Myles,
Is it still true that if you replace your Groove Tubes with the same Groove Tubes (type and hardness) Power Tubes that you do not need to re-bias your amp? I remember reading that some time ago and was wondering if that still holds true. Thanks for you responses to my questions so far.
Paul
Dr. Z 6545
Paul
If you have the SAME tube and the same rating that is true. This is the GT statement of policy. But ... there are exceptions.
What I mean about the "same" tube actually is a good example if we use your 6545. The 6545 comes with a GT #5 or #6 E34LS. Some folks think this is a standard EL34 but it is not. Some people think it is the same as a JJ E34L but it is not. If you hold the JJ E34L and a GT E34LS side by side and look at the plates you will see heat sink "wings" on the plates that are not on the JJ tube. The grid wire is different too.
The LS is a GT tooled tube built in the JJ plant. JJ was a spin off of Tesla. Aspen was in partnership with Tesla at one time and almost bought Tesla in the past. In any case, the E34LS and KT-88SV were GE developed tubes.
The LS is a 30 watt tube and all other EL34 tubes are 25 watters. The LS requires an additions -1.5 to -2.0 volts or so of bias to bias at the same point as a different EL-34 of the same rating. So .... they are both "EL-34" tubes but do require a rebias if going from one to another of even the same rating.
The EL34R (Sovtek/EH), R2 (Svetlana - real Svets ... winged C), and C (Chinese) are all 25 watters and tested on the same settings on the test fixtures. So ... if you have a #5 EL-34C and go to a #5 EL-34R2 you would not have to rebias in this case.
The 6L6's ....
KT66HP is not the same as a 6L6. At the same bias it will be up about 10mA at idle over a same rating number 6L6. I like to check bias on the 6L6S (JJ tube) as it can be a bit stronger than many other 6L6 tubes. The 6L6GE, R, R2 would all fall into the same bias range.
The 6V6S (JJ) is stronger than a 6V6C (Chinese), 6V6R (Electro Harmonix) The 6V6C and 6V6R are set up the same.
KT88s ..... all are very different .... KT88S, SV, C.
On the rating numbers ..... The 1-10 scale really spans about 200 numbers. It is reduced to a 1-10 scale to keep things from going crazy. Lets say (and these are not actual numbers) that a #5 EL34R2 will fall into being a #5 if the numbers are between 78-88. Starting at 89 the tube would be a #6 rating. So there is a bit of a margin here. A high #5 and low #6 would overlap.
Actually, you can usually go a point down and be in good shape. If you had 6's you can slap in 5's and not have to rebias.
The tubes are the same day in and day out. The computer controlled power supplies have had the same setting even when updated to newer hardware. The tubes are matched dynamically which means if you have a 5%-10% mismatch at idle (old day industry standard said 15% to as much as 20% was considered a match) they will be fine.
I typically go for less than 10% idle difference in one direction. That means if I have an amp I am setting with a 30mA idle I want the other tubes to be within 3mA.
Tubes too far off static match will hum and not sound as good. But, tubes that are dynamically matched across the entire operating range when viewed on tracer will show identical traces. There are other issues too. Sometimes a tube will be a bit off in current at idle when using something like a bias tool or bias probe. But ... this tube might be dead on when measuring transconductance and may be an even better match than the more simple method.
On GT stuff the rating number comes from a number of factors such as low vacuum is rejected (quite common in some runs of Svetlanas), grid leakage, gas leakage, and other factors. The rating is a compilation of all of these things but it's most important factor is how the tube distorts and when it distorts in specific areas of it's operating range.
In the old days tubes were more consistant. The 4-7 range is what you would expect to see and the numbers amp designers built around and expected.
Here is a chart that some of you may have seen at times:
Here is some stuff I have written that I copied off my website so [glow=red,2,300]
EXCUSE THE BAD FORMATTING:[/glow]
THE GROOVE TUBES RATING SYSTEM
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.
I hope this was some help.