Engl valve amp disaster
the standby switch opening the cathode path to ground makes me nervous.
Maybe the switch arcs when it opens.
I'd find a different way to put the amp in standby or at least put
a snubber cap across the switch.
Maybe wire a switch in series with R74 and let the negative grid bias
go way up to cut the tubes off.
** A switch in the supply to the output tube's screens works well - assuming beam tubes or power pentodes are in use.
Only a few mA need be switched at idle and no electros to discharge.