Ameritron tech support suggested that an AL-811H oscillation/RX noise
problem (arising when using the QSK-5 unit where the input floats during
QSK Rx times) might be helped by putting in a string of diodes (6
1N5408 rectifier diodes to give ~4.2 V bias) in forward bias along the
existing cathode line from the filament center tap. This will reduce
the resting plate current by providing a cathode to grid ~4.2V bias.
Some have written that this is desirable, even vital, for dissipation
reduction (and some 811A amps do have such bias using a Zener or bias
supply) independent of oscillation issues. Some writers have raised
concerns about the diode bias effects on IMD performance, but quoted no
data - others say not to worry. Spec sheets for the 811A do indicate
-4.5V for grid bias for a class AB2 (e.g., the claimed AL-811H class).
Anyone have any info on this topic, tried it or looked at the IMD (or
other) effects of bias use?
Bill, N6MW
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