Sunn Concert lead keeps blowing

[drewl]

I've got an early version of the Sunn Concert Lead that keeps blowing up after about a year, to two years.
The guys that use it admit they crank the heck out of it and understand why it keeps blowing.

I've replaced the output devices with MJ15022's instead of 2N3055's, that makes it last longer.
Everything checks out as it should after I rebuild the output section, but these guys keep killing it.

Any ideas to help make it more bulletproof?

The later version has some diodes on the base of the output devices, maybe something like that to help stabilize or limit the current?

Maybe a Zoebell network on the output?

[gbono]

Well do they use a 4 ohm load? You will fry the output devices using a 2 ohm load. I also believe there was a service note for the output board (circa 1972) - service bulletin 26 where you remove C302/301 and replace CC302 with a series connected .01uF and a 68 ohm resistor - also added a .0015 uF capacitor from R318 to C304. Also shows a series connected .0047uF + 470 ohm resistor across the collector base of Q1. Most Concerts have the board changes but I have never seen the RC network on Q1. Yours must be pre '72? There was another service bulletin (16?).

[drewl]

I'll check for the revisions, and they use all kinds of different bad speaker combinations.

I wrote out for them what ohmage to use for their different amps, but i doubt they follow my instructions.