A quick look at the schematic shows all of the typical things to check. Look for a shorted filter cap, bad tube socket, bad output transformer, etc.
I usually try and trace the high voltage path with my ohm meter. Ground the negative lead and take resistance readings starting at the secondary of the standby switch. Pull the center tap lead of the output transformer and see if the short goes away. If not, try lifting one end of R55, which will remove all of the driver/preamp high voltage circuits. This will isolate the short to the power amp or the rest of the amp. Keep looking until you find the cause of the short.