Hi Scott,
I think any PA using a properly air-cooled ceramic tube, will do the the job;
for 500~800W power levels even a single 3CX800 will work without problem - not
to mention a 3CX1500 or a pair of 3CX800.
The main problem with 811 PA's (and also with most 3-500z commercial
constructions) is that the cooling is just done by natural convection (just
with the help of a little blower) so all components are working at high
temperature after a while. A proper cooling would require to use chimneys, and
real air-forced cooling, which no manucturer is using... (because it would
become too noisy ...)
I recently repaired a drake L7 (2x3-500z) , and tried it one afternoon in TTY:
I would not try to use it at more than 300W for a TTY contest duration... On
the other hand, and though I am using it only at 50%, I am sure my homebrew
pair of 4CX250 is capable of providing a KW for a 100 or more contests
duration, with the advantage that I do not need more than 15W drive to get the
full output (but the PA has to be outside the schack, for noise reason - the
4CX250 are performant, resistant and unexpensive tubes, but they require high
pressure air flow...).
Most of modern "US legal limit" PA's are using a pair of Svletlana 3CX800
(rather than the very expensive Eimac 3CX1500) - Assuming they use a proper
blower, and a properly "sized" transformer, you should not have problem with
any of those (to give only a few brand names: ACOM, ALPHA, AMP-UK, QRO-
technology, etc...). Compaired to the a/m 4cx250 the 3CX800 requires low air-
pressure which allows to make really noiseless PA's.
Most of those manufacturers also produce a "single tube" version, which may
still be good enough, to work all TTY contests without problem for the next 20
years... (I built my PA with "second-hand " 4CX250, I am still using the same
pair of tubes since I built it 15 years ago...).
Good luck and happy new year!
Patrick
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