In this month’s Roger’s Corner, we will complete our vacuum tube trilogy with a discussion on power tube matching. One thing that always bothered Roger was the use of the term “auto bias” when describing the process where the circuit of the amplifier is designed to bias the power tubes. I cannot recall the explanation he gave me for how the term came about, but in my mind, it makes sense to say it became adopted by audiophiles because it was a simple way of expressing the concept of cathode self-bias which is really what we are talking about. On top of that, I have known audiophiles who felt “auto bias” and fixed bias were interchangeable terms. Simply, fixed bias means the user sets the bias manually. There is nothing “auto” about it.
So, let us examine these terms in a little more detail. Cathode self-bias is a method where a resistor in the cathode circuit automatically sets the grid voltage relative to the cathode without needing the user to make manual adjustments. This method is self-stabilizing, as the voltage across the resistor automatically adjusts to maintain the tube's operating point despite variations in tube current. On the other hand, with fixed bias a fixed negative DC voltage is applied to the power tube's grid to control current flow, while the cathode is grounded. The user can then set the negative voltage to spec using a potentiometer or trim pot while reading the voltage from a meter on the amp or by using a multimeter connected to test points. While providing more power than cathode self-bias, fixed bias does require periodic adjustment to maintain proper operation. In addition, if the amplifier uses pairs or quads of tubes per channel, it is crucial that matched tubes are used.
The last thing I would like to mention is that I believe there is a misunderstanding among some audiophiles that cathode self-bias circuits do not require the use of matched tubes because the circuit will somehow magically make them matched. This is just not true. I recall having Roger look at my OTL amplifiers once when I wanted to replace the Chinese 6AS7 tubes with the GE 6080 variant. He asked if the 6AS7s were matched and I said no, the manufacturer supplies them unmatched. Roger scoffed and proceeded to explain to me that while the amplifier would certainly work, the weaker tubes would be drawing more current to keep up with the stronger tubes and in the long term this would lead to premature tube failure, as well as increased distortion. He then popped the bottom off the amplifier, powered it on and showed me using his multimeter just how unmatched the quads were. Needless to say I had Roger insert four matched quads of the 6080s and went off on my merry way.
The Virtues of Power Tube Matching
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 (Gm). 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 they are 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 wide range of values so tube selection must be made from a particular part of the curve to achieve bias. In the "good old days," I am told, the spread of values was much less, allowing reasonable performance with random selection. Today, random selection is strongly not recommended, and I will 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 10V difference will cause a 70mA 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 0.5V or better and thus have only a 3.5mA (0.3 x 7mA/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 have 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 -32V 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 the current of widely unmatched output tubes. In our modern, larger amplifiers, multiple pairs are used in push-pull parallel to achieve higher power. It takes four of the popular output tubes to safely attain 100 watts, and here matching is even more critical.
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