>
>It's even tough to test for gas, because the gas can be gettered not
>only by the intentional gettering agents put in the tube, but also by
>the arc itself.
Interesting. Why would the tube suddenly arc when I was testing it on 15
meters while it was fine when I had tested it all day on 10 meters? Is
the tube still usable or should I send it back to the guy I got it from?
>If the cover hit the tube anode, or was too close to it, it could
>cause the POP.
>
>If the choke or tank was NEAR series resonance, and the cover's stray
>capacitance to the component caused the component to reach resonace,
>it could cause the POP.
>
OK. The series resonance of the plate RFC is around 23 to 24 MHz. Is
this perhaps a little too close to 21 MHz? Perhaps putting the lid on
lowered the resonance. I'll have to check for that. I was wondering if
that might be the case.
>It could also have been just a coincidence the tube arced
>after putting the top on.
>
>Whatever the problem, you had an arc because filament emission would
>not and could not cause the problem
OK. You're probably correct.
>
>> And I wouldn't expect a gassy tube to have more gain than a non-gassy
>> tube.
>
>Why not? Gain has nothing to do with gas. Gain is determined by the
>ratio of driving impedance to output impedance of a GG amplifier.
>Those impedance, as well as the mu, have little or nothing to do
>with the potential for a high vacuum arc.
Chalk it up to inexperience with tubes again. I always thought gassy
tubes are typically tubes nearing the end of their useful life and that
during that time, gain and such would begin to fall off.
>> From the blocking caps to
>> the anode in a straight line it's about 7". It's a little longer than
>> that when adding in the supressors, etc.
>
>That's a pretty long lead, no wonder the anode resonance is so low.
>
>The lower the anode resonance, the harder it is to de-Q the anode
>without adding loss at the operating frequency.
>
I began to think that myself. I would have to do some major
re-engineering to shorten it, but it might not be a bad idea at all.
Guess I'll just have to completely redesign the tank circuit from the
looks of it.
>> The vacuum cap is connected to the B&W 850A coil by about a 1/2" piece of
>> braid.
>
>Braid has several times the RF resistance of a smooth conductor.
>That's a high current area because of the VAR power in the tank.
>Braid should never be used at that point.
OK. I'll replace that then. It was done by the original builder, not me.
>Sound like a difficult tank to de-Q at VHF. Tuning caps should
>be solidly mounted on sheet metal connected directly and solidly to
>the chassis with as close to zero length as possible. The shorter
>that path, the less suppressor ESR required.
OK. In moving the tune C closer to the anode, I'll have to see if I can
lower it's position in the chassis as well.
>That's right. Build the tank considering VHF signals , and you'll
>have a very good HF tank.
Good advice.
Thanks for the help. I hope this discussion between us might be of use
to others on the reflector as well.
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@webspun.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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