Jim, with a little reverse engineering and thinking through this: 2500v /
1.88A = 1330 / 1.8 = 739 ohms (approx).
One would hope for 60% efficiency so power generated = 2500v X 1.88A = 4700
(w input) X .6 = 2820w.
The voltage at 739 ohms = sqr( 2820 X 739) = 1444 rms. Peak voltage would
be 1444 X 1.414 = 2042 v.
If the capacitor has a peak voltage rating of 3200 v then there would be a
safety margin of 3200 - 2042 = 1148 v peak. Or, room for a vswr mishap that
would cause an increase of about 56% of the transformed impedance.
While not bullet proof, it seems if you were to take care that the
amplifier would always have a reasonable load this would work just fine. It
probably is a good idea to roughly calibrate the tune and load settings vs
frequency to prevent a tuning situation allowing the plate load impedance to
rise
to a damaging level.
So, in my mind the plate load impedance is very important.
If my numbers are off I am sure an eagle eyed reader will catch it!
73,
Gerald K5GW
In a message dated 12/5/2012 3:03:42 A.M. Central Standard Time,
4cx250b@muohio.edu writes:
Hi All,
I'm trying to decide which air variable plate tuning cap to use on my
forthcoming 160m monoband amplifier. The one I'd like to use has .080"
plate
spacing, and I've tested it and it works up to 3200 VDC on my tester before
sparking across the gap. My question is whether that's enough of a safety
margin to use with 2500V plate voltage on three GU-74Bs? Not that it
matters
particularly, but the plate impedance of the three tubes is about 740 ohms.
There'll be no DC voltage on the cap.
I also have several air variables with .075 in spacing, and one with .140"
spacing. The latter I've tested to 6kV, but want to save it for a different
project. In principle, I'd like to use one of the .075" caps for my
monobander, because of their convenient size, but I think that's cutting it
a bit close for 2500V. Whaddya think?
73,
Jim W8ZR
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