A look at the data sheet shows a 5 V. filament. IMO, you should play safe and use current regulated DC for heating 71A filaments.
2A3s and #45s have 2.5 V. filaments and AC heating is frequently OK. However, the hum increases, when higher filament voltages are used.
2A3s and #45s have 2.5 V. filaments and AC heating is frequently OK. However, the hum increases, when higher filament voltages are used.
The 45 filaments carry the cathode (music) signal-current, as well as the heating current, so if you share ac-windings across filaments, there will be cross-talk from channel-to-channel, as well as crosstalk of the filament heating current (50/60 Hz and all the mains noise).
A hum pot can balance-out the idle heating currents out of each side of the filament, and into the cathode return circuit, which reduces the 50/60 Hz hum, but does nothing about the hum harmonics (100/120Hz) which the DHT generates. Also the ac-heating current produces huge amounts of intermodulation (50/60 and 100/120 Hz sidebands on every note in the music) - this is not high fidelity!
For a long time, DC heating had a bad reputation, because raw rectified dc causes more problems than it solves - mixing rectifier forward and recovery pulses with the music (shared with a large electrolytic capacitor, which also presents itself in the music signal path).
As Eli mentions, properly-implemented current-control is the way to go. I have DIY kits, if you need a ready-to-go solution ("Coleman Filament Regulator").
A hum pot can balance-out the idle heating currents out of each side of the filament, and into the cathode return circuit, which reduces the 50/60 Hz hum, but does nothing about the hum harmonics (100/120Hz) which the DHT generates. Also the ac-heating current produces huge amounts of intermodulation (50/60 and 100/120 Hz sidebands on every note in the music) - this is not high fidelity!
For a long time, DC heating had a bad reputation, because raw rectified dc causes more problems than it solves - mixing rectifier forward and recovery pulses with the music (shared with a large electrolytic capacitor, which also presents itself in the music signal path).
As Eli mentions, properly-implemented current-control is the way to go. I have DIY kits, if you need a ready-to-go solution ("Coleman Filament Regulator").
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Same here, three stage amp with 46 driver and 300B outputs, all push-pull. All AC heated, less than 1 mV hum at the speaker outputs, and very nice sound. Low IMD, not "huge amounts". I don't believe it is true to say "this is not hi-fidelity" on the basis of heating method. I have tried direct current heating with both better and worse results. Properly implemented AC can be hi-fidelity.
The 2.5 V. tubes are OK SE on AC. Higher filament voltage tubes are not OK SE on AC. When PP topology is employed, hum cancellation (in the O/P "iron") is occurring. An AC heated SE 6B4G hums like a bee hive. OTOH, an AC heated PP pair of 6B4Gs can exhibit satisfactory hum levels.
I suggest you read the following DHT filament regulator | Bartola Valves and Why filament regulators in DHT? | Bartola Valves entries and make your own conclusion if you are looking at implementing DC filaments 
Ale
Ale
Filter well before regulating. Don't use a huge capacitance at the filter's I/P, as that can be a source of "hash". A CRC filter, with the bulk of the capacitance in the 2nd position should get the job done. Using an inductive wirewound resistor is a good idea. You get a tiny bit of smoothing from the part's inductance.
I suspect that Rod uses a 2 stage setup. A 2 stage setup voltage regulates the filtered DC 1st, then it current regulates the feed to the tube.
I suspect that Rod uses a 2 stage setup. A 2 stage setup voltage regulates the filtered DC 1st, then it current regulates the feed to the tube.
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