----- Original Message -----
From: "Jim Thomson" <Jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Sunday, February 07, 2010 7:08 PM
Subject: [Amps] Converting a 76A to a pair of 3CX800's
> From: "Carl" <km1h@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.
>
> Carl
> KM1H
>
>
>
> ### 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
The ones Ive been buying from Ameritron are the same width but about a third
the height of the old ones; 270uF vs 160uF which was what I replaced last
month in a SB-1000 and 210uF in an AL-80A.
Carl
KM1H
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|