measures wrote:
>
>>
>>Tom Rauch wrote:
>>>
>>>You need enough capacitance near the screen to absorb the initial spike.
>>>Neither a MOV or gas tube will work well by itself. Even a modest
>>>overvoltage is too much for regulators. Slow the transient down.
>>>
>>>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,
>
>A tube that arced is already a proven leaker.
>
Not true, on two counts.
1. Sometimes the arc is due to an RF problem external to the tube, such
as a high SWR.
2. Sometimes the arc isn't any kind of RF problem at all. Many of mine
have been due to insects (or loose parts shredded by the blower) being
carried in the airstream until they lodge in the chimney between the
anode and the screen ring.
In both these common cases, the tube is completely innocent and
unaffected.
>>but if you blow that
>>capacitor, you need a whole new socket.
>
>The last two tetrode amplifiers I built were grid-driven and they had
>directly grounded screens. ....... Direct screen grounding is how large
>tetrode amps are normally built.
>
Yes, you can do that... but there are many more medium-sized amplifiers
that use the SK610-20-30-40 series of sockets with screen bypass
capacitors.
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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