So to confirm the basic operation of the power supply, disconnect the B+ from the preamp boards.
Now check the power supply board junction out voltage, your B+ should be ~315vdc and steady on both power supplies.
If that checks out, next will be to remove both tubes.
connect ONE B+ to preamp board and see if the voltage stays steady at ~315vdc.
If yes, connect up the other preamp board and check voltage.
If all good (probably), plug only one tube in and check for B+ ~240vdc, should be steady – after a wee bit.
unplug that tube and move it over and check the other channel.
By now you should see the logical approach to isolating the problem(s).
Let us know how this portion of the troubleshooting went.
Cheers,
Jim
