Tubes amps contain dangerous voltages and present a high risk of electrocution if you are not careful. Even when unplugged, the large filter capacitors in the power supply can store enough electricity to shock or even electrocute.
The .50 Caliber has a pair of bleed resistors across the filter caps to gradually discharge the caps.
This does not happen instantly. It can take several minutes for the bleed resistors to fully discharge the caps. Use your multi-meter on its highest DC voltage setting to measure the voltage stored in the caps and do not proceed with any repair until the meter reads zero.
This .50 Cal has a persistent 50Hz (line) hum even when in standby. It has already been looked at by a couple of local amp techs and the main power supply caps have been replaced.
The main PC board is showing extensive areas of heat damage causing blackening (carbonisation) of the fibreglass layers in the PCB. Once this occurs, the carbon can conduct signal between traces on the PCB. A highly likely source of the line frequency hum is leakage from the heater circuits into the signal path.
This is basically a PITA to fix, but I used a Dremel to remove all of the blackened parts of the PCB. With these gone, the amp is now whisper quiet.