[Amps] GS35b FWB issue

[Steve]

 >  My next thought, as suggested in this thread, was to hipot the HT 
 >transformer, though why that shorting under load would kill a diode 
 >string escapes me.

Sketch a circuit:

transformer secondary, label the ends A and B. Diode from A to the + end 
of your caps, cathode (arrow and bar) towards the caps. Diode from B to 
-ve on the caps, cathode to B. -ve of the caps to ground. The other two 
diodes in the bridge aren't doing anythig during this half cycle when 
end A is +ve, end B -ve.

Assume point A shorts to to the transformer core and thus to ground, 
thus to -ve on the caps. Draw in a wire from A to caps -ve. You then 
have a diode straight across A and B, trying to conduct the whole 
current the winding can produce. Phut.

Steve GSQ