By Roger A. Modjeski
My experience gained from testing thousands of EL-34's, 6550's, KT-88's, and other tubes has produced a wide bell curve of the two most important parameters, Bias and Transconductance. The center of this curve is the "bogey" value that the manufacturer is trying to hit, and the ends show the cutoff points of what he is willing to allow out of the factory. The tubes at the ends are still perfectly functional, but may not work well in all amplifiers, and will certainly not work well if mated with another from the opposite end of the curve.
Some amplifiers do not have enough range on the bias pots to handle the range of tubes so that selection-must be made from a particular part of the curve to ever achieve bias.
In the "good old days," I'm told, the spread of values was much less, allowing reasonable performance with random selection. Today, random selection is strongly not recommended-I'll tell you why.
Looking at bias voltage in a typical batch of EL-34's from the best producer, it will typically range from -32 to -42 volts for 50mA of plate current. At a typical transconductance for 7000 umhos (7 mA/volt) the 10 volt difference will cause a 70 mA bias difference. Obviously, one tube will be on and the other cut off if they are biased with the same adjustment pot. I match the bias voltage to. 0.5 volts or better, and thus have only a 3.5 mA (0.3 x 7 mA/v) difference in plate currents, which is only 4% maximum at 50 mA.
Now, those of you with individual bias adjustments are probably thinking that you've got it all handled because you can adjust for these different grid voltages. Well, you can-but in doing so, you will create a greater problem.
Say you put a -32 volt tube in one socket of a push-pull pair and a -42v in the other and bias them so that their currents are equal. Unless you can also adjust the a.c. balance of the driver to provide 32v of drive to one and 42v to the other, you will have very unsymmetrical clipping and reduced power output.
In most amplifiers with a.c. balance, the range of adjustment is nowhere near this, as that adjustment is meant to balance the driver and not current for widely unmatched output tubes.
In our modern, larger amplifiers, multiple pairs are used in push-pull parallel to achieve higher powers. It takes four of the popular output tubes to safely attain 100 watts, and here matching is even more critical.
To fully appreciate the problem, you must know a bit about the operation of a tube output stage. In push-pull parallel, we have four (or more) tubes connected so that two (in parallel) handle the positive half of the output signal, and the other two handle the other half. Even if you do find two tubes which are 5v off, there will be 35 mA different in bias current, which is still intolerable.
Ideally, they are all biased at the same negative grid voltage and have the same current through them. As we apply positive signal, the grid voltage goes up on the first pair of tubes, raising their current in a shared manner. For this to happen, they must have the same transconductance (Gm), which is the measure of how many mA/volt the plate current changes. Imagine if one has 7000 umhos (7mA/v) and its partner has 5000 umhos (5mA/v).
By the time they hit 0v (coming up from the -35v bias), one is conducting 175mA and the other is conducting 245 mA, which is neither fair, nor a good idea. Now imagine if these tubes didn't even start at the same bias, but were adjusted by individual pots to -32v and -42v as per the manufacturer's method of achieving equal bias currents. What then happens is that the -32v tube clamps the drive when it hits 32v (0 on the grid), and the -42v tube's grid never gets higher than -10v. Since most amplifiers require bringing the grid up to zero for full power, with unmatched tubes, you will never attain maximum performance.
Here's an analogy for the mechanically-minded. Imagine a VW-type engine where the cylinders are opposed and made separately, and the manufacturer puts together engines where the pistons, rods, and cylinders are not very uniform. In this engine there is only one arm on the crank (like the driver in our amplifier), and all the rods are connected to it so that they all have the same length of throw. One "pair" of pistons should top out together when the other pair hits bottom. If any of them has a short rod, it will achieve less compression and therefore less power. If he matched the rods and selected cylinders of the same bore, he would then get equal power per constraint of that one crank, as we have with our single (symmetrical) drive voltage. With unequal push rods, we're going to end up with very different compressions at the end of the stroke, and thus very different output power from each cylinder.
Fortunately, this nasty problem has a solution-simply match the tubes for Bias, and Gm, and use one bias adjustment. Then the amplifier will be working at its best, and the consumer will have the simplest possible set-up.
At RAM Tube Works, we match for Bias within 4% and Gm within 8%. We provide tubes in any size sets at no extra charge. We give you the full data for each tube so you may see the matching for yourself. We also reject tubes which have high grid leakage (gas), low power output, unreasonable bias or Gm, and other flaws.
We guarantee our tubes for ninety (90) days, and your satisfaction for as long as you are our customer.