Sir Mike. . . . . .

That sounds abnormal to me, as that would be indicative of upwards of 6.5 @1 amp being pulled thru that lamp in creating that bright of a condition.

Any chance that it is immediate . . . from a cold start . . . or only after the 10-15 seconds after the 83 and 5Y3 tubes have reached their filament warmed up temps ?

I would suspicion a normal condition of that fuse lamp to be only a very slight incandescing . . . and THAT even being in variance to the current demands of testing a tube . . . leaning more towards heavy current demands of a power tubes testing .

Just examining the schematic, any chance that the C2 capacitor is exhibiting ANY leakage at all ?

Probably not, since it is of a mere mid range capacitance, along with what seems an adequate voltage rating . . . but confirm anyway with one lead out of circuit.

If the 81 lamp was only coming on after the two power rectifier tubes warming up, next power down the unit and then unplug one tube only, each time, in making two exploratory trials to see which tube is associated with providing the power flow through that # 81 lamp circuitry.

Reference schematics in hand . . . . Standing by for feedback . . .

73's de Edd