[Amps] Air Variable Spacing Question

[Jim Garland]

Thanks for your note, Gerald. Your calculations are right on the mark.
Sounds like my .080" cap would have enough safety margin for my application.
It's a nice Cardwell capacitor, but only about 250pF maximum, which I would
pad for use on 160m. I just need enough capacitance range to tune the entire
band, and 250pF should be more than adequate. Carl KM1H suggested a 0.1in
custom OEP capacitor, and that's a reasonable approach also. I have a couple
of nice OEP caps on hand, though they're not right for this application. I
suppose I could use the .080 cap, but leave enough room for a larger custom
cap if the need arises.

73,

Jim W8ZR

 

From: TexasRF at aol.com [mailto:TexasRF at aol.com] 
Sent: Wednesday, December 05, 2012 1:16 PM
To: 4CX250B; Amps at contesting.com
Subject: Re: [Amps] Air Variable Spacing Question

 

 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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