>
>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.
>
? How did the insects get inside the tubes?
>>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.
>
Why trash a socket with a shorted bypass?, Ground the screen with
copper ribbon and the socket is still useable.
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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