OK. This is in response to PJ's instructions for addressing channel imbalance.
Something is not right here.
With the 60 Hz tone, and the volume adjusted so that the top center lug on the volume pot measures 100mV (bottom center lug is more like 60 mV) I get the following readings:
A1: 10.9 VAC
A2: Blinking between 250 and 340 VAC
B1: 1.75 VAC
B2: 22.7 VAC
Speaker terminals on A side, red wire: 5.6 VAC
Speaker terminals on B side, red wire: .88 VAC
So the B side readings seem somewhat close to what PJ indicated they should be, while the A side readings are 10 times what they should be.
I will power down and recheck all connections.
Thanks
In this post I will just address the channel imbalance.
The measurements at A1 and B1 (the grid of the power section of the tube) are 2.6dB different, indicating that the gain of the driver section is different. A difference is not unusual, but yours are at the normal limits of plus and minus. It's possible that is the entire problem, but let's make sure. As a first test, swap the tubes from side to side to see if the level difference follows the tubes. If the level difference switches channels and retains the same magnitude of difference, then a replacement set of tubes is the answer.
The above test assumes the power section gains are both equally matched. However, the AC voltage measurements were made at such a high input voltage level that the power sections are strongly overdriven, so the measurements at A2 and B2 (the power section plates) don't mean much. To get better information (hey, I'm an engineer, I LIKE more and better information!), provide the 60Hz tone but turn the volume control down until the middle terminal of the controls is at about 100mV (0.10 volt) AC. You should see around 2.0 volts AC at A1/B1 then, and around 30 volts AC at A2/B2, and around 1.0 volts at the speaker terminals. This will be a complete test of the gain structure of the amp in its linear range - specifically, the driver stage gain, the output stage gain, and the transformer ratio.