[Boatanchors] Tuning Capacitor Air Gap Information

[jeremy-ca]

I wouldnt expect any sane person to directly attach a scope probe to the 
plate tune cap of a KW amp.

For low power the probe is connected to the output cap which is somewhere 
near a 50 Ohm point, you can easily compute the expected voltage before 
attaching. Or use a T connector at the coax connector. The reading is 
relative, the difference between a CW carrier and modulation peaks is what 
is measured.

For high power use a directional coupler and pad down if necessary. The 
common VSWR/Wattmeter is a directional coupler, connect the probe before the 
diode.

You can even use a short length of coax as a low value coupling capacitor 
T'eed into the feed line.

Carl
KM1H


----- Original Message ----- 
From: "J Forster" <jfor at quik.com>
To: "jeremy-ca" <km1h at jeremy.mv.com>
Cc: <brianclarke01 at optusnet.com.au>; <Boatanchors at mailman.qth.net>
Sent: Tuesday, October 09, 2007 11:27 AM
Subject: Re: [Boatanchors] Tuning Capacitor Air Gap Information


>A great deal of caution is necessary when scoping HV RF circuits. You 
>really
> need one of the Tek special HV probes (P6015, filled with Freon) and even 
> they
> have to be derated at higher frequencies as I remember.
>
> Best,
> -John
>
>
>
> jeremy-ca wrote:
>
>> A scope works wonders for measuring peaks.
>>
>> You ever wonder why ham SSB amps often have bad habits of arcing the Tune
>> caps? Cheap manufacturers use spacings that equal or just a hair above 
>> the
>> HVDC and have no overhead for peaks. Thats just for SSB, imagine what
>> happens with plate modulated AM.
>>
>> I get customer amps in here on a regular basis that I have to either 
>> change
>> the caps or rebuild the type that use removable plates.
>>
>> Carl
>> KM1H
>>
>> -
>
>