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Over the years, on this board, there have been a few references to switching power supplies. I’ve mentioned that I use a converted computer power supply to power the heaters of my ST-35 amplifiers. I’ve also been asked how to do this, but it’s a very difficult question to specifically answer. Although the basic principle is all the same, circuits vary widely even between supplies from the same manufacturer.

I believe that I’ve devised a simple heater supply solution that should make modifying computer supplies, well, hopefully a thing of the past.
For anyone interested, here’s the link:

http://www.tronola.com/html/unique_tube_heater_supply.html

I may be reading it wrong but the LED in the limiter circuit schematic appears reversed.
That is if V+ is truely more positive than V-

Core32—You are right! I had it backwards! Funny how I can look at the same thing 10 times and not notice the mistake. It’s corrected now.
Nice catch! Thanks!

Uhh forgive my naivety if these heaters need alot of current this idea would be void but -

Would a little LM7805 set voltage 5V linear regulator (with heat sink) work well for this purpose? If more current is required then larger pass transistors could be used that could be driven by the LM7805.
One could take the auxiliary windings from their main transformer and rectify this to D.C for the reg.
Just seems easier than having a huge switch mode power supply from a computer sitting there just to heat some tubes.

Or am I missing something? Like the fact that maybe these filaments needs a shitload of current? o.O

Some of the switching supplies , are dirt cheap , and can be modified for less expense than a power / filament transformer and necessary parts for a proper DC filament supply .

Some people use what they have and others are just frugal .

Four EL34's need 6 amps at 6 .3 volts , plus another 3/4 of an amp for the other tubes .

Hmmm… I guess I’m no longer getting automatic notifications of posts---and it’s been awhile since I’ve looked at this one.

Yes, an LM7805 regulator will work, if you are using tubes with 5 volt heaters. If not, then an LM7812 or even an LM7824 may be a better choice. These of course, will give you close to 12 and 24 volts respectively. 6 volts is difficult to achieve with a fixed regulator. The LM7812 voltage output, if used as a stand alone regulator will be very close to 12 volts, not 12.6 volts. If 12.6 volts is desired, then a simple 1N4004 diode can be placed between the ADJ terminal and ground. The voltage drop across the diode junction will bias the regulator an additional .5 -.8 volts above ground.

This works pretty well for a tube or a combination of tubes with heaters drawing up to little less than one amp. Above that, a pass device is needed, which complicates the circuit some and adds expense and reduces efficiency, sometimes significantly. An adjustable regulator may be a better choice here, since there is additional voltage drop across the junction of the pass device.

The circuit that I showed is best suited for applications drawing a couple of amps or so. Below that, fixed regulator chips may be quite adequate, assuming that a rectified and filtered DC voltage is available.

Great stuff, thanks for posting. I'm planning an amp that uses a switcher for filaments. I have a 120/240 isolation transformer for B+.

Regarding linear regulators, Fairchild has a LM7806:
https://www.fairchildsemi.com/products/power-management/voltage-regulators/positive-voltage-linear-regulators/LM7806.html

I tried it with a current booster PNP BJT for the ST35 heaters. Couldn't hear any improvement over AC, but maybe that is just my old ears. I redid the ST35's all-over ground to star-ground and that had the best effect.

It looks like the LM7806 is now a common item. When I first started playing with these fixed regulators many years ago, the ’06 was always listed but nobody stocked them. To get one I would have had to order a thousand or more! I haven’t looked at these for a long time.

I neglected to mention; if a diode is placed between the ADJ pin and ground to raise the output voltage slightly, the cathode (stripe) is attached to ground.

I wouldn’t expect to hear any improvement running the EL84 heaters on DC. I think the only potential improvement in sound from using a switcher with the ST-35 could involve the 7247. Since it’s used as a direct coupled VA/PI, the added heater voltage stability should help maintain the relationship between the two tube sections. But even then, I wouldn’t bet money that the difference could be heard.