Mesa subway rocket power section

[Justin]

I came into a mesa subway rocket for cheap. It has a hole burnt through the main circut board. I was thinking of building a homebrew amp out of it since I have basicly all of the parts I will need. I was thinking of trying to use the same power amp design as the mesa used since I know that it will work well with the OT. After looking at the schematic I am a little puzzled. What are the 6 diodes right between the el84's and the OT doing? if anyone has a answer please let me know. here is the schematic
http://www.freeinfosociety.com/elect...iew.php?id=295

Thanks
Justin

[stokes]

Those are power tube surge protectors.If you dont have them it is no big deal.Most amps dont even have them,they dont affect the sound or function of the amp at all.

[Justin]

Those diodes are what burnt up. They got so hot the melted the board underneath them. It seems a little silly to me to make a surge protecter to protect the power tubes, in such a way that when it does its job it ruins something more expensive than the tubes it is protecting (the board). So anyway do you think that them failing that dramatically might point to another problem like a shorted OT perhaps?

[stokes]

Dont know for sure from here if it is the OT,but it is likely that something caused such a catostrophic burnout.If you are rebuilding the entire power section you will be able to find out when you do what caused it.

[Mark Black]

It may be that someone turned the amp on and wailed for a while with the speaker disconnected or failed, causing huge flyback spikes and shorting the diodes which then toasted themselves (and the PCB) by running the B+ through the OT primary to ground.

What kind of DC resistance measurement do you get from the OT center tap to each leg (shorted diodes removed of course)? And with the primary leads disconnected from the PCB what (if any - hopefully infinite) resistance from the primaries to either the output or the frame?

Are the speaker and its hook-up wiring OK?