My Mark III has died.....Help Needed.

[Direwolf]

So I played a gig last weekend with it and it sounded AWESOME. Finally got all my gear set up at practice and turned it on. I played for about 10 minutes and I started hearing a loud hum. I shut it off and checked the tubes and discovered my PI shattered. Never had that happen before. I replaced it and turned it back on and V7 immediately red plated. I swapped out power tubes and got the same result. I put new power tubes in and still got the same thing. It's only happening in this one power tube socket. I haven't pulled the chassis to look inside yet or to take any meter readings. Does this sound like a grid resistor or something else? I didn't smell any smoke and there was no electric smell. I'll pull the chassis tomorrow to get a peak inside. I'm just curious on some tech savy input. Thanks in advance for any help.

 

[Rob Lockwood]

I'm not sure but it does seem like that tube has lost its correct bias supply. The supply seems ok as the other tubes are not affected, so it could be one of the bias resistors - but thinking about possible connections betwen the failed PI and V7 - there are coupling capacitors between the PI tube and the power tubes. Looking at a schematic if one of those had failed short circuit or just partially leaking so it passes some dc it could affect the bias on V7 and the current through that half of the PI tube. Just an idea?