On Wed, 13 Jan 1999 23:36:09 EST KA2YKC@aol.com writes:
>
>Good evening;
>
>I am in the process of repairing a 76CA for a friend. It appears that
>the
>tubes are weak. Rathert than paying top $$ for 3 - 3cx400's, I
>remember seeing
>someone mention a web site regarding a convversion to 2- 3cx800's.
>Does anyone
>know the URL for the site ?
>
>Also, other than the heater voltage going to 13.5v, should the input
>and
>output circuits be compatible, or do I need to modify the tank or
>input ?
The conversion is rather straightforeward with little metal work.
If you run the 3CX800's at the same 1800W of a full bore CA there will be
no tank circuit issues. I would not run over that power by very much.
Several on the reflector have done the conversion with no difficulty.
>
>I thought I saw someone mention using a voltage doubler rather than
>adding a
>13.5v transformer. Is this possible ?
Yes, Ive done it several times and it is far superior to series
connecting another xfmr. A fault with all the 8874 Alphas is the filament
winding is on the plate xfmr and you have no control of the voltage. That
winding is rated 6.3V @ 9.6A so you have plenty of overhead for a 13.5V
@ 3A supply. There is plenty of room for a pair of 6A1 diodes, filter
caps and a TO220 size adjustable regulator or even a standard 12V one
with a pair of diodes to bias it up to 13.2V or so. All parts are real
cheap at Mouser. Breadboard it to make sure it works and then wire it in
the bottom of the tube deck.
A similar circuit is suggested even for the stock 8874's plus Dentron
DTR2000L, MLA2500 and other amps that use expensive indirected heated
tubes where voltage control is an issue.
>
>Lastly, in order to lower the resonant freq of the plate choke, do I
>need to
>add turns or remove turns ? Any rough idea as far as the turns/Mhz
>ratio ?
>(10/12m issue)
That choke does not like much playing around as it moves other resonances
also. I've posted a good homebrew design here several times. You can also
buy one from Ameritron, Ten-Tec, etc.
73 Carl KM1H
>
>Many thanks to all for the ongoing education in amplifiers via the
>reflector.
>I was born 20yrs too late (solid state era) to get a "traditional"
>education
>in tube amplifer design.
>
>73 Eric
>
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>
>
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