In article ,
Highland Ham wrote:

The crowbar is there to NOT blow up your equipment.

However operation of a crowbar should blow a fuse fitted 'upstream' of
the pass transistors. As already explained earlier in this thread ,the
crowbar should also be fitted 'upstream the pass transistors (but
'downstream' the fuse , while sensing the output voltage .

If the Astron voltage regulator would be suspect I would recommend a
very simple circuit with a 7812 voltage regulator and a PNP transistor
,its output 'steering' the pass transistors .

Ref : Experimental Methods in RF Design , by W7ZOI-KK7B- W7PUA
page 1.15


Instead of the 3 legged 7812 you can also use the similar LM317 enabling
output voltage adjustment
After all this is a homebrew NG

and if your really smart, you will dump the fuse, and replace it with
a Resettable Circuit Breaker, because Murphy's Law states the Crowbar
will do it's thing, ALWAYS at the worst possible moment, and the
appropriate replacement fuse will be a 5 miles drive away, at the
closest. You mileage may vary.....