[Amps] Air Variable Spacing Question

[TexasRF at aol.com]

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 at 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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