The following suggestions as to what should be done to the R-390 that you are restoring have been supplied by Roy Morgan. I support all of them.
1) Apply a fan to the side of the case next to the 6082 tubes. That section runs way too hot. Mounting a fan inside the radio is possible and with clever metal work can be done with no new holes. See Item 4 below, and also mount a line bucking transformer the same way.
2) Do NOT unplug the PTO and expect the B+ to remain regulated. The PTO tube and the DC amplifier tube are supplied filament current through the ballast tube. If the ballast tube is kaput or the PTO is unplugged, the B+ regulator will deliver full unregulated voltage to the whole radio.. This is bad.
3) Replace all, repeat ALL the 47 ohm cathode balancing resistors in the audio module (the B+ regulator section) and the power supply (rectifier cathodes). These are originally 47 ohm two watt carbon composition resistors and have almost certainly drifted with age and heat (see item 1 above). Replace them with 5-watt ceramic cased "sand" wirewound power resistors or metal film resistors. When carrying equal currents, each one dissipates about 1.8 watts. Unbalanced currents in the rectifier and regulator tube sections lead to early failure of these tubes, AND over-dissipation in the resistor in question. Both the rectifiers and the 6082 regulators are somewhat uncommon and both difficult to find and expensive.
4) Run your radio on proper line voltage. It was designed for 115 volts. Here in the US at least, common line voltages are 120 to 124 volts. See the below URL for information on a line bucking transformer to solve this problem. Such a transformer can be mounted under your radio and will be a very good thing.
5) Replace the copper oxide relay supply rectifier with modern diodes. It's likely ok to simply solder new diodes onto the old rectifier, since the failure mode of the copper oxide component is increased forward resistance. Be careful that the new diodes will not short against the radio frame when the module is re-installed. There is very little clearance.
6) Take the audio signal from the Diode Load terminals on the rear terminals strip through a moderate capacitor and feed any small HiFi amplifier you have with a good speaker. Details can be seen at:
and specifically, <http://www.R390A.com/html/diode_load.html>
7) While you are at it, read in its entirety the above web page.