[Amps] Air Variable Spacing Question

[Carl]

Jim,

The 250pf OEP used in the SB-221, HL-2200 and as a wider spaced SB-220 cap 
would be ideal. Im all out at the moment but anyone who has converted those 
amps to 6M should have one laying around, its a .1" I believe.

The Cardwell cap you have, if its a 154-9, is listed at .075" and 3000V peak 
without breakdown at 70 deg F and 50% humidity iffn I remember.

Thats cutting it mighty close.

Carl



----- Original Message ----- 
From: "Jim Garland" <4cx250b at muohio.edu>
To: <TexasRF at aol.com>; <Amps at contesting.com>
Sent: Thursday, December 06, 2012 1:15 AM
Subject: Re: [Amps] Air Variable Spacing Question


> 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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