What is the real capability of the PA211 power trafo used in PAS preamps?
The spec's provided by Triode Electronics for their PA211 replacement show 15 mA. of cap. I/P filtered B+ and 800 mA of 12 V.
I think the RMS capability of the rectifier winding is approx. 25 mA. However, the 12 V. winding gives me pause. Hafler's heater circuitry uses a voltage doubler to energize pairs of series connected 12AX7s. That requires 300 mA. at "25" V. and, in turn, at least 1 A. of "12" VAC feeding the doubler. When you add in the needs of the incandescent pilot lamp and 12X4 rectifier, the head scratching starts.
The spec's provided by Triode Electronics for their PA211 replacement show 15 mA. of cap. I/P filtered B+ and 800 mA of 12 V.
I think the RMS capability of the rectifier winding is approx. 25 mA. However, the 12 V. winding gives me pause. Hafler's heater circuitry uses a voltage doubler to energize pairs of series connected 12AX7s. That requires 300 mA. at "25" V. and, in turn, at least 1 A. of "12" VAC feeding the doubler. When you add in the needs of the incandescent pilot lamp and 12X4 rectifier, the head scratching starts.
The heaters in one dual-triode 12AX7 can be wired in series or parallel. Internal sections in series for 12.6v for each 12AX7, each tube requires 150mA at 12v and two tubes (four sections) in series requires 25v at the same 150mA. How many 12AX7 tubes are there? You say "pairs" so I assume 4, which would require 300mA at 25v. Is the doubler that inefficient that it wastes more than a third of the power? If it was 100% efficient it would require 600mA at 12v, + pilot + rectifier heater. The 12X4 needs another 300mA at 12v. That's 900mA + pilot + doubler inefficiency. Hmmm...still not good.
Eli,
For what it’s worth, I dug out some notes that I made a few years ago, when I was trying to “squeeze just a little more” from the Dyna PAS transformer. I concluded it can’t be done, at least at 120 VAC line voltage, anyway.
4 (12AX7) X .15A heaters running at (only) 11VDC =.524 ma
1 (12X4) @ .3A heater running at 10.5VDC =.250 ma
1 #53 lamp @ 10.5VDC =.088ma
TOTAL= .862 A
I found a note, but am unsure where it came from, that the absolute max. heater current is .9A.
I did some wire resistance measurements on three variations of the transformer:
PAS 2---Primary: 40.6 ohms
Heater: 1.1 ohms
HV: 3114 ohms
PAS 3 (carries different part #)
Primary : 41.62 ohms
Heater : 1.12 ohms
HV: ? (probably the same)
Triode Electronics
Primary: 34 ohms
Heater: .65 ohms
HV: 1190 ohms
With the “Triode” transformer, I’m able to power 4 tube heaters at 12 VDC through a 7812 regulator (substituting an LED for the lamp, and SS HV diodes) down to an AC line voltage of about 106VAC, but, adding a 5th 12AX7, is a big compromise, even with a LDO regulator.
HV? who knows? I don't! I'm running 5 12AX7's, but only at <300 VDC, unregulated.
For what it’s worth, I dug out some notes that I made a few years ago, when I was trying to “squeeze just a little more” from the Dyna PAS transformer. I concluded it can’t be done, at least at 120 VAC line voltage, anyway.
4 (12AX7) X .15A heaters running at (only) 11VDC =.524 ma
1 (12X4) @ .3A heater running at 10.5VDC =.250 ma
1 #53 lamp @ 10.5VDC =.088ma
TOTAL= .862 A
I found a note, but am unsure where it came from, that the absolute max. heater current is .9A.
I did some wire resistance measurements on three variations of the transformer:
PAS 2---Primary: 40.6 ohms
Heater: 1.1 ohms
HV: 3114 ohms
PAS 3 (carries different part #)
Primary : 41.62 ohms
Heater : 1.12 ohms
HV: ? (probably the same)
Triode Electronics
Primary: 34 ohms
Heater: .65 ohms
HV: 1190 ohms
With the “Triode” transformer, I’m able to power 4 tube heaters at 12 VDC through a 7812 regulator (substituting an LED for the lamp, and SS HV diodes) down to an AC line voltage of about 106VAC, but, adding a 5th 12AX7, is a big compromise, even with a LDO regulator.
HV? who knows? I don't! I'm running 5 12AX7's, but only at <300 VDC, unregulated.
Art,
Thanks for digging those notes out.
Replacing the pilot lamp with a LED is a given. I'm leaning towards using a series wired combo of LED, UF4007, and current limiting resistor powered by the AC mains.
The idea of moving the power trafo to the outside of the back panel, for improved ventilation appeals to me. I was reminded of the tweak, while doing some web "surfing". Also, AVA uses it as part of their PAS rebuild package.
FWIW, I'm thinking Schottky diode full wave bridge rectify the filament winding and using the DC to energize only signal tube heaters. A small "12" V trafo would energize the 12X4 heater.
Thanks for digging those notes out.
Replacing the pilot lamp with a LED is a given. I'm leaning towards using a series wired combo of LED, UF4007, and current limiting resistor powered by the AC mains.
The idea of moving the power trafo to the outside of the back panel, for improved ventilation appeals to me. I was reminded of the tweak, while doing some web "surfing". Also, AVA uses it as part of their PAS rebuild package.
FWIW, I'm thinking Schottky diode full wave bridge rectify the filament winding and using the DC to energize only signal tube heaters. A small "12" V trafo would energize the 12X4 heater.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.