EL84 short

[Heavenamps]

Does anyone know what might cause a short between the control grid and the screen grid on an el84? I just replaced the power transformer in an AC30 style amp and when I turned it on, one of the power tubes looked like a new universe was being born inside! All kinds of arcing and sputtering! I tested the tube (which was working fine before) and found a short between pins 2 and 9. The transformer is a classictone 40-18050.

[tubeswell]

The control grid and screen grid are right next to each other. EL84s are pentodes in a 'miniature envelope', so the grids are even closer than they are in an EL34 etc. If the bias supply fails (e.g. a failing/leaky coupling cap from the PI on that side pulling the bias voltage more positive etc), then the tube will invariably red-plate, which if bad enough and long enough, can melt and buckle the grids or the grid support rods, so that the grids end up touching and shorting. Even just an old worn out tube is more likely to go this way (which is why output tubes should be changed regularly (and EL84s more regularly than most other output tubes).

[Heavenamps]

Thanks for the response. I'm fairly certain that it was in fact a failed bias supply. I pulled way to much current through the tube and it shorted.

[sluckey]

Isn't your AC30 style amp cathode biased? IE, no bias supply?

[tubeswell]

In which case, it could simply have been a bad tube where one or more screen and/or signal grid support rods failed from overheating/wear and tear.

[shooter]

All kinds of arcing and sputtering!

I've fixed 2 amps that did just that, both were cathode biased, both had bad OT's

The results are almost always obvious, the cause not so much, which came 1st the tube or the OT?, I just swapped both n went outside n played

[hesamadman]

I’ve had this happen with JJ el84 multiple times in different amps

[Heavenamps]

I guess I misspoke. I don't know what I was thinking when I said bias supply. What I meant was that I believe I inadvertantly put too much plate voltage on the tubes to the point that I pulled far too much current through at least one of them. It's all good now though :)