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In Reply to: RE: I realize specs aren't everything... posted by Eli Duttman on March 25, 2015 at 13:19:22
30WPC should work nicely given the OP's stated listening conditions/preferences. So an El Cheapo Grande is another possibility.
I recall you (or Jim?) referring to the "Grande" recently but I didn't realize it had been "released to production".
Follow Ups:
The Grande idea has been out and about for quite some time. Putting together something "official" has not been done.
Compared with the "12" W. tube "El Cheapo", the "taller" B+ rail needed by the 7591 allows the 'T7 plate load to increase to approx. 80 Kohms. 33 K carbon film and 50 K inductive wirewound parts in series could be fine.
Combination biased "finals" would be employed. "Stand" the tied together cathodes of the O/P tube pair on a 470 muF./100 ohm bias network. A single "idle" current set trim pot. is fine and the 100 ohm resistor is a very convenient test point. By making part of the total bias voltage self generated, protection against runaway, which the current production EH 7591 is known to occasionally do, is provided and 330 Kohm grid to ground resistors will be safe. Keeping the 7591 grid to ground resistor value up prevents splitter/driver gain from becoming insufficient.
For monoblocks, AnTek AS-1T300 power trafos should be quite adequate for B+ and heater power.
Give me some time and I'll concoct a scheme for the requisite negative voltages.
Eli D.
Two negative rails are needed: the 1st is "short"/instant on (for bias/C-) and the 2nd is "taller"/slow starting (for LTP power/B-).
Start with an AnTek AN-0240 power trafo.
Phase the 2X 40 VAC windings up and connect them in series. Hybrid bridge rectify the composite using a MBR20200CT common cathode twin Schottky diode as the connection to ground and a 6AL5 forming the connection to the B- supply filter. This bridge is like that in the original "El Cheapo", but the rail voltage is a bit lower (NBD).
Connect the cathode of a MBR1100G Schottky diode to the junction of the 40 VAC windings. Connect the anode of the MBR1100G to the C- supply filter.
Both rails are full wave rectified, with the MBR20200CT providing the path to ground. The "tall" rail is bridge rectified and the "short" rail is FWCT rectified. Zero switching noise too. ;> )
Thank Pete Millett for the 2 rails from 1 CT winding concept.
Eli D.
Not to take anything away from Pete, but isn't this the "economy" supply that appears in technical manuals going back many years? It's a great way to get two different voltages of the same polarity, although it was traditionally employed for B+.
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