[AMPS] Bias for SB-220

[Dick Green]

> OK, I'm confused.  I'm building an electronic bias switch and vacuum relay
> QSK into my SB-220, having finally been jarred out of procrastination by
> the failure of the changeover relay this past weekend.  In the process, I
> have become aware of (apparently) wide variation in the bias voltages that
> various modifiers recommend.

Pete,

I built Rich's electronic bias for my SB-221 ten years ago, then recently
rebuilt it after a powerful gassy tube arc destroyed a couple of components
on it. I think it works very well (as does his QSK mod.) A diode string in
the circuit allows adjustment of the bias voltage. You simply short out
diodes until the idle plate current is in the 160mA-200mA range. I set mine
for 180mA and always run the amp in the SSB position, even for CW. I think
the idle plate current drops to about 140-150mA when the CW position is
used. Setting idle current to 200mA in the SSB position will probably result
in a usable bias voltage for the CW position.

73, Dick, WC1M



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