[AMRadio] 6U8 Subststute and the 20a

Jim Candela jcandela at prodigy.net
Sun Nov 11 23:07:09 EST 2007

-----Original Message-----
From: amradio-bounces at mailman.qth.net [mailto:amradio-

Glad to hear someone is still flogging those old CE's. I was always
to phasing rigs and would love to find a 100V again.


Reply by Jim, WD5JKO:

Carl, Yes I have been flogging the 20a's for about 5 years now... In
fact I was on last night on 3890 LSB running legal limit SSB. The same
setup can run about 300 watts AM.

The tube lineup was:

1626 Osc
1625 Buffer
6MU8 9 Mc Osc and audio driver
6BA7 Mixer
1/2 12BH7 Driver
2 x 7591  Linear output tubes
8877  Dentron MLA-2500 substitute tube for a pair of dead 8875's

Part of my QRO 20a campaign has been to settle down the stock 20A heat
issue. Here is an excerpt of my report: 

The unregulated voltages +550, +290 are both from choke input circuits,
so the regulation with these is decent. The Key up high B+ only rises to
about 600 volts. Funny thing here is that with choke input filtering,
the power transformer runs cooler than a stock 20A, which uses a
capacitor input filter circuit. Lowering the B+ on the low level stages
also cuts way back on the heat produced. Recall that a stock 20A will
run the low level stuff at ~ 400 volts B+ (idle) with today's 120V +
line voltages.

QRO 7591 output tubes:

QRO single E34L output tube:


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