I've been working on my homebrew '68 era non master Marshall for a while. I've had two instances where two different tube sockets have arced between pins 2 and 3 on the el-34's. The first time I noticed a little scoring on the v5 socket when I got the amp but didn't figure out what it was until it was too late (an expensive lesson as it took out the OT) The second time it was (v4) a brand new NOS amphenol socket. It arced so badly, the socket actually felt lighter than the others from all the carbon displacement....Luckily the ot seems fine. I've just replaced the socket and have talked to a few people but I'm looking for some thoughts. About the amp.... This was a mkII suberbass. It now has a ptp metro board, new power tube sockets. New caps a MM OT and a MM 10H choke. ***The original PT is in there. It has some outside rust and is pretty covered in varnish on the outside of the laminates **** The last time this happened. The amp was biased at 35ma (490 volts on the plates) I had my OCD pushing the front end of the amp. the amp was dimed and going into a Hotplate....then fzzzzzzt! both times this has happened the amp has been on for a while. I can play regularly for a while, 2+ hours with no problem. I've talked to a few techs...here's what I've heard I bet it's parascitic oscillation I'd bet you have too much current going through the heaters shrink wrap the heaters replace the PT, that will solve it It's more likely an impedance issue. If the tubes are arching it may be a mis match when the amp is under an extreme load.......check to make sure your impedance selector is wired correctly. (it's very easy to do this in correctly) any thoughts???