This is the method I use to bias my amps. It’s not the only way to bias your output tubes, but I think it’s the safest and easiest method.
Essentially, you want to bias your amp such that the plate dissipation rating of the output tubes is not exceeded. Plate dissipation is measured in watts (but don’t confuse this plate dissipation rating with the audio output power of your amplifier, which is also measured in watts). By Ohm’s Law, power in watts is given by the following:
P = E * I
So, in order to determine the actual plate dissipation of a given output tube, you multiply voltage by current. In this case, E represents the actual plate voltage, and I represents the total current draw of the tube. But, although we can easily measure plate voltage, we need a way to determine how much current a tube is actually drawing.
In order to determine the actual current draw of an output tube, we can introduce a resistance in series with that tube. Because current is common in a series circuit, the current draw of the resistor will be identical to that of the tube. But we can easily determine the current draw of the resistor. By Ohm’s Law, current in amperes is given by:
I = E / R
In this case, E represents the voltage found across the resistor, and R represents the value in ohms of that resistor. We can easily measure the voltage across this resistor. And, if we make the value of this resistor 1 ohm, the total current becomes E divided by 1, or simply, E. Since the resistor is in series with the tube, the current drawn by the resistor is identical to that of the total current drawn by the tube.
Putting this all together: in order to determine the actual plate dissipation of an output tube, we need to solve P = E * I. E is simply the plate voltage, and now we can determine I by simply measuring the voltage across resistor R.
Here is an example. The maximum plate dissipation rating for the 6L6GC is 30 watts. Suppose I want to bias my amp such that my 6L6 output tubes run at a conservative 20 watts. Suppose also that the plate voltage of my amp is 400 volts. Recall that P = E * I. In this case, 20 = 400 * I, so we simply solve for I. A quick calculation gives 0.050. So, I simply adjust the bias of my amp such that the voltage across R reads “0.05” to reflect a total current of 50 mA.
In order to implement this method in your amplifier, you’ll need to install a 1 ohm, 1% resistor between ground and the cathode of each of your output tubes, as illustrated in the diagram below. Although I have heard of people using 1/2 watt resistors in this position, I recommend using a higher wattage device.
For reference, here are the maximum plate dissipation ratings for some popular output tubes:
- 6L6GC / 5881 — 30 watts
- 6BQ5 / EL84 — 12 watts
- 6CA7 / EL34 — 25 watts
- 6V6 — 14 watts
- 6550 — 35 watts
For more information on biasing your output tubes, check out THIS article by Lord Valve.
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