[Amps] Drake L-7 failure...amp vs L7-PS power supply?

[Torrey L. Mitchell]

Good evening all.

As I was tuning up my Drake L-7 amp this evening there was a very loud bang and a visible flash behind the plate voltage meter on the left.  Foolish or not I reset the power supply circuit breakers and tried again with low RF drive.  Same story again.  I took the top off the L-7 and could see no evidence of arcing or burned components.  Final tubes and attached resistors look normal.  In short, everything looks normal topside.  I have not removed the bottom panels to look.

The remotely located L7-PS power supply reveals charring and a burn-through of the R12 resistor (2 watt 0.82 ohm IRC type BWH).  The charring put a big black mark on the adjacent large tubular capacitor, but the capacitor does not look damaged and all the large resistors across these capacitors appear intact.

The R12 resistor appears to function as a protective fuse in the high voltage output line.   If so, where could such a impressive short circuit have occurred in the linear amplifier without any evidence of damage topside?   There is a good chance that I was over driving the L-7 with the TR-7 which easily puts out 160w instead of the nominal 75w.   If so, what could have caused the large bang & flash in the linear (the R12 resistor is located in the remotely located power supply)?   Does it make sense to replace the R12 resistor, make sure the idling current is nominal, then try tuning up with 25-30w drive.  How can I determine whether one or both 3-500z finals are shorted or shot?  

I may have to send both units out for repair, but I certainly would like to avoid it...given all the weight, 3 boxes, etc.

Specific recommendations for L-7 repair?

Any thoughts or help will be most appreciated.

Thanks,
Torrey N9AUJ
cell 515-778-2354