Tom Rauch wrote:
>> >Actually, the MOV is a last resort just to protect the socket
>> >capacitor...as would be a gas tube.
>>
>> The MOV or gas tube at the screen terminal is actually the *first*
>> line of defense because that is where the arc hits. Tom correctly
>> identifies that the screen bypass capacitor in the socket is at least
>> as valuable and important as the tube. The tube may survive, but if
>> you blow that capacitor, you need a whole new socket.
>
>It is last because the voltage is too high for it to be "first". Hopefully
>it is last, and not the socket!
>
I see what you mean now.
>> However, slowing the transient down - stretching and lowering it -
>> requires some kind of series impedance. This is where it gets tricky,
>> because in most designs the series impedance will tend to spoil the
>> voltage regulation in normal operation.
>
>Ian, the impedance is normally in the anode.
>
Oh, that one... sure, you always need that. I thought you meant another
impedance between the screen and the screen supply, which would help to
protect the supply.
>> One word of caution. Don't connect the electrolytic directly in
>> parallel with the screen bypass capacitor. The self-inductance of the
>> electrolytic will parallel-resonate with the bypass capacitance,
>
>I doubt that will happen with any large impedance peak because
>the RF Q of the electrolytic so low,
That may well be. The experiments I had done were with polyester
capacitors, whose self-inductance gave a sharp parallel resonance at HF.
The only complete answer is to check the actual amp. Connect the power
supply and all the wiring, and then hook up an antenna analyser to the
screen pin and sweep it from MF through VHF, looking for any frequencies
where the impedance doesn't read zero. It would do no harm to repeat
that exercise at other places, eg to check that the "cold" end of the RF
choke is truly cold at all frequencies... including the ones you *don't*
want the amplifier to operate on!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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