In restoring and upgrading an Alpha 76A to use a pair of
3CPX800A7 triodes there was a choice to be made over
continuing to use the OEM bias source, which is based on a
fixed zener, and meant for the OEM 8874 tubes, or to
convert the whole shebang to the bias supply provided
by the Paul Hewitt WD7S Triode Controller.
The CPX's are conservatively rated at 800 mils apiece
Anode Current, and 60 mils apiece for Grid Current.
The WD7S controller instructions say to set up the
Bias Supply with a bench power plant running some
current through the Cathode Bias resistor network.
The instruction set says to crank up the bench supply for
a tad more Bias Voltage than desired, then to key the
Amp without HV or drive applied and adjust the
potentiometer on the Triode Controller's Bias
pot to the same value seen on the bench supply.
I need a sanity check here, please.
With either 2400 or 3400 Anode Volts at
a maximum of 1.2 amperes anode current,
is around 25 volts an adequate bias voltage?
The Triode Controller adjustment sequence
works well up until this point and I e-mailed Paul
this morning, but if any of you have thoughts on
setting up the Bias for a pair of 800's I would
surely like for you to share them.
Thanks,
Hal Mandel
W4HBM
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