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5) Repeat step 4 for V5 through V7, moving to each responding red probe jack for each
power tube. Write down your measurement for each
To calculate bias, there are two pieces of information you need to know: your amplifier‟s
power tube plate voltage, and the published value for maximum plate dissipation for the
power tubes used in your amplifier. To save you some time and energy, here are those two
values:
- Approximate V4-7 plate voltage for Plexi 51 amplifier
= 430VDC with Solid State Rectifier chosen
- Maximum plate dissipation for EL34s
...and now some math. The formula for calculating bias is as follows:
In most cases, amplifiers are biased between 50% and 75% dissipation. We bias the the
amp to approximately 35mV on a DMM, which is 65% dissipation.
An example is as follows:
You might wonder why your DMM is set to millivolts and not milliamps – simply, we have a 1
Ohm resistor placed between your probe jacks and ground to convert a current reading to a
voltage reading. That way, a bias current of 35mA measures as 35mV on your DMM.
NOTE – Only set your DMM to mV for measuring bias on the amplifier. Not doing so may
damage your DMM.
Now that you know how to calculate bias, all you need to do is:
1) Follow steps 1-5 on page 9
= 25W
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