As I understand vacuum tubes (like triode / pentode) usually are designed to be heated with certain current through the filament.

And usually it is about low-voltage and comparatively high-current. E.g. 6.3 V at 300mA.

Schematics I usually find around use transformer to get this heating voltage. I wonder, whether I can instead connect heater filament directly to 220V AC via suitable capacitor to limit current. E.g. I think 4.7 uF should provide about the said 330mA. And it can feed several lamps in chain.

What could be the drawbacks? I can suppose that temperature will be pulsating slightly (like incandescent bulbs do about 5% light pulsation) and this can affect the anode current... But vacuum tubes have slower temperature time-constant...

UPD: some clarification - this scheme of course should only be considered for isolated filament (how do we call it... indirect cathode heating?).

Also "simplified" calculation I=Uac*(w*C) which I mean is correct only while total voltage on filament (or chain of filaments) is small in comparison to full Uac.

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    \$\begingroup\$ Think about it for a moment. You are using the capacitor as a voltage divider, with the heater filament as the second part of the divider. If you add more tubes in series, you change the divider ratio. Also, heater filaments aren't simple resistors. Like light bulb filaments, their resistance is low when cold and higher when hot. \$\endgroup\$ – JRE Aug 27 '19 at 10:48
  • \$\begingroup\$ Interesting approach. Be careful with placing un-isolated 220 volts between filament and cathode. \$\endgroup\$ – analogsystemsrf Aug 27 '19 at 10:50
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    \$\begingroup\$ The RC highpass filter disagrees. \$\endgroup\$ – JRE Aug 27 '19 at 11:05
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    \$\begingroup\$ And, yes voltage divider. \$\endgroup\$ – JRE Aug 27 '19 at 11:07
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    \$\begingroup\$ Long ago, there was a 5-tube radio whose filaments were wired in series, driven by 120V AC: 50V + 35V + 12V + 12V +12V (all indirectly heated cathodes). I recall that one or more flared alarmingly at turn-on until all thermally stabilized. Have also seen a series resistor to take up the slack...never a capacitor. \$\endgroup\$ – glen_geek Aug 27 '19 at 13:31

AC voltage on the heater will drive the filament to a positive voltage every half cycle. Any voltage difference with respect to the cathode will draw electron current away from the cathode, thus modulating the voltage gain of the tube at AC frequency. Likely highly undesirable.

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    \$\begingroup\$ Many tube circuits use AC for the heater filament. They are designed for that. \$\endgroup\$ – JRE Aug 27 '19 at 11:04
  • \$\begingroup\$ Ah, yes. I clearly missed this. 300V swing to both positive and negative - comparable or more than anode voltage.Thanks a lot! \$\endgroup\$ – Rodion Gorkovenko Aug 27 '19 at 11:04
  • \$\begingroup\$ @JRE they are designed for roughly 6V AC, not 220AC... Interesting to check how AA5 was designed in this respect, I think they have higher-voltage filaments. \$\endgroup\$ – Rodion Gorkovenko Aug 27 '19 at 11:05
  • \$\begingroup\$ Filaments are usually driven from a separate winding on the transformer so they're isolated from the anode and cathode and won't cause a current to flow. \$\endgroup\$ – Finbarr Aug 27 '19 at 12:21
  • \$\begingroup\$ simply tie one end of the filament to GND, and other end to the voltage-dropping capacitor. The cap handles most of the 220 volts. \$\endgroup\$ – analogsystemsrf Aug 27 '19 at 13:51

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