On Mon, 20 Apr 1998 17:56:30 +0100 Peter Chadwick
<Peter_Chadwick@mitel.com> writes:
>>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.
If the air flow is available I run to almost full rated
dissapation....why play around? Once I ship an amp or a repair I prefer
to not see it again.
>
>Then if it's stable unloaded, as Carl says, it should be OK. Unless
>(until?), of course, after time on 10 metres, the suppressor resistor
>cooks
>and goes high, and the parasitics appear.
With overstressed 2W carbons for R, I agree. But with 5W metal oxide
available at a fraction of the cost of 2W carbons why not change when the
cover is off? If you want to spend the time measuring the R of an
antique, why not just upgrade at the same time? I dont even bother
wasting my time to measure the old ones any longer.
Then it's conceivable that a
>problem could exist leading to arcs on bandswitches and tuning caps.
I doubt it Peter. Yes parasitics would be possible after R changing value
but please explain the path from the plate blocking capacitor to the
output connector. I fail to see a connection between a strictly tube
phenomona and a tank circuit problem.
>For
>zero grid dissipation tubes, one could conceive of the parasitic then
>leading to excess grid current causing the grid to melt - a scenario
>that
>explains some of Rich's observations on ceramic tubes.
That is a real stretch of the imagination since most of the time we have
been discussing 3-500Z's which have a fairly robust grid. I would
suggest that we stick to one category of tube and not drift off to
confuse the issue as Rich does so often. The zero grid dissapation tubes
are tetrodes anyway and should be another chapter down the road.
73 Carl KM1H
A different
>scenario
>to the flash arc effect, which does throw enough energy around to bend
>filaments and grids.
>
>73
>
>Peter G3RZP
>
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