I would not suggest that mod as your first attempt at QRO. To do it
correctly requires a bit of sheet metal work , circuit mods and designing
protection circuits.
An alternative is to replace with a pair of 3CX800A7's which many have
reported is quite simple. Yes the 800 is the same price as the 8874 but
they often show up as full output pulls from medical equipment.
Svetlana has announced their 8874 but I have not heard a accurate
delivery date.
GL Carl KM1H
On Fri, 21 Nov 1997 10:29:44 -0800 "J. Allen" <jallen-yecl@polarcom.com>
writes:
>
>I am planning a conversion of an Alpha 76 CA from 8874s to 4CX400As.
>The
>price difference between the two, and the lack of availability of the
>triodes, makes the 4CX400As very attractive as a modification. I have
>been
>reading the articles in the handbooks and lately on the reflector
>regarding
>screen supply and am uncertain now what I will need for a supply. It
>does
>not appear as simple as it did at first glance.
>
>Has any one done work with these tubes or used them in a conversion
>from
>8874s? How can I determine the best voltage to run the screens at
>when I
>use the Alpha's existing plate supply?
>
>Does anyone have a protection circuit which I can use that will shut
>the
>amplifier down in the event of high screen current? Is there a common
>circuit for protecting the amplifier from high SWR? I believe that
>the
>triode finals which are in it now were ruined by the high grid current
>that
>is associated with high SWR, and I do not want the same thing to
>happen to
>the tetrodes. Are there any other things that I should be considering
>or
>changing when I modify the 76?
>
>I am handy with a soldering iron, and work with industrial
>electronics, but
>have never worked on an amplifier in this power class before. Any
>help
>that I can get to avoid pitfalls into which others have fallen would
>be
>appreciated.
>
>Thanks,
>
>J., VY1JA
>jallen@yecl.ca
>
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>
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