Biasing of Tubes: Part 1.
If you and I went back in the “Way Back Machine” * and visited an engineer or a radio repairman (called “radioman” in the 1920's-40) and started a conversation on bias we might not get past the first sentence with this competent fellow. If we said “I adjust the bias on my amplifier to be 50 mV or 300 mV or any number under 20 Volts he would look at us very strangely. For him “bias” is typically –20 to –100 volts. No millivolt bias in 1929. He would call your bias zero and wonder how you kept your tubes and amp from burning up.
Positive reading low voltage bias came into the amplifier world in the 1955 with the birth of Dynaco. See:
http://en.wikipedia.org/wiki/Dynaco for more information on that company. My dear friend, Bruce DePalma (brother of Brian) worked for David Hafler in the early years. We had many interesting conversations over gin and tonics. He told me some humorous stories about David which I will relate if there is interest.
I had worked on many a Stereo 70 by then and I knew why 1.56 Volts was the chosen numbers because I had built several meters by that time. Anyone who has built and calibrated a Heathkit VTVM or most any other meter, will recognize that number, it’s the voltage of a “fresh flashlight cell”. In the age before accurate digital meters that was the best “voltage standard” one could get. In that day the most sensitive scale on common meters was 1.5V with just enough swing at the top to allow 1.56. Some meters even had a little mark up there.
DePlama told me an interesting thing I would never have known otherwise. It turns out that a common cathode resistor of around that value also reduced the distortion appreciably. DePalma was 16 when he started working for Hafler. Another early starter.
I had better warn you now. I go off the subject sometimes to get a few details in, my intent is to fill in the gaps and history for the younger readers who may have never seen an analog meter. You know, the kind with the needle that swings up and down and all those numbers on the scales. I will say here that analog meters are still the best for many audio applications and the good ones are quite accurate. One thing to know about them is the 1/3 scale rule. The rule is you should not read below 1/3 scale. Most meters had a 1,3,10,30 series so you could range down if below 1/3 scale. If the needle barely moves, forget it. Analog meter accuracy was specified as a percentage of full range. So if you have a meter that is 5% accuracy and you are on the 5 volt scale you can be .25 volts off. Add .25 volts to 1.56 and you will be 16% high, or low if the meter is off in the other direction. Bias does not have to be dead on, but 16% is a bit much, especially if on the high side. The ST-70 tubes are already running at full dissipation, and 16% over would shorten their life considerably.
David Hafler undoubtedly knew all this. Not all meters had a 1.5 volt scale. Some had 3 or 5. The manual told you to get a fresh flashlight cell, see where it read on your meter and just adjust the bias to the same spot on the meter scale.
Now what does all this have to do with our visit back to the old radioman and why don’t you find numbers like this on a tube data sheet that he would be looking at? Look up an EL-34 data sheet and you will find typical bias is around –35 volts (note that’s a negative voltage). What’s even more confusing, when you BIASET the Dynaco you read a positive 1.56 V (all with respect to the chassis which is typically ground). By now there is great confusion in the conversation with the radioman. BTW, I have found that many students have trouble getting a feel for voltages, both magnitude and sign. I play this little game: 1 volt = $1 and you know that $1.56 is not the same as $35. You also know that if you balance your check book and you have -$35 you are going to get a note from your bank. If you have +$35 you can at least get dinner. To a technical person voltages are as real as money and reporting them should get the same care as your bank takes reporting the status of your account.
There is one other reason the radioman thought in terms of bias voltage and not plate current. The tubes he was dealing with, types 71 and 45, were all low Gm tubes (1.6 to 2.5 mA per volt) so variations in the tube parameters or the setting of the bias voltage made little difference in the plate current. When the 2A3 was introduced in RCA manual RC-11 (1933) there was no mention of bias matching though they did mention, rather proudly, the high Gm of 5.25 ma/volt. By the publication of RC-12 (1934) they had inserted a very important paragraph, undoubtedly based on things learned from applying the tube. It’s a long paragraph but the most telling sentence (paraphrased) is this: “The very high value of transconductance makes the 2A3 somewhat critical to grid-bias voltage since a very small bias change produces a very large change in plate current” In push-pull they strongly urged that the tubes had individual bias pots and the ability to read the plate current and make it the same in both tubes. Please note that both plate current and cathode current are the same in triodes and both terms are used interchangeably. In pentodes the difference is usually no more than 5% due to the screen grid current, but that’s another story. Note also that today’s common pentodes; EL-34, KT-88 etc. have Gm’s of 6 to 10 mA per volt.
So what are we measuring if we are not measuring bias voltage? We are measuring the current in the output tubes as a voltage drop across a resistor. We don’t really care what value of negative voltage is applied to the grid via the bias pot that you are adjusting. As long as it is enough to dial the tube in. One maker, whose initials are ARC, purposely limits the range on bias adjustment thus requiring output tubes to fall in a rather narrow range (less than 10 volts) of grid bias. Because we get all the bias and Gm data from our tube tester, Ram Tubes can select tubes in the required range for these amps. (We sell the same tubes at lower prices). When I test 6550’s only half of the tubes will fall within a 10 volt range. I designed the RM-9 to give the user a very wide bias adjustment range from –32 to –64 volts (32 volt range). This range also allows the user to use EL-34’s, KT-77’s, KT-88’s, 6550’s and any other tube that can be biased within that range of negative grid voltage.
The ST-70 had a 15.6 ohm 1% resistor in series with the cathodes of each output pair. That’s a rather odd value. Why not 15, that’s a standard value. Well David wanted to have exactly 100 mA through the two tubes and 15.6 ohms yields (by Ohm’s Law) 1.56 volts in that situation. He even bothered to get a precision resistor. When its all biased up you can take your meter and read the negative grid voltage that you adjusted on the bias pot and it will be somewhere between negative 30 to negative 40 volts with average tubes. Now if you said to the radioman. “I bias my tubes at –35 on the grid for 50 mA of plate current per tube.” He would say… gotcha.
End of Part 1.
Stay tuned for Part 2. Where we will look at the terms: autobias, fixed bias, cathode bias, self bias. These are terms I feel need clarification. It makes discussing things easier.
* The “WayBackMachine was a creation of the cartoon show “Bullwinkle”.
