[Hunts smoothing bomb]

Is the O/P measuring to chassis from pins 4 &5 in turn I wonder?
This may account for no voltage indication if the heater supply is isolated from chassis?

Measure the voltage by setting the meter to ACV and placing one probe in pin 4 socket and the other probe in the pin 5 socket.

You should have heater supply around 6 to 6.5vac

Cheers