Groove Tubes - the rating system and tube selection
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Which tubes should I use?   I get this question all the time.   Tubes selection is something which is very much personal taste.  
Elsewhere in this website are all sorts of reports on the characteristics of various tubes.   In the Tube Primer area is a 200+ page
two part document that you may download for much more information on various tubes and their characteristics.   In other areas
of this website are technical data sheets on most of the common guitar amplifier tubes.

There are also good descriptions of each tube on the GT website:

Preamp tubes

Output tubes


Bias tools etc.
Hopefully this chart 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 characteristics 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 versatile, 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 versatile 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.

The Fender tube scale is
Blue = GT 1-3 range
White = GT 4-7 range
Red = GT 8-10 range

In a amp with a given bias, a higher GT rated tube will have a higher idle dissipation if no adjustments are made.   
This aspect can be used to fine tune amps such as older tweed era Fender amps, Mesa Boogie Amps, Hiwatt,
Orange, etc.   If you find as an example that a #5 tube has an idle of 26mA and you wish something closer to 30mA
you may want to try a #7 rating.  
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 Groove Tubes high end audio equipment such as The Brick, the ViPRE (award winning mic preamp used now by many major
studios), Glory Comp, and the Speaker Emulator II,

For the Groove Tubes SFX systems, CLICK HERE
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 dynamic.

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.
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Tube Matching

Over on the Dr. Z Forum in July of 2007 somebody asked about converting a vendor number on a box to a GT rating number.  This was my reply:

You cannot convert a GT dynamic number to a static number. The GT number is based on where the tube distorts not just the idle draw. The only way
you can do a close version of conversion is to have the latest or same batch of raw stock from which the tube was pulled (as each batch is different)
and have access to the GT computerized GT stuff and run a side by side measurement. This will get you in the ballpark but there are many other
factors considered on the GT scale.

The GT system also makes sure the tubes match under actual operation rather than at idle over the entire operating range of the tube. This is one
reason that you might see a static match difference in a matched set at times of a milliamp or two but under operation on a curve tracer the two tubes
will match perfectly across the entire amp's spectrum.

When you have numbers such as "43" you need to know what this number is. Is it plate current? Usually that is the case. But if it is plate current you
need to know what the plate voltage setting on their test equipment was and also know the bias voltage used. Otherwise this number means nothing at
all to the end user.

As a side note .... when I do tube tests I always use the original test spec settings as published in the RCA, GE, Sylvania, GEC, Mullard, etc., books. By
the way ... all these specs were the same and had the same target.

As an example ... a simple tube matching unit such as the Maxi-Matcher will test a tube at 400 plate volts and -48 bias for 6L6 tubes. The numbers you
see will be from 25-55mA or so. Using RCA book spec 250 plate volts is used with a -14.0 volt bias. A spec tube would produce 72mA. The tube is
driven much harder as the bias is so much lower. This would be somewhat the same as testing at 500 plate volts and a -28 volt bias all things being
equal. Most 6L6 amps have their bias at about -48v to -52v. -52 was the standard on many Fender amps and is the fixed bias of Mesa Boogie 6L6

Tesing at true book spec pushes the tube much harder into it's true operating range than the devices out there that seem to use higher plate voltages
but with the higher bias keeps the tubes running at a much lower output. Less throw out when you do it that way.

Some folks think GT stuff is too pricy. Well, the labor time in testing is much higher as is their reject rate which is above 50%. The cost and upkeep on
the testing equipment is insane as well as the calibration frequency to assure that a #5 tube from 20 years ago is the same as a #5 tube today.

For anybody in my general area .... if you even want your tubes tested just bring them to me. I do not accept mailing by the way. I will test them for free
for you. I can also do this on afternoons, evenings and weekends as I have testing equipment at home. I can also tell you the true and actual gain of
your preamp tubes which cannot be done with simple common testers including all of them that you see for sale on ebay. I can also tell you
transconductance, current output, plate resistance and run curves on the tubes for true matching rather than just matching a number like
transconductance which is what most folks do.

Once you know what the tubes in your amp are actually doing it is no longer a crap shoot at +/- 50% variability or more in the case of preamp tubes.
You can then replicate your tone of you change tubes, move to a more gainy tube, less gainy one, tighter one, more easy to compress faster one, etc.
This is what blueprinting is all about in some of it's areas .... knowing the specs and documenting what is actually going on in the amp.
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
clicking here.