The hum  
 
balance control is as named. It is a 100 ohm  
 
1/2 watt pot with the center lug grounded,  
 
in that respect if the control is centered  
 
it should act as if it were the pair of  
 
fixed resistors found in post Blackface amps,  
 
prior to that, the pwr. xfmrs. had center  
 
tapped filament supplies.

To solve this  
 
problem replace the pot, as it probably was  
 
damaged by a plate to htr. short.

Hope  
 
that answers your question.