[AMPS] 4XC400A Screen Supply

[J. Allen]

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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