5 48
11 25.4
You very likely do not have the cathode bypass capacitor in the circuit on terminal 11 side. This the 220uF cap on the front of the board that feeds to the "K" pad and connects to terminal 20. Be 100% sure that this wire is well soldered on both ends and not broken. Also check the solder joint on the 220uF capacitor itself.
Could the left output transformer going bad be the cause of the volume imbalance? If so what part # can I replace it with?
This isn't very likely. With the amp off and the high voltage drained off, you can measure the DC resistance between terminals 5 and 10 on each output transformer.
What I have seen happen occasionally is that the center lug of one of the hum pots will get smooshed down into the chassis, which sets a very high bias current for the 2A3. If this happens for long enough, the plate choke will get damaged, and gain and power will decrease. To test for this, you will need to measure from HV+ to pin 2 on the 4 pin socket on each side of the amp. (So DC resistance between HV+ and pin 2 on the left side of the power supply board/amp, then DC resistance between HV+ and pin 2 on the other side of the board/amp) A damaged plate choke will have a significantly reduced DC resistance compared to a healthy plate choke.
If the plate choke impedances are off by ten or twenty Ohms, that's nothing to worry about. If one is a hundred Ohms less than the other, then you'll want to look at those hum pots and at the orientation of your 220uF/250V caps on the board to be sure everything is in order, then replace the choke.
-PB