6.3v Regulator?

[BlueApple]

I need a 6.3v supply (at least 0.6amp) to run vacuum tube filaments on. I can't seem to find a regulator that outputs 6.3v though. Does anybody know of one?

[wraper]

Any that is adjustable will do. Just select right voltage with 2 resistors.

[richard.cs]

They're also not that tightly toleranced, and 6.0V would almost certainly be fine. 6.3 V is simply the nominal on-load voltage of a 3 cell lead acid, which once was a common way of powering the heaters.

Do you mean heaters or filaments? Filaments implies directly heated cathodes, heaters implies indirect. Directly heated cathodes can sometimes be problematic with DC heating as the voltage difference along the length affects biasing.

[BlueApple]

I see, thanks! The tube I will be using references it as a heater in the datasheet, and says it will run off AC or DC.

[mag_therm]

Tube shortwave Rx  here has regulated plate and heater supplies for the RF section ( 2 of 6BE6, 1 of 6BA6 and 1 of 6AV6)
The plate is by a pwm booster.

Heater Regulator:
Heaters are regulated by a 7805, TO220 , standing on resistors to give 6.12 V
R_upper = 270 Ohm, R_lower = 56 Ohm
The (4) tubes + pilot lamp draw 1.04 Amp DC.
The receiver has run like that, DDS locked almost continuously for 2 years.

To reduce temperature of my (too marginal, 100 C + ) heatsink, there is 1.5 Ohm 10 Watt resistor in series with 7805.
The residential supply here varies from about 126 V AC down to 118 V with fluctuations by 4 houses on the pole pig.
The input to the 7805 is from a shared 12 V unreg supply, and after dropping resistors is 8.6 V at 123 V AC. and verging into drop-out on low utility excursions.

Hope these data help you do a better job.
If doing it again, I would use a switching regulator.

[Vovk_Z]

to run vacuum tube filaments on. I can't seem to find a regulator that outputs 6.3v though.

Vacuum tubes come along with 50 Hz transformers, so there are (or there were) a lot of unified transformers with 6.3 VAC secondaries. And we don't usually use tubes with switching power supply (because of common mode noise etc). That's why there are not SMPS with 6.3 VAC or 6.3 VDC output (the market is very small).

[Benta]

It's advantageous to use AC for vacuum tube filaments. The relevant term is "electromigration".
Somewhere in a container there must be tons of old 6.3 V transformers lying around (think: electronics flea market or surplus stores).

[TimFox]

Note that the relatively small "filament transformers" with 6.3 V secondary are rated to deliver 6.3 V at a specified current.
At currents below that, the voltage can be substantially higher, which can shorten the life of a 6.3 V heater in a vacuum tube.
Also, a lot of vintage equipment was designed for US operation at 110 VAC, but the modern specification is 120 VAC (having evolved through 115 V and 117 V), so the heater voltage may be 10% higher than intended.
This is a complicated question.  One useful reference is R B Tomer "Getting the Most Out of Vacuum Tubes", H W Sams 1960, reprinted by Audio Amateur Press 2000.
That reference does not discuss DC heating in detail, but does mention the connection of input tubes' heaters to the output tubes' cathodes in audio amplifiers to reduce hum with DC heating, since the 12AX7-type tubes are run at plate current far below saturated cathode emission, so a low cathode temperature is not harmful.
In my recent builds with 7586 Nuvistors, I wired three heaters in series (nominal 18.9 V at 0.135 A) with an additional 39 ohm resistor, to work with a 7824 voltage regulator.  This has the advantage of greatly reducing the surge current on turn-on with cold heaters.  (I used such a network with each of the three-triode amplifier stages.)

[MikeK]

Doesn't the 6.3V have to be AC?  I have an old power supply kit that has two 6.3VAC outputs, which I assumed were for vacuum tubes.  No regulator, it comes straight from a tap on transformer secondary.

If DC, why not an LM317?

[TimFox]

Here is a datasheet for a typical 6.3 V "receiving tube" with indirectly-heated cathode ("heater"), the 6SN7GTA:  https://frank.pocnet.net/sheets/049/6/6SN7GTA.pdf
Note that the specification for the heater voltage is "6.3  AC or DC volts".
Most vacuum-tube era equipment used 6.3 VAC from a suitable winding on the power transformer, but AC was not mandatory.
Some tubes with directly-heated cathodes ("filaments") required AC, such as the 5U4GB rectifier  https://frank.pocnet.net/sheets/049/5/5U4GB.pdf

[james_s]

You can use an adjustable regulator as already noted, or you can use a 5V regulator with a resistor in series with the ground pin. I remember using that trick years ago when I didn't have an adjustable regulator.

[Benta]

Most vacuum-tube era equipment used 6.3 VAC from a suitable winding on the power transformer, but AC was not mandatory.

No, but it increased life time of the tubes.

[BlueApple]

You can use an adjustable regulator as already noted, or you can use a 5V regulator with a resistor in series with the ground pin. I remember using that trick years ago when I didn't have an adjustable regulator.

Between the output and ground?

[mag_therm]

Need 2 resistor and 2 capacitor.
Use 317 or 7805 .
Search "adjustable 3 terminal regulator tutorial".

[magic]

You can use an adjustable regulator as already noted, or you can use a 5V regulator with a resistor in series with the ground pin. I remember using that trick years ago when I didn't have an adjustable regulator.

Between the output and ground?

No, between the chip's GND pin and actual ground.
The chip has ~5mA quiescent current flowing to ground.
Add 270Ω and this lifts its GND pin 1.35V above ground.
The output follows this fake ground and becomes 6.35V.

It's a kludge and the exact value of "5mA" may vary from chip to chip, but it may be close enough.

[Benta]

It's a kludge and the exact value of "5mA" may vary from chip to chip, but it may be close enough.

Yes, the 7805 is a total kludge. The correct device to use is the LM317. I'm fully with @magic here.

[floobydust]

DC-powered filaments are much quieter (for AC hum) if the tube's cathodes are unbypassed, or it's a single-ended type of amplifier. So PP output tubes I don't see any benefit there to DC power on the filaments. DC is pretty much mandatory for tube phono/electret mic preamps.

I use the transformer's 6.3VAC winding, Schottky bridge rectifier and filter cap to an LM317 to generate 6.3VDC filament power.
I find ordinary silicon bridge rectifier has higher losses giving less Vin to the LM317, you do lose around 1.5V in it.

[james_s]

It's advantageous to use AC for vacuum tube filaments. The relevant term is "electromigration".
Somewhere in a container there must be tons of old 6.3 V transformers lying around (think: electronics flea market or surplus stores).

Transformers with a 6.3V secondary are still widely available new. they're common for making 5V with a linear regulator. Iron transformers aren't as widespread as they once were but they are still widely available.

[TimFox]

Most vacuum-tube era equipment used 6.3 VAC from a suitable winding on the power transformer, but AC was not mandatory.

No, but it increased life time of the tubes.

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.
There is a lot of information about biasing the heater power positive or negative with respect to the cathodes, heater-cathode insulation voltage rating, use of DC for hum-critical circuits, proper use of AC to reduce hum, and effects of high or low voltage operation.
For directly-heated cathodes ("filaments"), there is an important question for DC excitation with the variation of cathode voltage across the emitting area.
Personally, I suspect that "soft-start" and similar operation to minimize thermal shock during turn-on may be more important to heater lifetime.
Electromigration is a real effect, and the literature shows it can be extremely important in solid-state electronics with extremely high current density (A/m²), analogous to the Wisconsin glaciation shaping the landscape in my part of the US.
Do you have any quantitative reference about this lifetime question?

[mag_therm]

Since I converted the RF tubes to 6.12 V DC heaters in the old Lafayette HE30 in March 2021, it has been running almost continuously on wspr and ft8.
That is about 22000 hours.

About that time I replaced the original 2* 6BE6 and 2* 6BA6  with NOS ruggedized equivalents 5750 and 5749W respectively.
The receiver open gain only changed by 2 dB or so at that change, indicating that the original tubes were still good.

Surplus Sales state they have 1 million NOS tubes for sale.
Don't worry about tube life. We'll be long gone and there will still be tubes.

[james_s]

Surplus Sales state they have 1 million NOS tubes for sale.
Don't worry about tube life. We'll be long gone and there will still be tubes.

That depends on the specific tubes in question. Some tubes are still common as dirt and relatively worthless, it seems they must have made enormous quantities of tubes for the last generation of vacuum tube based (not just the CRT) TV sets. A lot of the earlier tubes and anything audio related are considerably more scarce.

[floobydust]

The only time I have seen DC causing ion-migration andf short lifetime is with incandescent light bulb filaments, called "DC notching". But that's pretty hot tungsten.
In vacuum tubes, I imagine directly heated cathodes thoriated-tungsten filaments would suffer, but have never seen it.

[TimFox]

Indirectly-heated cathodes (with appropriate coatings) usually operate at about 1050 K, according to the RCA RC-24 Receiving Tube Manual.
RCA claimed their "dark heater" construction allowed operation of the heater at lower temperature than earlier units that used Al₂O₃ insulation between heater and cathode sleeve.
Directly-heated cathodes with thoriated tungsten operate at about 1970 K, but alkaline-earth coated filaments run much lower, about 1000 K (same source).
Traditional domestic incandescent bulbs operate at about 2800 K to get a reasonable white spectrum.

[floobydust]

Only mention of DC-powered filaments having less life I could find with X-ray tubes and I'm not sure about their construction.
"For filaments heated by direct current, a phenomena of notching occurs especially for thin filaments. In this case some tungsten ions form from the evaporated tungsten atoms and are attracted toward the negative end of the filament and deposit themselves forming a series of “notches”. These notches are thinner than other sections of the filament and lead to hotspots with accompanying greater evaporation and ultimately burn-out. Filament life reduction of two to ten times is reported by operating with DC rather than AC... "

I think it's a non-issue for tube preamps, I've never seen a failure and LM317 offers inrush limiting. Some filament lead-ins flare up due to cold inrush.
Wurlitzer 1015 juke box which had a 503 amp, for rapid start they hit the 6L6's with >9VAC until the cathode current flows...  the relay coil is the cathode resistor lol and it pulls in to switch over to 6.3VAC they did eat up 6L6's it was quite hard on them, tubes lead-in wires melting due to very high inrush. It's all AC though.

[TimFox]

X-ray tube directly-heated cathodes are operated in "saturated emission" (rather than "space-charge limited") mode, where the beam current is controlled by varying the filament current (and cathode temperature), and may run at a higher temperature than the indirectly-heated cathodes in "receiving tubes".