[Amps] Air Variable Spacing Question

[Han Higasa]

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