This is a friend's amp, a vintage Teisco Checkmate. The last time the amp was on was when this video was taken, and only then turned on for the purpose of quickly documenting the problem. I have built a few amps, and I generally understand what the OT does and how it works, but what's going on here is above my pay grade other than to say that when loud chords are played, the OT seems to be be internally arcing or shorting. So: 1) What's your take on what the issue is? 2) Is it likely internal to / limited to the OT..? Any opinions, input, suggestions welcome. Thanks.