Hi Steve,
> All has been fine until about 6 weeks ago. Hit the key on 20 and the
> amp went 'FLOOOOOOP'. Loud, but not like before and kinda drawn out.
Although it sure sounds like a tube arcing (no one can be sure
without being there, hi) I'd like to suggest some other things to
consider.
If the failure happens only on one band, it might be a series
resonant plate choke. Did you test the choke properly while it was in
the final location?
ANY anode to chassis arc can appear like a grid plate arc.
As a second suggestion, run the filament for a long while (with
blower on of course, but no high voltage). The 8877 getters itself
via little discs on the cathode, and filament heat activates the
getter.
Copper also tends to grow "barnacles" or whiskers in the presence of
a strong vacuum.
Tubes also out-gas from elements when the elements are heated.
Outgassing is a NORMAL part of a tube's life cycle. Getters don't
always work well with all gasses, and outgassing may get ahead of
gettering. It might pay to operate the tube at normal current in
actual operation but with reduced anode voltage for a period of time,
in case an element is outgassing.
Finally the 8877 went through some very bad production problems in
the 80's (and on occasion in the early 90's). Gold sputtering was
common, and the heat dam was a constant source of trouble. Eimac
worked long and hard to fix the problem, but many older 8877's are
probably floating around with potential problems.
Adding the surge resistor (especially if it is a energy absorbing
high voltage type) is a good idea, but it's no cure for a bad tube.
I assume you have the grid ring grounded directly to the chassis
with finger stock.
> The only thing I can't accout for is that when I tune up, if I hold the
> carrier for 10-15 seconds, I can watch the grid current slowly creep up
> maybe 10-15 mils.
I once received a bad batch of 8877's. The cathodes were
contaminated with gold, because the grids were defective. They had
a plating problem. Perhaps you have the same problem.
73, Tom W8JI
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