If no tubes are heating up, yet some show OK on the tester, then you seem to have lost the power to the heater chain. It looks like there is no power transformer in this radio, and the chassis is live. Be VERY careful when poking around inside, testing voltages!
The heaters are in series, so if one heater burns out, nothing works. You have identified that the audio output tube, 50A5, has a dead heater. That will be why the radio stays cold. If the other heaters are all OK, then when you replace the 50A5 you will have the radio lit up again.
It’s good that you will soon have a multimeter, because troubleshooting any electronics is virtually impossible without one!