Phil Allison wrote:

"TimPerry"

what fires the 2n5064 in the crowbar circuit? looks like the 2 gates are
tied together and go nowhere else. how does that make a diac?

** It doesn't - as a rule.

Just tried two GE brand C106D1s ( 400 volt, 4 amp sensitive gate SCRs)
connected as shown in the QSC schem.

Functioned like a pair of soggy, 18 volt zeners wired back to back.

Hardly the best substitute for a low voltage diac !

Graham