Any cure for Microphonic power tubes?

[Chuck H]

I only build a little. I don't have a repair shop to maintain, but...

WRT microphonic power tubes I've certainly had my fill. So what I do now is buy my tubes in quads or "lot"s and test them for microphony when they arrive. Ruby tubes are about the only el34's that'll take any real voltage, but tend toward microphony. So I test tubes when they arrive and pick the tubes that exhibit the lowest microphony for combo's and those that exhibit higher microphony are labeled "head only". Unfortunately the amount of tubes appropriate for combo amps vs. heads is disproportionate.

I hope this helps, but I can't imagine it will much.

[nevetslab]

Just a couple weeks ago I had an amp in that gave out a low moaning "moooooooo" soon as it warmed up. No rattles, just the cow imitation. Bad output tube. Like bells, tubes can ring at a frequency appropriate to their size. Yes some rattle too, but not that one. I'm sure in time you'll run across Ferdinand-in-a-bottle, and you can have a big smile as you think "yeh, there it is all right, I finally found one!"

Pre tubes can make all sorts of outer space noises, in fact I've saved about a dozen or so prime offenders just for fun. Not so for output tubes. Just rattles & moans like Marley's ghost. Boring... mooo, clank!

I've found external fixes like O rings & shrink tube may help a tube that's marginal, but the heat/cool cycling they undergo drives that marginal tube into unacceptable range soon enough. Maybe that can work in hi fi gear, but not so much in combo amps or heads that are stacked atop speaker cabs.

The same Fender Twin Reverb today ALSO had a 'ghost' in the system.....this eerie sounding haunting noise that was singing in the background....chased it all the way back from the driver to V2 12AX7. Keeping that tube.......might find a customer who give me such a hard time, he'd deserve it!

I might try some of the hi temp silicon rubber tubing that I use on hold-down clamps.....slit the tubing open so I can run some lacing cord thru the center, cut the tubing so it joins without overlap, and tie it onto the glass to see what that does. I hadn't thought about the hi temp O-rings.....or kinda did....figured I'd see what ya all said.

[J M Fahey]

I think the market is ripe for a floating/damped tube socket .