at the plate assembly), notice the heat sinks on the plates of  the GT version),
and other tubes such as GT's own, USA self made 6L6GE.   The KT-88SV is a different tube in regard to plate
assembly as another example.  These are all tubes built off GT developed and produced tooling.  
UPDATE as of
January 2004:  On the horizon is the GT-6CA7GE and 12AX7M.  See elsewhere on this website for info on
these and their progress.

For tubes that you may think are the same, look at the bases.  GT re-bases many of their power tubes with a
different assembly.  Look at some other companies blunt pins with dipped coatings.  Feel the resistance when you
plug those into your vintage Marshall or Fender as the female inserts in the tube sockets either feel like they are
being deformed, or just as bad, the insert comes out of the socket?  GT uses polished and tapered pins that are not
pot metal.  This is yet another change often overlooked.  Vintage tube bases are a lot happier with these pins.

This is not the early 1960's anymore, and tubes are no longer widely used in medical equipment, scientific
measurement equipment, or the Minuteman missile program.  Quality control is not what it was four decades ago.   
In the past, a 12AX7 at 250 plate volts with a 2 volt bias would produce 1.2mA.  Today, even the same maker, same
batch, and run, will have tubes that vary from 0.6mA to about 1.8mA.  Over 80% of these fall at below 0.9mA.  This
is one of the reasons that some of you have put
new preamp tubes in your amp, only to have it sound worse than
before.  Maybe you had a 1.1mA tube in V1, and the new tube was 0.7mA.  Groove Tubes has less than a 50%
acceptance rate on their preamp tubes.  The are tested for low output, microphonics, and noise.  They carry a 180
day (6 month) warranty.  I found other vendors in many cases had a high degree of low output, microphonics, and
even one of the two sides of the triode not even working.  Most of them replaced these tubes, but it took time and
effort to send back the bad tube (after making a call or two, or sending letters), and then waiting for the "new" tube
to arrive.  It became quite obvious, that certain vendors did little more than put the tube in a box and ship it.

MATCHING and associated cost

There are a lot of ways to match tubes.  The most common and simple method is done using a fixed voltage, a fixed
bias, and looking at the output of the tube.  If it is within a certain range, the tube is labeled as "matched".  This is
very cost effective, taking a matter of a minute or so to check tubes.  I call this the static method.

Over the years I noticed, that when I set bias on a performers amp, after a week or so, I would have to reset the
bias, as things changed.  I also noticed that when I used Groove Tubes, I did not have to re-bias the amps as had to
be done on amps using non GT tubes.  This was due to weak vacuum that was not discovered in tubes from
vendors that did not test for vacuum, grid leakage, gas leakage, etc.   If you suspect your vendor is saying they do
test for this, and perhaps may use nothing more than a static current draw match, ask them what they use to match,
and ask for this in writing if you ever plan to get them to replace tubes that died in the first few weeks.  Even if a
vendor has a long warranty, without testing for the above, the are banking on the odds that the tubes will hang in
there, or you will hopefully not notice the difference, or it will be too much trouble for the customer to send the tubes
back.  (That is, if they even have a return policy for the reason of not liking the tubes).

I initially rarely thought all that much about all this, but over time, other things came to my attention.  After a
performance, the output transformers on GT equipped amps seemed to feel cooler than amps with statically
matched tubes.  I had less output transformer failures on clients amps that used GT tubes.  I became more curious.  
It became apparent, that a mismatched output section had not only sonic issues, but perhaps also drove the output
transformer harder.  It was like using cheap oil in a good car versus good oil ... the engine lasted longer.

The static method was fast and simple, but it had some areas of concern for my clients.

The second method of matching uses what I call the dynamic method.  Dynamic matching takes the static method a
few steps further.  It runs tests at various voltages and looks at factors that determine reliability of the tube, and
what the character of the tube is over its entire operating range, like an amp works.    Tubes that were statically
matched within one milliamp, when thrown on curve tracers, were far from being matched in actual operation.  Tubes
that were dynamically matched, but from a static match were as much as 10% off match in a static idle match, when
current, signal, and voltage sweeps hit the tubes,  had their curves and match line up beautifully.

Think of it in this way ....  A car that has not been tuned at all will idle roughly and sputter and stall.  In a static
matched application, the idle may be set to be very smooth, or perhaps if a different voltage and load is used, then
the car runs fantastic at say, 4000 rpm.  In a dynamic match, the car is run on the dyno, and you know how it will
perform over it's entire operating range.  I can make my VW idle great, or run down the road at 60 mph nicely.  
Somebody else can do the same with a BMW M5.  But, when we both step on the gas and hit that first corner, there
will be a difference.  Another more simple way of looking at this....  I can make my VW Jetta idle nicely at 800 rpm.  I
can make a friends M5 also idle at 800 rpm.  By some tube vendors definition, these cars would be "matched".  
When my friend and I take off, hit the gas, the brakes, and the turns on Mullholland, what do you think will happen?

There were other factors too.  GT also tests for gas leakage, grid leakage, low vacuum, as said earlier, as just a few
points in their testing process..  There is also a burn in period.  This was where I learned even more.  Today?s
cathode coatings are not as pure perhaps, as those in the past.  There are impurities.  Now,  I am just guessing
here, I have NO basis in scientific fact, but I feel that what is happening is that tubes that are not burned in before
matching have gassification of the impurities in the cathode coating, and this changes the tube characteristics.  
Factors such as grid leakage also contribute to decreased reliability.  That is why tube suppliers have guarantees...
there are factors that are outside of some aspects of control.  The guarantees were all well and good, but did not do
a lot for my clients on stage when a tube failed.  The GT tubes are tested for these aspect, and for me it has shown

If you think all of this is hype and does not matter, that is your opinion, and either you are open to additional
information or not.  There are tubes like the new USA GT 6L6GE, that whether you think it is great or not, you may
want to give it a listen before you jump on the word of some lone individual.  Perhaps look to see what tube folks like
Eddie use in his 5150s.  Whether you like his music or not, its hard to miss that he has either a firm understanding
of tone and a matched output section, or he is pretty darn lucky.  He can get his harmonics and sounds anywhere
on the fret board.

Tubes tested in a matter of a minute are going to cost less than tubes tested many times longer.  Tubes that use
premium components will cost more.  Tubes made in the USA with USA parts and labor will cost more than Chinese
or Russian tubes.  Good oil costs more than cheaper oil in your car.

If you have learned something here, or if this gives you course to seek more information, than that?s great.  If you
think its all hype, and have not at least looked at some the unique GT designs, than I guess we will just go our
different ways.  It was nice meeting you informally here, as I am sure we won't be seeing each other in the future
face to face, as we don't seem to run in the same sonic or tone circles.  I am not saying that Groove Tubes are the
only tubes that are any good.  There are some terrific tube suppliers out there, and I support their products too.  I
support quality, whether its tubes, amps, guitars, or studio equipment.

If you want to contact me, I can be reached via the various forums in which I participate.  You can also reach me via
email from this website, or give me a call at Groove Tubes, where I have been spending time on a more formal basis
since May 2002.  When you go to their website at be sure to take a look in their preamp tube
section at the bottom for the
SAG tubes.  I did not have enough tubes at my own hands to find one in 20-50 that met
my needs for graded and classified preamp tubes or matched phase inverters, so as of May I started working with
GT and formed the SAG (Special Applications Group).  I guess that means you may also call Groove Tubes most of
the time if you want to speak on the phone one on one, or drop me an email there at  and if you are in the area and want to see how tubes are made in the USA (even
though some folks say this is
hype and GT does not make anything), drop me a line, maybe I can show you a few
things, and have you listen to a few things.

Lower Cost Groove Tubes - If you still feel that tubes which are tested with more simple testing
methods work great in your amp, GT has a line of tubes that are less costly than just about anybodies out there.  
They are called
"Silver Series" and are tested in the faster way that other vendors test their offerings.  These
are great for a lot of folks.  These tubes carry the same long warranty as the premium tested and selected GT tubes.
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