Flyback diodes in a MusicMan...and otherwise

[Le Basseur]

Got this HD One-Thirty (the SS PI version) on repair and one half of the OPT is shorted (the amp came with a shot EL34 wich took it's toll on the PI and driver transistor).
Schemo here:
http://www.ernieball.com/mmonline/te...s/2100-130.pdf
My question is:why the flyback diodes didn't do their job?
(...however,the mains fuse was gone too,but...too late! )
The diodes are original (in the schemo,marked as "D8 MR250-5 5 KV!) and measure OK.
Is the "5KV" a too high value for the purpose of a spike protection in that place?
If yes,what's the reccomended diode type(s) and their "voltage threshold" in order to protect your average OPT?
Thanks!
Regards,

[Steve Conner]

Well, if the OPT shorted because it overheated from too much current due to a runaway tube, the flyback diodes wouldn't do anything about that. They only clamp voltage spikes.

And the blown mains fuse suggests that a runaway tube was indeed the problem.

[Le Basseur]

Thanks!

Well, if the OPT shorted because it overheated from too much current due to a runaway tube, the flyback diodes wouldn't do anything about that. They only clamp voltage spikes.

Thank you very much Steve,THIS is the kind of an answer I hoped and waited for.

And the blown mains fuse suggests that a runaway tube was indeed the problem.

More than the blown fuse,the overall landscape of an EL34's cracked glass envelope and it's melted anode WAS indeed the problem.
...too bad that the owner kinda "insisted" by replacing the blown fuse with a bigger one and fired it up once again,without even taking a look at the head's back...but now's too late...
Regards,