Yes, you can unhook on of each pair of the wires on the PT-3 you'd like to check (other than the STP that comes from the power switch and IEC inlet), then measure your AC voltages carefully.
If the AC voltages measure OK, then reconnect the 6.3V winding and measure POS-IN and verify that the tubes on that channel glow.
If you pass those two tests, then you have an issue with the HV supply. Let me know if you get there, and we can continue breaking the chain to test each part on its own.
-PB