There needn't be any kind of delay for HV. I place full B+ on 4CX250Bs (and similar tubes) all the time, even if they're turned off (no heater). Never had any kind of failure. The tubes in my old P-P 2m amp are from 1979 and work the same today as they did back then. You just can't "drive" the amp until they're warmed up. The only important delay is the one that "keys" the amplifier into "operate" from "standby" and allows RF to drive it. Simple is usually best.