Pass transistor voltage handling question

[Chris Wilson]

Large linear bench supply, 60V @ 50A. SCR control of primary transformer voltage. issue with SCR control stuff. Bank of eight 2N3055 transistors in pass transistor assembly, fed by 2 more 2N3055's as driver. When SCR control fails the pass transistors can see up to 103V. The 2N3055's don't like that... One or more go short and the expensive 50 Amp internal fuse pops. If I change the 2 driver 2N3055 transistors and the 8 2N3055's in the pass assembly itself for 140V @ 20A capable MJ15003 devices should they survive whilst I do some probing? With a shorted pass bank there's no feedback to the SCR control stuff, so diagnosis is made extremely hard. Mechanically the changing of the pass transistors is tricky and time consuming, plus they aren't FOC **broken link removed** So will MJ15003 devices tolerate this and allow me to do some tests, as well as give extra overhead over the rather old hat 2N23055's once fixed please? Any issues swapping to these I might not have considered? Thanks for reading.
http://www.gatesgarth.com/H60005.jpg should bring up the pass assembly schematic if it doesn't attach here

 

[ronv]

There is probably one more preceding stage tied to the high voltage.
You would need to be careful that the load is low during your troubleshooting to stay within the SOA.
I think you might be better off with an over voltage disconnect circuit.