This may be an offbeat question, but I have been wondering about this a while. It seems that where I live the AC voltage is around 125v, not 117. Working on some old phono and audio amp circuits from the 40's and 50's lacking much in the way of pre-amp power stages, I find that sometimes it's good to have a little pre-amp, so I can try it with a microphone or something really low-power.
I'm thinking in terms of a single tube like a 6AV6, for example, which has a high amplification factor, and I can run a plate voltage onto it of, for example, 250 volts using a voltage doubler diode circuit coming from an line-to-line isolation transformer with appropriate filtering. This would all be done on a separate hand-drilled little 4x6 chassis with a jack on one end and the other, and just the 1 tube.
Now for the filament, can I put the 6.3v 0.3 amp filament in series with perhaps a 4 or 7 watt night light bulb (perhaps with an extra low-value resistor if need be) and run the filament from the AC this way, being sure eveything is properly grounded? This way, it would avoid a separate filament transformer.
I read somewhere (perhaps here) that the filament voltage is a bit liberal, perhaps even if off 15-20% (like putting perhaps 5.0 - 6.0 volts on a 6.3 volt filament) wouldn't affect the thoughput of the HV DC that much?
An interesting question... thanks in advance

I would still use a transformer with a winding for filament and plate voltage. Some high end equipment uses 5 volts DC on the filaments of a 6 volt tube with no apparent loss in gain and reduced hum. The tube also lasts longer. I prefere the isolation that a transformer gives and I have never been a fan of voltage doubler circuits. These are just my preferences.

MRO

I heartily second the motion of disavowing the utilization of Acky-Dack operation / design. Since common power transformers are getting to high
priced for my blood.
I now universally use this procedure:

Consider the circuit requirements of the B+...hi voltage circuitry
Also the filamental voltage and current specs and its power requirements.

Taking that info that you supplied, of using a 6AV6...6.3 V filament...300 ma...HIGH Mu triode...100 gain..Woooo Wooo !

Just about the simplest x-formers that Radius Shackamus stocks should full fill that power requirement...soooo..
get two units and wire one units primary up thru a line cord,fuse and switch and that units secondary (6.3VAC) feeds into the like
secondary (6.3VAC) of an identical transformer.
Now you have your 6.3 V AC for your tubes filament supply available at the co-joined 6 VAC junctures, whilst, as if by magic, you have
your 117, 125,...(BTW..I have 126VAC here too !) etc AC output available at the output of the second transformer. Take that AC output
and rectify and filter and under a very low loading ,such as that sole tubes requirement will exhibit, you could find upwards of ~150VDC
available to you for its B+ supply.
Also you could, of dubious / minumal extra value, upgrade to a voltage doubler circuit, but with the utilization of a series
filter/dropping resistor in the dual electrolytic capacitive filter output loop to keep that then much higher voltage; down below that
6AV6 max plate voltage spec.

That's it ! Saved an arm and a leg transformer cost....A complete AC line isolation for your filament supply and a DOUBLE AC line
isolation for your high voltage supply. Now you can feel safer when interconnecting into other auxilliary equipment.
Of course there are variants of the utilization whereupon 12.6 V centartapped inits could be tied together and a mix of 12.6 or
6.3 VAC tubes be supplied their center / interstage derived filament power source.

The same is true of using 24VAC center tapped transformers ....In one case I was feeding a radios 12VAC filaments off the 12VAC tap and
using the raw 24 VAC into a FWB and filtering it and using it to feed a Transistor Williamson design to replace the receivers puny old
35/50L6 tube output stage that typically...I should say universally...uses a 4-5" speaker that is bottoming out and accruing splattering
distortion at just about the 1 Watt RMS level.
The second transformer was voltage doubled to eventually drop down and feed ~ +250 VDC plate supplies to the 12BA6-12BA7-12BA6-12BA6-12AV6 tube
circuitry. The unit being re organized / redesigned by its using a "Full house" Rf front end Pre Amp, hi gain lo noise mixer,dual cascode
IF stages and 12AV6 Audio detector with audio cathode follower output, feeding into the quality Williamson SS amp, that feeds into a
modern "deep throw /excursion" main speaker.

73's de Edd

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Yes, thanks, I've done one circuit already that way, the cheapo filamnet xfmrs definitely do the trick, back to back. Thanks.