You know those pictures on the internet - somebody taking a picture moments before the accident....
Here is such a picture:
Switched the voltage switch to 12V since I am now trying a 1962 Sylvania 12AU7 (branded Baldwin) supported by a small army of Foton 6H8S tubes - sounds very good. And look at that beautiful tube glow!
I usually leave the GOTL on for many hours; with fans it runs very cool. The amp played for an hour or so and I went outside for an errand and came back inside: Horror! A terrible room shaking hum!
Threw myself on the blue halo elevator power switch - the hum went away but it continued playing for a little while. Never a dull moment! What's going on????
Yanked out the adapter with the 12AU7 - could it be a bad adapter? Nothing smells burnt...
Switched back to 6.3V and put in a known working adapter w/ tube - everything is fine! Big sigh of relief....
But what happened? I have this box of tubes with the same pinout and using the same adapter:
12AX7, 12AU7, 12AT7, 12AZ7, 12BH7, E180CC (courtesy Phantaminum), 5693.
Let's see, 6SN7 is 6.3V, 12SN7 is 12.6V - let's check these tubes:
Why in the world would a 12AU7 be 6.3V and all the 12s listed are 12.6V??? (The only other tube listed above that is not 12V is the E180CC).
So the nice tube glow came from a tube struggling with a tube-lethal dose of voltage....
Luckily I have a spare.
All is well again.
Did I mention that it sounds very good?