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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 (red arrow). This is GROUND in the amplifier.
4) Insert the red probe tip into the V4 red probe jack (green arrow) on the left. This measures bias for V4. Write down your
measurement
5) Repeat step 4 for V5 (red probe jack on the right). Write down your measurement
6) Adjust bias adjustment (blue arrow) SLOWLY until your DMM reads approximately 36mV (see calculations, following
page)
7) Remeasure V4 bias, and adjust as needed
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 Plexi 50W series amplifiers
- Maximum plate dissipation for EL34s
...and now some math. The formula for calculating bias is as follows:
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In most cases, amplifiers are biased between 50% and 75% dissipation. We bias the 50W Plexi series amplifier to
approximately 35-37mV on a DMM, which is 65% dissipation.
×�������������� ���� �������������� ����������������������×1000 = �������� �������������� (����)
10
=
420-435VDC
=
25W
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