This video unlocks the deepest mysteries of amp tube filaments. What are they? How should they be wired together? How are they powered? Kiss your insomnia goodbye as these and other troubling questions are tossed to the four winds.
At 4:02, a hum-balance pot mounted to the rear of the amp. This video didn't discuss common hum reduction techniques for tubes being heated from an AC supply (probably encompassing 95% of the amps that techs and DIY tube amp enthusiasts will encounter), such as grounded centertaps, artificial centertaps using a pair of fusible resistors (or a hum balance pot), twisted-pair wiring, phasing all heaters the same way, and biasing up the AC filament supply above ground with a DC voltage (in a cathode-biased amp, simply steal the necessary voltage from the output tube's cathode) I also have to wonder, what will happen to a switching power supply being used to heat tube filaments if the tube develops an internal short? Appropriate fusing is called for. The artificial center tap using a pair of small, low wattage resistors always struck me as a simple solution to protect the filament transformer winding from shorted tubes.
Some big solid state hifi power amps (Nakamichi PA-7 for instance) similarly use a bank of power resistors in series with the power transformer primary, and a normally-open relay on a timed circuit to shunt around the resistors a couple seconds after turn on. The resistors limit the current surge through the power transformer at turn-on as the big filter caps charge up, and then the relay closes and takes the resistors out of the circuit.
I have recently started using Chinese-made switching power supplies which are specifically designed for tube amps. They produce DC filament power, high-voltage (usually 260V), and also a handy 5V DC source. Outputs are sequenced and ramp up slowly. Pretty much solves all the main concerns with powering tube amps. I haven't used enough of them for long enough yet, though, to swear by the reliability; all I can say is "so far, so good". But anyway, if you're already using a switching supply, rather than (cleverly) kluging around with a failire-prone mechanical relay, why not put some timing circuitry into the control side of the supply, to ramp up the voltages as desired. Or even your klugy relay, might be better applied to reduce the voltage regulation value within the supply, rather than passing all that filament current through the relay contacts. Just an idea. Your work is very interesting and no negative criticism is intended.
The power supplies you describe sound like an interesting option, especially if their MTBF proves to be up to the challenge. SMPSs tend to be small and light enough that installing two with a failsafe relay is another option. Please keep in mind that not all relay circuits are "klugy", and relays properly chosen for the application can have very impressive reliability figures, as borne out by their continued use in many industrial settings. Also note that the two-component relay-based warmup circuit I described is actually quite elegant: if the relay fails in any way (open coil or faulty contacts), the circuit will continue to pass current to the filaments. The components get ungainly in a big amp, but in a 12V string in a smaller amp that circuit is a good solution.
For a 6.3 V filament tube the coreect voltage is .............6.3 VOLT ! Any deviation , higher and also LOWER will impair life time expectancy..... This is ONLY valuable for VOLTGA CONTYROLED filamants ! Current controled heaters ( string connected heaters like PCC etc ) MUST be driven by the correct current , not voltage ! Some loxer the voltage to,oçbtain a smoother sound ............resulating in a cathode operating at a lower ( = uncorrect ) temperature...........The getters don't work at a lower temperature and do no longer do,their job corectly and the electrodes as well ast hecathodes become poisoned .....................cathode barrier may become sooner an issue as well ( degenartion ). The ONLY corcect way to power the heater is to respect as close as possible the correct voltage .
After reading your comment, I again checked for data and research regarding the relationship between tube filament voltage and longevity, but I couldn't find anything authoritative that supported your viewpoint. One paper found that running 6.3V filaments at 6.1V extended their life by 50%. Transmitter maintenance personnel often run new tubes at slightly higher than their rated filament voltage, then back it off to a slightly lower than nominal voltage to maximize their life. Keep in mind that 6.0V is less than 5% lower than 6.3V, and that filaments fed by a power transformer winding typically see much wider voltage deviations due to normal variations in the mains voltage (6 to 10%), historical changes in the mains voltage (5 to 10%), and normal variations in transformer windings. Also keep in mind that filament failure is a relatively minor cause of overall tube failures. Of course, there's no harm at all a running audio amplifier tube filaments at precisely 6.3 or 12.6V (and fully current regulated) if you can find a way to do that, but in practical applications this pursuit would soon give way to diminishing returns.