The Ultimate Red Plating EL34

[plexi50]

I have a Musicman HD-130 and here is what i found. Haven't seen one of these situations in quite a while.
This HD-130 has the 12AX7 PI.

[Willabe]

The glass melted and cracked?

From what I've read, the early 1's had a tube PI, that they got rid of.

EC and J. Winter were both using them for awhile on tour, both preferred the tube PI models.

Both stopped using them as they were catching on fire on stage.              

[plaidzebra]

Why does this happen with a tube PI? Why does this happen at all, for that matter?

[plexi50]

Yeah melted glass as well. The bias caps and coupling caps are good. The bias trimmer pot is functional. Power supply caps are shot.

[Willabe]

Why does this happen with a tube PI? Why does this happen at all, for that matter?

It's not because of the tube PI.

The amps ran too hot.

[vibrolax]

Many moons ago I ran into Rich Koerner's website. This page on the Fender PS-400 shows some melted 6550's:
http://timeelect.com/6550a-ex.htm

[plexi50]

The good part is there are a pair of Sylvania  6CA7 in the amp that are golden good.  So i need to check the amp out tomorrow with a quad of  known good tubes. Maybe a bad coupling cap. The PI tube (JAN) is excellent. Anytime i see mixed mismatched tubes it's generally the problem to start with. Man that melted glass though is a wall hanger/

[PRR]

The tube PI is fine (with all the usual failures, like leaky caps.)

Plexi noted that, because MusicMan's next design had transistors to bias and drive the output tubes. These (several variations) have new and unfamiliar failure modes, that a classic tube-tech can't diagnose on sight. They actually do make sense to any fool who serviced many transistor power amps, except the MM does it all different from any straight transistor amp, and throws-in some op-amps for extra fun.