OK. Something to address that's popping up as of recent:
"Stability of heater power supplies"There seems to be some misconception that there needs to be some "elaborate" schemes for powering the filaments of tube projects.
IN THE BEGINING, there was darkness. A deep, cold darkness. We lit the way with candles or oil. Then a great inventor gave birth to the electric light bulb. Shortly following, that same man invented the vacuum tube, first the diode, then the triode. These were early filamentary cathode types. Course, we know this man was Thomas Edison. Course, he had to have a power source, right? Yes, and it was DC (direct current). Then, one day, another famous inventor introduced us to a different form of voltage: Alternating Current (AC). As time went on, AC became the standard power source, much to Edison's dismay. You see, Edison, using a DC generator built by the man who would give us AC, felt DC was the best way to power the world. He was wrong. But I digress. However, there is a point here. While DC, in the form of batteries, was the predominant power source early in tube tech. With the success and growth of the AC power grid, battery powered "sets" slowly became replaced with AC powered sets. Now, granted, early radio and TV did not put a whole lot of design of super clean power supplies, they were clean enough for the average user. The one place they focused very little design effort was in the filament supply. Why? IT WAS NOT NECESSARY. The same holds true today. Filaments are nothing more than resistive loads, nothing more, unless you're dealing with 5U4GB or 300B or the few other popular DHTs. In those few types (DHTs), the filament is also the cathode. Which opens up a whole other can of worms. However, DHT filament supplies need not be extravagant, though there are some design considerations that go with having to combine filament power with cathode bias. But since most of the tube set being hobbied together now are non-filamentary cathode types, that is they have separate filament and cathode elements, it is only necessary to provide either AC or DC at the appropriate voltage and current capability for the filament. 6 - 6.5v for the "6volt" types and 12.3 to 12.8 for the "12volt" types. Too much shortens filament life, to low sets up a condition called "starvation" - the tube isn't quite warm enough for efficient operation. So, folks, when designing your filament supplies, just focus on getting the proper voltage. Don't worry about "stability". Don't worry about "cleanliness". Is the voltage correct and are you providing enough current for the total draw. How do I know my total draw? Simple. Using the tube data sheets for your particular tube (type
and manufacture), find the filament current. Then take those values and add them all up, that's your total draw. Multiply that sum by 1.5 and that should get you a good source current you need to provide. For example, if your total draw is 1 amp, you need to use a power transformer that can muster 1.5amps. That 1.5 multiplier is the absolute MINIMUM value. You DO have to consider inrush current. Cold filaments pull slightly more current than when they're warm. It doesn't hurt to go bigger. Now, I'm not going to get in to the AC vs DC argument here. I have my preference and my own reason for that. Just remember the KISS principle (no, not the killer rock group):
Keep
It
Simple,
Stupid. The most KISS way is a dropping resistor in series with the rest of the circuit. It'll even absorb some of the inrush, thereby easing the initial load on the power transformer.
Oh. And that "other" inventor I referenced? None other than Nicola Tesla. The
REAL father of radio, and just happens to be the father of the modern AC electric power grid. Not to mention a myriad of other inventions we use every single day.

Cheers!