I took the following information from my 500C receiver (serial# 74907X):
Assuming that a single CL-80 type current limiting device has been installed in one primary lead of the power transformer, then 120.0 vac as received at the unit (i.e., before the CL-80, and therefore as measured at the unused auxiliary AC power receptacles) will produce 6.30 vac at signal strength meter's illumination lamp socket terminals after 1 minute of operation from a cold start (allowing time for the CL-80 device to stabilize). This lamp socket represents the "end of the line" for the audio related 6.3 vac heater circuit, and is therefore the best place to measure the voltage of this circuit. With 6.3 vac appearing at this point, it ensures that the other devices connected to this circuit will be receiving at least 6.3 vac, and/or minimally error slightly to the high side of it, which is the way any error should lean from optimum, with all voltages on the circuit then being well within a -0/+5% ideal range.
When the power transformer heats from extended operation, all voltages will drop slightly due to the properties of copper used in the windings of the transformer. As a result, I operate my 500C directly from my AC line (typically 122-123 vac), with no other voltage adjusting devices used other than the drop produced by the CL-80 current limiter. This typically results in no more than 6.45 vac at the meter lamp's terminals after a stabilized cold start, with it hovering just above 6.30 vac when the power transformer reaches its stabilized operating temperature, and 122 vac is applied as measured at the auxiliary outlets.
I hope this helps!
Dave