[AMRadio] 3-500Z vs 4-400A

Patrick Jankowiak recycler at swbell.net
Mon Dec 13 21:37:43 EST 2004

True, but the rig had a pair of 304TH's in it before, and is 
capable of running 3600V at 1A CCS on both the modulator and PA HV 
supplies (separate and individually adjustable from 0-4000V), so 
I want to preserve the maker's intent by not downsizing the 
modulator tubes. The modulation transformer is from an RCA 1-K 
broadcast transmitter. It was built back in the "DC KW" days and 
has a special 'band' for 2726KHz, a Texas State Guard frequency. 
Durward Tucker W5VU spent 2 years building that 24x7 KW rig and I 
must avoid doing anything that would diminish its specifications, 
out of respect.

There are no simple and inexpensive replacements for 304TH except 
3-500Z or 4-400 (and blower). So many of the radiation cooled 
tubes like the beautiful 250TH are rare and costly, and I did not 
want to use 4CX250B's or smaller radiation cooled tubes, wanted 
to stay with large glass. Here's the rig's page, so you can see 
what I am trying to restore.

In the Binder, there is a page showing 2160 watts tested as a DC 
input at carrier condition. There was no real power limit for the 
Guard's transmitters.

All I can say is that with today's restrictions on peak output 
power, the tubes in the rig will just idle along, and last forever..


From: "Jim candela" <jcandela at prodigy.net>
Subject: RE: [AMRadio] 3-500Z vs 4-400A
To: "Discussion of AM Radio" <amradio at mailman.qth.net>


       For a "legal" AM plate modulated ham rig, running the 
tubes you
mentioned as you mentioned is way over kill. Heck, ~ 300 watts or 
so of
audio is all you will ever need to modulate the a rig designed 
for 375 watts
carrier output. If I had 4-400's I'd go AB1 and run it as a 
tetrode. Sure,
that requires a bias supply, and a screen supply. The drive will 
be simpler,
and a lot less swing, enough simpler to justify the extra 
complexity of the
extra power supplies. A 12AX7 driver would do fine.

       As for your question, yes it's workable. I would use 
negative feedback
however to linearize those tubes a bit since distortion will be 
fairly high.
The hardest part of your plan is getting undistorted drive from 
the driver.
Maybe use one of your 2 X 8005 PA amplifiers driving a quality 
audio output
transformer in reverse.


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