My Fender Bassman was showing zero current at idle on one of the two 6L6GC tubes. I knew it was isolated to the socket because swapping the tubes showed the same zero current on the same socket. To make a long story short, the screen resistor (470 ohm) was fried. This is a 1 watt 470 ohm resistor that goes across pins 4 and 6 on the output tubes. The real culprit was a bad tube that would draw too much current.
Replaced the screen resistor and tube and all is good.

The symptoms of the bad tube/burnt screen resistor was low output volume and extremely low/none current across the 1 ohm cathode resistor.