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Re: EXO-173 progress

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keto

In Reply to: Re: EXO-173 progress posted by Sector-7G on May 10, 2004 at 19:05:27:

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

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progress - Sector-7G 17:17:47 05/11/04 (2)
Hey-Hey!!!,
The DHT amp I have on the boards will run 300B's but much else is similar. Two filament windings, a hum pot on each cathode and a current sensing resistor from wiper to ground. Say ten Ohms or so. Fixed bias, supplied through the split CT, PP grid choke, to each power valve.
5687 input tube, configured as a common cathode diff amp, feeding a PP, common cathode diff amp of two 12B4A's, loaded reisitively, and feeding the ends ov the grid choke the AC signal.
regards,
Douglas
oh yes, MQ-431 output Iron, Hammond 300BX power Iron, with the 10 Hy/500 mA choke, configured as a choke input supply, B+ in the conservative 310-320 V range for 80 mA per valve. CCS-es by DN2540
- yet another comment on filament windings - keto 22:16:05 05/15/04 (1)
  While I can't completely separate all the factors involved, its pretty clear that with a CCS or bypassed resistor in the output cathode, a common filament winding sounds and measures best. I also think I understand why Gary Pimm uses an 11-ohms resistor from each filament potential to current sink, since I'm now using a pair of 12-ohms in that position (instead of 25-ohms) and it sounds the best yet. Have you had any luck dialing things in, as described?
  - it's just a sack of bits now - Sector-7G 03:49:15 05/16/04 (0)
    Soooooon though, very soon. Nothing has jumped out of the Long Grass to bite this idea in its tender flanks and I have been bashing it about for several months now.
    Its not actually mine, and a bit of distance has gotten in the way of a marathon build session between me and the parts buyer.
    regards,
    Douglas

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