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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 GROUND probe jack (green arrow). This is GROUND in the amplifier.
4)
Insert the red probe tip into the V5 probe jack (red solid arrow) on the far left. This measures bias for V5.
5)
Write down DMM reading
6)
Remove the red DMM probe tip from the V5 probe jack (red solid arrow), and insert into V6 probe jack (blue solid arrow).
This measures bias for V6.
7)
Compare readings, they should be fairly similar (within 2-4mV).
8)
Adjust bias adjustment (orange dashed arrow) SLOWLY until your DMM reads approximately 55mV (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 6550 plate voltage for AFD #35 series amplifiers
=
450VDC
-
Maximum plate dissipation for 6550 (depending on brand)
=
35-42W
-
…and now some math. The formula for calculating bias is as follows:
𝑚𝑎𝑥𝑖𝑚𝑢𝑚
𝑝𝑙𝑎𝑡𝑒
𝑑𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑖𝑜𝑛
𝑎𝑚𝑝𝑙𝑖𝑓𝑖𝑒𝑟
𝑝𝑙𝑎𝑡𝑒
𝑣𝑜𝑙𝑡𝑎𝑔𝑒
×
𝑝𝑒𝑟𝑐𝑒𝑛𝑡
𝑜𝑓
𝑚𝑎𝑥𝑖𝑚𝑢𝑚
𝑑𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑖𝑜𝑛
×
1000
=
𝑏𝑖𝑎𝑠
𝑐𝑢𝑟𝑟𝑒𝑛𝑡
(
𝑚𝐴
)
In most cases, amplifiers are biased between 50% and 75% dissipation. We bias the AFD to approximately 50-55mV on a
DMM, which is around 60% dissipation, depending on brand.