[AMRadio] 4D32 rig testing

[Donald Chester]

>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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