And....... if you do use the two diodes/octal socket like Primo suggests, you can make your tests with a lot less than 3/4 wall voltage because you will get instant high voltage from volt 1. You won't be needing enough voltage for the 5AR4 to conduct, cuz you're not using it. Checking negative bias voltage, functionality of bias pot adjustments, all can be done at lower voltage. As can plate voltage to preamp tubes. I didn't realize you had a Variac or I would have suggested this.
I made my SSR out of a broken octal tube, soldering the two diodes right to the pin leads in what had been the tube. This came back to bite me, literally, as I was doing THE SAME tests you were doing on the ST70. I brushed my hand on one of the diode ends on the top side of the amp and zappp I got my high voltage. What I have done since then, since this amp is right on the bench with the soldering station etc, is I solder the diodes temporarily from pins 4 and 6 to pin 8 of the rectifier socket. That way its there with all the other high voltage stuff that I am used to staying clear of. I remove them when I am ready to pop in the rectifier tube.