5F6-A losing power

[Chris Scott]

... or so it seems.

Playing a gig on fri. when all of a sudden the volume drops about 80%. I start nosing around and decide the fil on V2 looks a bit weak, (technically, a lame hunch by any estimation, but it got me to the right tube) so I pop another one in, and all is back to normal. Both tubes are Sovtek 12AX7LPS.

Check it the following day - old one back in, and no real power, replacement back in, and all is good, so I decide it was a bad tube.

That night, same deal occurs, and this time I pop a NOS spare in there. By this time I'm thinking that it's gonna be iffy to finish the gig, as I've only two other spares, yet the following 3 hours go by uneventfully.

This morning I check all voltages, and all seems OK.

Fluke?

...or am I just not looking deep enough?

Best,
Chris

[WaltC]

The Sovtec LPS tubes have a mostly-hidden spiral filament wire that, on a good and normal day, looks a *lot* dimmer than "normal" 12AX7s. So it may well be a loose sleeve in the tube socket, weak solder joint on the socket or some such problem that is "worked around" by removing and replacing the tube. Doesn't account for the initial tube not working properly, but might be as likely as two LPS tubes failing in a row.

Definitely worth checking out further (or a trip to your local tech's).