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.