On the one hand, this is excellent news because it means the conditions I am seeing are normal. On the other hand, I am ticked at Ameritron for not putting this information in the manual. Seems kind of strange they would intentionally get the turns ratio wrong at 125V, but the least they could do is "fess up", particularly when it appears the unit will not make full power as advertised at a typical 120V input line on the 125V tap as shipped.
I was going to get a Variac and take some measurements at reduced voltage, but then I realized that I can't make any sense of the schematic they provide. They show one stage of the HVDC filter cap circuit completely shorted out. I can't believe it's actually wired this way. Has anyone ever done a mark-up of this schematic to make it intelligible?
I really don't think you need to do any troubleshooting or testing on the PS. The 125V tap and your 120V line voltage along with perhaps a slightly out of spec multiplier resistor are probably the reason for your lower than expected indicated plate voltage. I did take a look at the schematic that is in the on-line manual from Ameritron. That manual was dated 1995 and I would think that they would have fixed that error by now as they do show C124 shorted out.