[MrFixr55]

The above ARF link makes for very good reading. A few interesting takeaways:

One contributor added 3 diodes in series across the filament output, anodes to positive, cathodes to negative. This is a great idea as it acts as a "clamp" to protect the rather valuable 1.4V tubes from burnout in case of regulator shorting or other failure that causes an overvoltage. While the octal 1.4V tubes are not as rare as UV199s or the dreaded WD11, the original Sylvania design "G" taller skinnier tubes (small shell octal, T9 envelope, about 3 7/8" length) are more scarce than the newer style with the medium or wafer octal base, which will not clear the shield mounting in some sets.

A 4 tube 1.4V / 90V farm or portable radio draws 250 mA. A standard alkaline D cell has a capacity of about 12,000mA Hours to discharge. Therefore one D cell can run the filaments of the radio for about 40 - 50 hours. This simplifies design so that you need only a "B" supply.

Building the B Supply should be very easy. The circuit from Philip Nelson's "Phil's Old Radios" in the following link from David:
https://antiqueradio.org/bsupply.htm
would cost less than $15.00 for the handful of parts and perfboard to build. Any transformer that you can scrounge that has 2 separate primary windings for 120/240V can be used as an isolation transformer for safety. Even if you have 2 decent size transformers that are 120 V primary and secondary voltages that match, you can connect the secondaries of the 2 transformers together and Voila! you have an isolation transformer

Another contributor had recommended using a "nixie tube power supply" for the 90V. There are many of these kits of Chinese origins from eBay for very little money. However, I have tried this myself, with extra filtering both with electrolytics and mylar caps (as electrolytics don't filter high frequency very well. The experiment was an epic failure. Much noise. I was hoping that it would work because I wanted to make the radio portable by powering from 8 AA cells for 12V to run the nixie supply.

I remember someone here at the Phorum discussing powering tube radios from the "portable" 120V AC supplies, such as the one from Jackery. He stated that his Jackery supply ran the AA5 set cleanly where other cheaper brands do not. I have one of the cheaper portable supplies with a "modified sine wave" output (the Jackery supposedly has true sine wave output), and interestingly enough, when powering a Philco Transitone AA5 with this, there was much noise but when using the same supply on a transformer powered radio with a full wave rectifier, the result was clean sound (and this was on AM, with a wire antenna instead of a loopstick. Therefore, I think that full wave rectification of the output of a switcher may be part of the solution. However, in almost any switcher that I have looked at, the output of the transformer is rectified using half wave rectification.

Vibrator power supplies as used in auto radios are old school examples of "switching" power supplies. However, I believe that almost all auto radios of the era that used vibrator power supplies used a full wave rectifier or were "synchronous (Secondary winding switched in synchronization with the primary winding, yielding DC).

I have used a recycled computer power supply for 5V for filaments very successfully. I have used a 3V Wall Wart supply with resistance in series for Farm sets using 2V tubes, also successfully. This is interesting that switchers don't cause noise when heating filaments (or solid state electronics for that manner; they are widely used in modern audio).

Sorry, this turned out to be long, but these are my "brain droppings" (Sorry Mr. Carlin).