Really hoping someone can help me out.
I just got what appears to be a mostly stock '69 plexi-panel Super Lead. Consistent across all four power tubes: I measured 482v plate voltage (pin 3, with the tubes in). I measured -44v on pin 5 (with the tubes out). Tubes bias fine to 35mA (no input, volumes turned down). I've dropped the bias as low as about 20mA just to check.
At idle (guitar plugged in but not playing): everything seems fine.
But if I play for about 60 seconds or so, one side of both V4 and V5 starts to red plate (the side toward the back of the amp; the other side of the tubes is fine). As soon as I stop playing, the tubes cool and return to normal.
I've swapped the tubes around: the problem says with the sockets for V4 and V5.
Any ideas what's going wrong here? The amp sounds glorious. But it would be catastrophic to lose the output transformer from red plating tubes.
Last edited by cardinal
on Fri, Jun 14, 2019 8:04am, edited 2 times in total.