[Boatanchors] PA tube biasing?

J Forster jfor at quik.com
Sun Dec 16 20:38:36 EST 2007


I personally don't know, but there is a book, "The Care and Feeding of Power
Grid Tubes" published by Eimac that will, most likely, answer your
questions.

I suspect it's available on the web, but the Eimac site didn't work for me.

Best,
-John



eugene at hertzmail.com wrote:

> Hi there,
>
> Working on my harris RF-110A (urt-23) and had a question about biasing.
> I am reading the procedure to properly set the bias of the two finals
> (4CX1500B).  The instructions explain that the difference in idle plate
> current of the two tubes should not exceed 40ma.  To assist in getting
> the two plate currents to within 40ma of each other, there is a diode
> voltage divider that allows you to change the screen voltage of each
> tube from 250v to 320v. There are two jumpers with 6 positions to allow
> each tube's screen voltages to increase or decrease based on the
> position chosen.  The two tubes are currently set for the first position
> for V1 and the next to last position for V2.
>
> I have noticed that the two tubes are not identical, meaning they must
> have been acquired at different times (one says eimac, one does not,
> etc).
>
> My question is what causes the requirement of different screen voltages
> to produce the same plate current? Is it simply differences in
> manufacturing? Or does it indicate that one tube is older than the
> other? If age is the cause, my concern is that with only one jumper
> position left on V2 that I might be running out of life on the tube. I
> suspect a 4CX1500B is very $$$.
>
> So, is age the primary cause of the need to change screen voltages in
> order to balance the tubes? Or is it mostly mfg differences in which
> case, I could conceivably keep these screen voltages for a very long
> time to come.
>
> Thanks!
> Eugene
>
> _______________________________________________



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