AL-80B Dynamic Bias Defeat
If you listen closely to someone running the stock dynamic bias
circuit you may hear a gritty first syllable or a mushy first cw
note. If you own an AL80B with the dynamic bias circuit in place,
listen to your own signal in another receiver. When you key the
mic without speaking you will hear the sound of an amplifier at
cut-off with a few pops and clicks blaring through now and then.
Personally, I can't fathom why anyone would want this circuit
active in a linear amplifier. The amount of power consumed between
syllables is incredibly small, and the amount of heat given off by
having the tube drop to its normal resting current between
words
rather than complete cut-off is not enough to warm a
teaspoon of water. After a few months of operating the AL80B I
decided it was time to bypass the dynamic bias circuit.
This is very easily done with a single resistor to turn Q101 on
all the time. What they have done with the dynamic bias is to
couple RF from the input circuit through C102 and R101, which is
then rectified by D101 and D102 to produce a small positive
voltage to only turn Q101 on when RF is present at the input. The
solution is to insert a 47k resistor (actually anything between
10k and 80k works fine, but I had a pile of 47k resistors so I
grabbed one of those) between the VCC line at the junction of R105
- R103 and the base of Q101. This causes Q101 to switch on all the
time and defeats the dynamic bias circuit. If you trace things out
you will find that those connection points are actually accessible
at several different convenient spots in that area of the board.
The amp will now function like a normal amp and you will see the
correct resting current immediately when you key the amp. The
modified schematic below shows where the new 47k resistor is
inserted. The photo below that shows the 47k installed in a
convenient way.
The 47k resistor will bias
Q101 to turn on