Burnt power tube grid stoppers - Why?

[Gaz]

I took in a friend's MV Orange OD100 amp because it was sounding "weak." It puts out 95Wrms clean, but the scope trace looked a little fuzzy so I opened it up.

Sure enough I smelled the typical burnt smell and noticed two of the power tube control grid stoppers (2.2K 1/2W) were burnt. They measure correctly when the amp's off, but I doubt they're functioning normally with voltage across them (I haven't had a chance to look too deeply into that yet).

What causes an amp to burn up it's grid resistors? The amp seems to be operating normally otherwise.

[Diablo1]

Remove the resistors from the amp and test them while applying heat to them from either a heat gun or soldering iron. If the resistance doesn't drift much from heating, the resistors are good. Those grid resistors don't see a lot of current in normal use. If they really did burn up, then I would look closely at the coupling caps from the phase inverter. If the caps aren't blocking the DC plate voltage from the PI, that will cause trouble at the grids of the power tubes.

[Andy Le Blanc]

whats the tube type?

some types develop a control grid current when you push them into the AB's

el84 / el34 family I think, I had a build based on a very old inverter type that worked great with 6v6/6l6 but not with el84/el34. they developed enough of a grid current that it played hell with the paraphrase variation I was trying until I installed and extra blocking cap to protect the inverter.

I'm surprised at the cooked grid stops, even though I know that control grid current can develop, if its not the coupling caps, your gonna have to look at dynamic conditions under a power test to see if anything else is amiss

[Firestorm]

whats the tube type?

some types develop a control grid current when you push them into the AB's

el84 / el34 family I think, I had a build based on a very old inverter type that worked great with 6v6/6l6 but not with el84/el34. they developed enough of a grid current that it played hell with the paraphrase variation I was trying until I installed and extra blocking cap to protect the inverter.

I'm surprised at the cooked grid stops, even though I know that control grid current can develop, if its not the coupling caps, your gonna have to look at dynamic conditions under a power test to see if anything else is amiss

I've seen that happen with some of KTs, too.

[Gaz]

Not sure what really caused this in the amp, but Andy's hypothesis sure is intriguing. Needless to say, I replaced the resistors, and the amp seems normal besides the worn power tubes which are about 15ma off from eachother per side. Oranges sound pretty good, btw, but not as good as Marshalls

[Reeltarded]

Oranges sound pretty good, btw, but not as good as Marshalls

^^^^ the truth is out there somewhere .. oh wait! here it is! ^^^^

[Gaz]

Haha, I knew you'd like that.

[tubeswell]

Cooked resistors are ones that have seen too much power. In electronics, power comes from voltage x current. In normal operation, your typical 1/4W to 1/2W grid stopper wouldn't cook, so there must've been a heck of a lot of current through them at one point. This could happen through bias failure and/or shorted output tubes - anything that results in lots of current flowing through the grid. Look for clues about where the current came from.

[sepulchre]

Don't know if this means anything, but I thought it was curious when I saw that the grid stoppers on a TW Express were 2 watts. Guess KF was expecting some juice through there? Just sayin.

[jjman]

Grid stopper are sometimes near heat. Higher wattage units would stand up to heat better. Current should be very low. They are "stopping" current after all. Assuming stable bias and coupling caps, I would be concerned that bad tubes were/are in there.