Gallien Krueger 400B woes

[nateflanigan]

Good morning, for sentimental reasons I picked up a Gallien Krueger 400B, it's an earlier version of the popular 400RB but it's all discreet along the lines of an old Sunn or Acoustic. 


The amp shut down the other day in the middle of practice.  Opening it up R50 had burnt up.  Here's a list of what I've done and what the results were thus far...
-Replaced R50, started smoking immediately
-Replaced Q8, 16, 12, R50 is fine but R40 started smoking immediately
-Replaced Q13, R40 and 50 are fine but another resistor (R32) started smoking immediately. 


Any insights would be appreciated.  I'll keep replacing resistors but if anyone could help me figure out a way to keep it powered on long enough to measure some voltages that'd be a big help.  I've attached a screen shot of the schematic problem area.and here's a link to the complete service manual.  My version of the power amp is page 7.
Thanks!
https://www.dropbox.com/s/2o17kkxgrk15w0m/G-K%20400B%20Service%20Schematics.pdf?dl=0

[smackoj]

hi; did you try using a simple current limiter and/or a variable AC transformer? That might get you some breathing room?

 

[sluckey]

In a dc coupled amp such as that, transistor failure is usually a catastrophic domino effect, ie, one falls down, they all fall down. Replace only one bad one and it will usually pop as soon as you apply power. You've got to replace all the failed transistors. Plus you gotta check/replace all the diodes.

You can't reliably check the transistors and diodes in circuit. Pull every one and check out of circuit. An analog ohm meter makes this check easy. Replace every one that is bad BEFORE powering on. Recheck the resistors too.