WOW 38 years is a lot of time, surely you will need use a VARIAC if you have one, and maybe more. It is good start the Variac with 10V, and after afew hours rise slowly to 20V and so on.
I do not have a Variac and use various autoformers.
Capacitors in use last 10 to 20 years, so with 38 candles on this cake you may need extra care with this amp, Iam not informed about these procedures and I will let it to a amp expert, as there is various in this good site.
Good Luck, Gustavo