[Amps] Converting a 76A to a pair of 3CX800's

Jim Thomson 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?
> 73
> 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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