[AMRadio] RF Amp

ARS W5OMR ars.w5omr at gmail.com
Wed Dec 6 20:46:04 EST 2006

There's been a lot of good comments in hee, and having been witness to
the 'cookie and ether warmer'  rig of Johns when it was in it's
hey-day and running the full 1kW DC legal limit input power allowed at
that time, all I can say is, that was an 'awesome' rig.

But, pay attention to what John said...  the 304's were modulated by
(4) 813's in Push-pull parallel. (Count 'em - That's (4)!

Otis/K5SWK has -great- audio (when that rig is on) and he's running a
pair of 833's in the modulator, modulating a -single- 833 in a
link-coupled balanced circuit.

in MY rig, I'm running around twice the voltage on my 250TH
modulators, as I am on the final.

That said, since you have the 250's and the 450's.. were I you, I
think I'd run the 450's in the Modulator, and the 250's in the final.
The legal limit is 1500w PEP output, period.  You could run 1kW
output, and change the phase of the audio, where the peaks don't reach
up, and hit 1500w PEP, or you could probably run 250w with the 450TH's
in the modulator, and hit 1500+WPEP with no problem.

That's what I -really- wanted to do here, was to run a pair of 450TL's
in the modulator, to modulate the 250TH's.  I may still, but it's
gonna take some re-configuration of the modulator deck.  the sockets
are too close to the modulation transformer, to allow the 450's to set
in there.

If you've got the iron, I'd build up a 2kVDC supply, to run at 250mA
on the final, for 500w DC Input to the final.  at those voltages, you
don't need nearly as big of value components as you would if you were
running 4kV on a pair of 450's in the final.

Find a B&W Butterfly tuning cap, with nuetralizing caps on the end,
for cross nuetrization of the grids to plates, and just resign
yourself to the fact that you're probably going to have to parallel
another capacitor across that one, to resonate the final on 75m.  As
John said, it -will- need a lot of Capacitance.

450TH/L's run 7.5v @ 12amps, each.  A pair is 25 amps (thereabouts)
and a single 4-1000 filament transformer will handle that, just fine.

Operating your AM rig without a scope
is like driving our car at night without headlights.(~K4KYV)

73 = Best Regards

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