6.3v Regulator?

[james_s]

The filaments in x-ray tubes are plain tungsten, directly heated and operated at high temperature compared to other vacuum tubes. They glow white, pretty close to that of an incandescent lamp. Filament voltage is used to control beam current.

[cowasaki]

I build valve amplifiers. 6v will probably work just as well and increase the life of the tubes. Is your reason that you want to add a dc power supply to an amp to reduce noise?  What type of amp? Is it for prototyping?

[peter-h]

7905, with 2 x 1N4148 (in series, anodes pointing towards the 7805) in the GND wire

That will give you about 6V. If you need a bit less, use 2 x 1N4001.

[magic]

This is not even remotely thermally stable, possibly worse than the resistor trick, though I'm not sure.

But it has occured to me that one could use LM385-1.2 instead, for a fairly decent 6.235V output.

[BlueApple]

I build valve amplifiers. 6v will probably work just as well and increase the life of the tubes. Is your reason that you want to add a dc power supply to an amp to reduce noise?  What type of amp? Is it for prototyping?

I tried running it on 6v, it works just fine. I'm planning to make an amp at some point. I'm just experimenting right now.

[cowasaki]

In addition to running it on 6v you can actually use a 5v 7805 and a 1.3v Zener diode plus a resistor to get exactly 6.3V.  What you can do is feed in your DC voltage, say 9v, into a 5v 7805 then take the ground pin to ground via a 1.3v Zener and a resistor from the input to the ground/Zener node.  That would give you a regulated 6.3v BUT as you have found 6v is fine and your filaments will last longer :-)

[Vovk_Z]

7905, with 2 x 1N4148

When we use the same power transformer and its 6.3 VAC windings we typically don't have much headroom DC voltage after rectifying.
So I would recommend Shottky diodes and LDO regulators, kind of 1083, 1084.  But not fixed or adjustable regulator ICs (7805/317). 

[cowasaki]

I was assuming he didn’t have a 6.3v secondary. If he has one I wouldn’t go DC at all. Correct layout and attention to detail and it’s not really necessary.

[oz2cpu]

Tim said : I have looked through my personal library of vacuum-tube era technical books, and have not been able to find any quantitative discussion of the lifetime of heaters (in indirectly-heated cathodes) powered by DC or AC.

me too, I might add almost : ALL real sensitive front stages run at DC and 50-60 years later, those tubes work just as good as any AC driven heaters,
it is really need to have ONLY dc into the first stages of low frequency range equipment, look at RIIA and such as a very good example.

[cowasaki]

Tim said : I have looked through my personal library of vacuum-tube era technical books, and have not been able to find any quantitative discussion of the lifetime of heaters (in indirectly-heated cathodes) powered by DC or AC.

me too, I might add almost : ALL real sensitive front stages run at DC and 50-60 years later, those tubes work just as good as any AC driven heaters,
it is really need to have ONLY dc into the first stages of low frequency range equipment, look at RIIA and such as a very good example.

Same here.  I specialise in building mildly to heavily modified clone guitar amplifiers so super hifi is not really a prerequisite. 

I have heard reports that some valves using DC filament heaters stop working properly when you try to go back to AC after a lot of use but it isn't really something I make use of.

I am planning a stereo hifi amp and a cello amplifier soon so might give it a go as an experiment.  6.3V AC should give 6.3X1.41 volts via a full wave rectifier then the trick with a 7805s and Zeners is an option.

[bdunham7]

This is not even remotely thermally stable, possibly worse than the resistor trick, though I'm not sure.

It's certainly more reliable (not dependent on the ground pin current) and how 'thermally stable' does it need to be for a 6V heater?  It might vary from 6.1 to 6.4 volts at most, probably on the low side if it is in a warm spot with 1N4001s.

[bdunham7]

me too, I might add almost : ALL real sensitive front stages run at DC and 50-60 years later, those tubes work just as good as any AC driven heaters,
it is really need to have ONLY dc into the first stages of low frequency range equipment, look at RIIA and such as a very good example.

Those devices are designed knowing they are using DC heater drive.  I think the issues (allegedly) arise when you have a long string of filaments driven directly from the mains where you have to worry about the bias between the indirect heater filament and the other components of the tube.  If this is large, apparently having this bias being AC negates some of the migration of filament material or whatever happens. The concern would be what happens when you convert an AC mains heater string to DC.  But I have no idea whether or not that is true and am not aware of any data in support of this theory--it's just something I heard or read somewhere.  I actually converted my Zenith tabletop AM/FM radio to isolated DC operation trying to eliminate hum and it now has a DC heater string.  So far it work (but still hums) just fine.

[TimFox]

Tim said : I have looked through my personal library of vacuum-tube era technical books, and have not been able to find any quantitative discussion of the lifetime of heaters (in indirectly-heated cathodes) powered by DC or AC.

me too, I might add almost : ALL real sensitive front stages run at DC and 50-60 years later, those tubes work just as good as any AC driven heaters,
it is really need to have ONLY dc into the first stages of low frequency range equipment, look at RIIA and such as a very good example.

Same here.  I specialise in building mildly to heavily modified clone guitar amplifiers so super hifi is not really a prerequisite. 

I have heard reports that some valves using DC filament heaters stop working properly when you try to go back to AC after a lot of use but it isn't really something I make use of.

I am planning a stereo hifi amp and a cello amplifier soon so might give it a go as an experiment.  6.3V AC should give 6.3X1.41 volts via a full wave rectifier then the trick with a 7805s and Zeners is an option.

For a guitar amplifier, the hum on the first stage might be important.  I am still looking for a real quantitative and documented discussion about the dangers of DC on indirectly-heated low power tubes.  Is that urban legend about not going back to AC about 12AX7-level tubes, or 6L6-level tubes?
For DC voltage regulation, you must go past the 6.3x1.41 calculation.  The voltage drop across even Schottky rectifiers (two in series for a bridge) must be included, and with reasonable filter caps the minimum value of the ripple waveform may well cause regulator dropout.  Again, this is quantitative.  I can’t find any Zeners between 1.0 and 1.8 V in Mouser.  An LM317 or low-dropout adjustable regulator is preferred here.