> Such a progressive bias scheme is completely benign in its action and
> introduces no leading edge distortion artefacts, maintaining a spectrally
> clean amplifier SSB output signal.
In the ACOM 2000A, three-state bias is used. -124V on standby, -75V during
plate idling when the PTT line is pulled low, and -52V when input RF is
sensed. When using VOX, the amp is quickly transitioning back and forth from
deep Class C to Class AB. This EBS system produces non-linear, bandwidth
consuming distortion that is immediately heard -- and seen on a S/A or one
of the better panadapters (e.g., SDR-IQ, LP-PAN), especially when the
received signal strength is high. Several ACOM owners have altered the bias
to move the standby state closer to the conduction point. R42 is bypassed.
After the EBS-modification, BIAS is fixed at -52V during transmit and -124V
in standby. The improvement is dramatic.
But there's no free lunch, and the mod does result in greater heat
dissipation since the amp is not allowed to pass to the mid bias level upon
input RF sensing. Users report higher tube temperature as reported on their
remote RCUs. The ACOM 200A has the ability to monitor stack temperature
with LM34-type devices in a TO-92 package.
Similar issues with my Alpha 70V and 77Dx. In the case of the 77Dx, a
user-adjustable pot allows for the conduction of a bit of plate current to
minimize EBS distortion. I do not hear this when using the Alpha 87A and 89
models -- and never investigated why as they both use EBS as well.
Paul, W9AC
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