Collins runs them at over 700 volts in class C plate
modulated service (32V series).
If they don't arc over, and you don't exceed the plate dissipation,
what would be the problem?
That's a good point. I recall an article pre-WW2 in QST which described
getting high peak audio power from modulator tubes by running unusually high
voltage on them, but staying within the rated dissipation rating. A point
was made that for example, the 807 is rated for a maximum of 600 volts plate
modulated. That means the peak voltage would be at least 1200 volts, and
that doesn't hurt them, and is still within factory specified ratings. So
for intermittent service, you should be able to run as much as 1200 DC volts
on an 807. I think the limit on plate modulated voltage would be the point
where arcovers would begin.
However, I do recall using a quad of p-p parallel 2A3's in my audio driver.
I didn't have a proper driver transformer at the time, so I ran them @ 400
volts/30 m.a. each tube. They are rated at 300 volts/40 m.a. maximum in the
RCA manual. That way I could get plenty of peak driving voltage to the
class B grids, despite having a driver transformer with too much stepdown,
but with exactly the same plate dissipation as per RCA recommended ratings.
They worked great, and drove the modulator tubes with very little
distortion. But I noticed that within a few months, the 2A3's would get
weak. I went through a couple of sets that way. Finally, I obtained a
proper driver transformer with the proper turns ratio, and slowered the
plate voltage back to 300v, and changed the bias so that each tube would
draw 40 m.a., and now I have had the same set of 2A3's for almost 10 years,
and they still test good. Maybe just a coincidence, but at the current
price of a new 2A3, I don't feel like trying to find out.
My conclusion is that some tubes may not take overvoltage very well, while
others will not be harmed, and the only way to find out which ones is to try
them and see. If your tubes seem to be short lived, go back to the
recommended parameters.
-K4KYV
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