[AMRadio] 6U8 Subststute and the 20a

jeremy-ca km1h at jeremy.mv.com
Mon Nov 12 09:00:42 EST 2007

Nice setup Jim. Ive had the full range of CE transmitters from 10A to 200V 
and everything in between.

How did you handle the 8877 conversion as far as mounting and cooling?  The 
DTR-2000 had the tube horizontal and blowing all the heat right on the tank 
circuit. That caused a bit of tuning drift as the fixed caps for 80/160 were 
right in the path; more so when the PS was upgraded to 3500V.

Ive a pair of MLA-2500's here being converted to 6M for customers, 
thankfully the 8875's are full power. But Ive been looking for tube swap 
ideas that get rid of the heat better.


----- Original Message ----- 
From: "Jim Candela" <jcandela at prodigy.net>
To: "'Discussion of AM Radio in the Amateur Service'" 
<amradio at mailman.qth.net>
Sent: Sunday, November 11, 2007 11:07 PM
Subject: RE: [AMRadio] 6U8 Subststute and the 20a

> -----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
> partial
> to phasing rigs and would love to find a 100V again.
> Carl
> KM1H
> 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:
> http://pages.prodigy.net/jcandela/Central%20Electronics%2020a%20QRO%20%2
> 32/
> QRO single E34L output tube:
> http://pages.prodigy.net/jcandela/CE20AQRO/
> 73,
> Jim
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