All preamp tubes except the PI are heated by DC.
ALL of these that I\'ve had in (quite a few) have cooked solder joints on the little round (don\'t know package desig.) bridge rect. The htr. rect.cannot handle the current draw. It will over heat and melt the joints. Repeated episodes will cause the joints/ traces to become cold & cracked. If you play one in this condition, you will suffer from (slow) decreases in volume & then it will either shut down, or as the rect cools (during the drum solo), vol. will come back up.
Besides beefing up the traces & joints at this component, I have been disconnecting the dc heater feed to V3 and hooking it to the PI AC htr. I cannot hear any increase in 60 cycle hum by doing this, btw. The mod reduces the DC heater current (.6A ?) and seems to be enough to solve the problem. (no "come backs" ever) Although it may not be a concern (lots of things were designed with starved heaters/ fils, i.e., see Fisher Hi-Fi stuff for one) when V1-V3 are in the original state, (DC) heater voltage is around 5V. When you remove V3, the level jumps up to ~around~ 6.3. (This one was 6.5) '; 1;