MagneQuest/Peerless Forum

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Re: EXO-173 progress

Posted by keto on May 11, 2004 at 15:53:44:
Hey, Douglas. The 6.8uFs ground to the output cathode? and the 50K ct to earth?
Revisiting my own EXO-173, the WW pot in the shared, PP2A3 cathode is a very bad idea, methinks. At least it sounded that way. A current source (or bypassed resistor) and shared filament secondaries is better, provided you have a means of adjusting current, which is what your "dealie" would do, right?
My perceptions are definitely on the move, and I look forward to reworking the EXO173-loaded (the "reference") of my two amps.
A simple solution would be shared cathodes, and following Gary Pimm's lead, measuring exact DCRs of the two OPT primary halves, at operating temperature (not voltage!) and adding a precision resistor to even them up. Then with a well matched pair of tubes, you'd be very ballparky wrt to DC offset, alternating between the pair to see which configuration is quietest. Quietest in my universe means DC offset null, which may be completely wrong. I don't have a way of measuring it otherwise, or the interest in carefully and extensively probing and recording high voltages.
Or, how about a 60mA CCS (or appropriate bypassed resistor) on each of the two filament potentials, to null filament humm with a high impedance "center tap"?
Thanks for the explanation of the 4-tap item. --keto