>>The way I was taught is that you key the amp with no drive and vary the
>>caps all around and look
>>for a kick in the plate or grid current.
>
>Also decrease the bias so the tube is running a fair bit of plate current -
>say 1/3 to 1/2 rated dissipation so the gm is high.
>
However, unless the anode current is being pulsed, the VHF-resonance in
the anode circuit will not be generating damped-wave VHF signals that
could possibly lead to an oscillation.
>Then if it's stable unloaded, as Carl says, it should be OK.
At VHF, with a conventional Pi-output, there is No VHF load on the
amplifier, even if a 50 ohm VHF-rated termination is connected to the
output.
>
> Unless(until?), of course, after time on 10 metres, the suppressor resistor
cooks
>and goes high, and the parasitics appear.
What if the amplifier is being operated on 20m, there is a big-bang, the
suppressor resistor looks like new, but the resistance is 400% or so
higher than the marked value?
>Then it's conceivable that a
>problem could exist leading to arcs on bandswitches and tuning caps. For
>zero grid dissipation tubes, one could conceive of the parasitic then
>leading to excess grid current causing the grid to melt
I have never autopsied a tube that had a melted grid.
>- a scenario that
>explains some of Rich's observations on ceramic tubes. A different scenario
>to the flash arc effect, which does throw enough energy around to bend
>filaments and grids.
In a grounded-grid amplifier, how could an anode arc to the grounded-grid
damage the filament?. I have never autopsied a tube that had a bent
grid. Bent filament, yes. Gold-sputtered grid, yes.
- later, Peter
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|