12
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 (black arrow). This is GROUND in the Chupacabra amplifier.
4)
Insert the red probe tip into the V4 red probe jack (green arrow) on the far left. This measures bias for V4.
5)
Adjust V4 bias adjustment (yellow arrow) SLOWLY until your DMM reads approximately 36mV (see calculations,
following page)
6)
Remove the red DMM probe tip from the V4 red probe jack (green arrow), and insert into V5 red probe jack (orange
arrow). This measures bias for V5.
7)
Adjust V5 bias adjustment (blue arrow) SLOWLY until your DMM reads approximately 36mV (see calculations below).
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 V4-5 plate voltage for Chupacabra 50W series amplifiers
=
420-435VDC
-
Maximum plate dissipation for EL34s
=
25W
…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 Chupacabra to approximately 35-37mV
on a DMM, which is 65% dissipation.
An example is as follows: