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Re: [Amps] Air Variable Spacing Question

To: <Amps@contesting.com>
Subject: Re: [Amps] Air Variable Spacing Question
From: "Han Higasa" <higasa@plum.ocn.ne.jp>
Date: Thu, 6 Dec 2012 13:29:15 +0900
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Hi Jim san

The 0.080" gap is equal to 2.0 mm. I think it is not
enough for such a HP amp as your opinion.

It is said that, using a safety factor of 200 % an air variable capacitor with a gap of 1mm stand 1 kV peak and 2 mm for 2 kV peak.

Generally the plate potential can be estimated as DC plate voltage (HV+) plus 1.414 x rt(P x Rp) Where P is output power [W] and Rp is plate load impedance [cf. 740 ohms]
the 1.414 represents peak RF voltage.

The DC plate voltage is stopped by a blocking capacitor
so your plate variable capacitor should stand the peak RF voltage: if P=1500 and Rp=1000 then E=1732V;
if  P=3000 and Rp=740 then E = 2107V

So your 3200V DC variable is enough provided the
nominal plate output impedance is low.

But, when a failure of connected load or de-tune of
the pi circuit can put high voltage on the plate VC and
an arc over the gap may happen.

I recommend to use a VC with a gap of 0.14" or 3.6 mm
or, a vacuum variable of 7.5 kV or higher.

de Han JE1BMJ
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