Here is my take on some things to look at first.
First off, you can perform all these first checks with the power off. As the B+ heads down from the rectifier tube it gets dropped down at R220, then R221 and then by R216, R211, R209 and R202.
On your measurements you are showing us values at R222 which comes after the final signal cap. I would need to know if R219 is showing 155 volts. This would indicate the function of the R220 and 221 step downs. With the power up then R 219 should be 155v and R 216 should be approximately 327 volts.
As your first step I would recommend testing the values of R220, 221, 202, 211, 209 and 202.
Second, I am not sure why you have a dead channel unless one of the tubes is bad, or one of the resistors in the direct circuit pathway is blown. Jim uses a series of 220v resistors between each component juncture in the direct circuit pathway so if something does go wrong the issue stops at that resistor.
I want to thank Jim for taking extraordinary care and thinking of safety first. Safe circuit design is evident in every aspect of his board.
The resistors in the direct signal pathway are R201, R204, R208, R212, R214 and R219. You should check these and see if one has sacrificed itself for the team so to speak.
If you find a dead resistor make sure to let us know its place on the board.
Speculating a bit, let's imagine your build was perfect and a new tube failed. If the voltage rose, it might take out a direct signal resistor.
Do you have fresh or known tubes you can swap in if the resistors all check out on the B+ line? You might be the silly victim of just a random tube failure. Don't get over anxious that you might have something wrong with your workmanship. Remember it worked perfectly for a time before the failure occured. A resistor or two swapped out and a fresh tube might see you playing music for decades without further incident.
In summary, with the power off check all your B+ resistors.
Then check the direct signal path resistors. If these all check out, power it up and check the voltage values at every test point. Then power down, swap out some different tubes and fire it up again. Check your values.
Finally, the H+ is high IMO and could contribute over time to early tube failure. With a tube expecting 6 volts on the heater I feel pretty comfortable between 5.75 and 6.20 volts. I have friends running their tubes at 6.3 but 6.38 is seriously pushing the envelope. Just add some resistance to R223. If you are at 1.8 now, try a 2.0 ohm resistor. 6v would be a lot nicer on your tubes than 6.38v from my seat.
Interestingly enough as the rectifier burns in over time the voltages might change ever so slightly . Mine came down a bit and I ended up soldering the original R223 value back in place.
I hope this helps.
