[MrFixr55]

Hi Murf,

Do this:
Monitor between ground and one plate of the rectifier while playing the radio.  Do the same with the other plate.  If your meter can measure a high enough voltage, measure across the plates.  CAUTION!!  This is about 750 Volts or so.  If no voltage drop from start until distortion occurs, then the power transformer is good.

While radio is playing, especially when the distortion starts, observe the 6F6 tubes for "red plate" or any abnormal glow between elements. (Hopefully they are not metal).

Power down, unplug and immediately feel the transformers.  Feel the caps, C44 and C43.  Likewise, the speaker field coil.  They may be warm but not hot. 

What voltage rating did you use for the filter caps?  Fr C44 in particular, ensure that you used either 450 or 500 WV (Working Volts)

You can measure the current draw by placing the meter between the 5Y3 cathode and the rest of the circuit.  If the current increases with time as the B+ voltage drops, then suspect a component shorting.  If the current decreases as the voltage drops and the AC output of the HV winding of the power transformer did not change, then suspect the rectifier or the B- C- resistors (R22, 23, 24).  You can also measure the DC voltage drop between the chassis ground and the HV secondary center tap.  If this voltage increases with time when the distortion starts, then these are your culprits.  Ensure that you used sufficient wattage.  At least 5 W, preferably 10 or 20W.

I do not expect that output transformer to be the issue.  They either work or don't.  I don't think that it is shorting as it gets warm.  It is possible, but I don't think probable.  The speaker frame (and therefore the output transformer frame and field coil frame) are grounded.  If a ground develops, there will be a lowering of B+ along with a B current increase.  However, there would be smells, burning, etc. if intermittent grounding of transformer and / or field coil windings is occurring.

Hope this helps.