14
To measure your power tube bias, carefully follow these steps
with the amplifier in
OPERATE and connected to a speaker
load
(not doing so may damage your amplifier!)
:
1)
Turn on a digital multimeter (DMM), and set it to read millivolts (mV) in the 100mV range (this will vary from DMM to
DMM)
2)
Plug a black probe into the color-coded jack on your DMM, and do the same for a red probe
3)
Insert the black probe tip into the black probe jack labeled COM on the rear of the amplifier. This is GROUND.
4)
Insert the red probe tip into the red probe jack (green arrow) on the far left according to the picture on page 13. This
measures bias for V6. Right down the value your DMM reads. You might expect a value between 35mV and 45mV.
5)
Repeat for the next red probe jack to the right (V7), again the red probe jack one more to the right (V8), and finally the
red probe jack on the far right (V9).
Okay, now I’ve measured my bias. Now what?
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 plate voltage for SSS amplifiers
=
450-460VDC
-
Maximum plate dissipation for 6L6GCs
=
30W
…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 SSS to approximately 35mV-40mV
reading on a DMM.