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
