Heat/cool cycle in one tube in my ST-120

Hi All,
My much-loved and very much used 5year old ST-120 has just started dropping the left channel....
On closer inspection, the heater in the second-stage 12AU7 on the was going out.
At first, I thought this was just a burnt out heater (and showed the kids a disected tube) only to discover the problem was still there with a new tube... but only once the system warmed up.

So what I've found since is:
a) It's not the tube. A bit of rolling showed that it's always the tube in this socket.
b) It doesn't appear to be the heater circuit: The power is wired in from the from KT-series power tube and it's not missing a beat.
c) It is intermittent. Sometimes the problem will occur and black-out the heater, other times, it cycles a bit with heat/cool - almost like a bad connection.
d) I don't think its the wiring - I've even warmed up the solder joints with the iron to make sure there wasn't a dry joint.
e) The tube socket all looks OK and I've got the component tweezers in to tighten the connections on the heater pins.

Right now, it's been running without a fault for 2 hours, but prior to that we had an hour of the left channel dropping out.

Any suggestions?

-Anthony

My guess for the most likely source of the fault would be, as Bob mentioned, poor contact at one of the tube pins. I gather from your mention of the tweezers that you've re-tensioned the socket contacts. Has it been behaving since then?

-Pat

Crack in the wire? It happens. Also, cold solder may not necessarily be visible ... best bet is to suck the connections dry and go fresh. Even a slight crack can cause problems as the amp heats and cools. I'd also look real close at the circuit traces on the driver board for lifted pads or cracks. You're putting a lot of pressure on those when you roll a tube.

Fire it up and give it the ol' poke 'n wiggle with an insulated probe (popsicle stick works). That would certainly point to an issue with the wiring anyway.

PS ... rather than pliers, I use a small bladed jeweler's screwdriver that slips in between the socket body and finger, then give it a slight twist. Doesn't take much. Do a close inspection and make sure none of the fingers have broken off as well.

All else fails, just replace the socket and see what happens. Not sure if the older boards had the copper on both sides, but definitely make sure both top and bottom of each pin are dressed properly if they do. Ditto with all the other components on the board.