[Amps] Converting a 76A to a pair of 3CX800's
Jim.thom at telus.net
Sun Feb 7 16:08:27 PST 2010
From: "Carl" <km1h at jeremy.mv.com>
Subject: Re: [Amps] Converting a 76A to a pair of 3CX800's
> As I mentioned in an earlier post, I need a 3CX400. If I don't find one
> I'm considering converting to a pair of 3CX-800's which I already
> have. Although I'd have to replace the transformer as the one in there
> is a bit on the light side and there is little room for a filament
> transformer. That's notably less than the cost of a new tube. The
> conversion looks straight forward and quite simple as the new
> transformer has the proper filament voltage (IOW it's a drop in
> replacement with the filament winding for the 3CX-800's) ...BUT, when I
> look at the transformer with a 1900 VAC RMS winding into a full wave
> bridge and then at that stack of 6 450 V caps it looks a bit marginal.
> IOW 1900 X 1.414 - 2686 volts out of the bridge while 6 X 450 = 2700 V
> for the capacitor ratings. If I figured that correctly that's a whole 14
> volts to spare if all the caps are at least 450 volts.
> Any thoughts or suggestions?
> Roger (K8RI)
Buy the 470V rated caps (thats whats claimed and marked on them) from
Ameritron to give you some headroom. You could also drop the input AC with a
fat bucking transformer as long as the filaments stay within ratings.
### Cornell Dublier also makes caps in 500 vdc versions, mind you, not too many.
Another thought is is to use 3CPX-800A7's.... which are rated for 3.5 kv. Eimac came
out with a bulletin, telling commercial FM broadcast stations using 3CX-800A7's..
to use 3CPX-800A7's. Reason was, line voltages in some country's would drift to
the high side, add some spike on the drop wires, and tubes would flash over.
## IMO, the stock OEM cap values on all these amps are way too small. There isn't
enough energy storage to get the ripple down, nor provide for good dynamic regulation.
## ask anybody, who has replaced 200 uf caps, with 400-600+ uf caps. PEP out is
higher, and dynamic V regulation is much better. Ripple typ drops from 3%...down
to 1% .
## You might also be able to retrofit 7 x caps, where the original 6 x stood. The new breed
of caps are way smaller these days.
Later... Jim VE7RF
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