My SRR-13A had its 6X4s replaced by top hat diodes sometime around 1963 (according to part date codes), resulting in the B+ being higher than desired. This annoyed me somewhat, so I decided to add a resistor and chain of four 30V zener diodes, to keep the B+ at the rated 120V as well as provide regulation. In the process of doing this I discovered that in addition to replacing the rectifiers they had also replaced the LO ballast tube with the circuit shown below. The 6.8 volt zeners (1N5342B) clip the 14 VAC (RMS) to a 15 volt P-P trapezoidal waveform, which measures at 6.3 volts RMS with my meter. The inductor and bypass at the tube were original, the additional bypass at the zeners was added. The way it works out, the tube heater draws 150mA, 98mA is flowing in the zeners, so the resistor sees 1.9 watts and the diodes 340 milliwatts. If the tube heater burns out the zeners still see only 1.56 watts total, or 780 milliwatts each.
In concept, this circuit should work for any ballast tube, the 3TF7 used
in R-390A's comes to mind. There are other ways this is handled, from
jumpering the tube socket and using 12BA6's to building a whole solid state
module using a 3 leg regulator. I think you could build the circuit
shown here in an old relay can, for instance. For the R-390 the circuit
is the same, except that you would use 1N5351B zeners (14 volt). The
resistor value is the same, the resistor sees 4.2 watts and the zeners 845