[gary rabbitt]

The 7B7 is a Loktal tube. They have been know to have oxidized pins and sometimes don't make good contact.

Find the dimple on the tube base, then rock it in that direction toward you. It should come right out. Clean off the pins and reinstall.
As Terry says, the tube filaments are in series, so If the other tubes are lit, we can assume your filaments are ok. However one of the pins that carries the B+ voltage might not be making contact. For good measure, I'd clean all the pins.
Yes, you are going to have to have all the tubes installed for voltage measurement.

You can see on the diagram what and where to measure the voltages. Start just after the rectifier tube, measuring the B- and the positive lead of each 20uf cap. As far as I can see, the B- or negative side of the caps is a floating ground, not the chassis.