As PJ suggested, it sounds like your meter is set to AC voltage (incorrect) rather than DC voltage (correct). If your meter is set to AC voltage, I expect you'll get good AC voltage readings from power input through to the secondary (output) side of the power transformer, but negligible readings after that - except for maybe at the tube heater pins if they're AC-heated.
Double check your meter is set to DC voltage.
cheers,
Derek