Flipped Transformer Powers Budget-Friendly Vacuum Tube Amp

If you’ve ever wondered why something like a radio or a TV could command a hefty fraction of a family’s yearly income back in the day, a likely culprit is the collection of power transformers needed to run all those hungry, hungry tubes. Now fast-forward a half-century or more, and affordable, good-quality power transformers are still a problem, and often where modern retro projects go to die. Luckily, [Terry] at D-Lab Electronics has a few suggestions on budget-friendly transformers, and even shows off a nice three-tube audio amp using them.

The reason transformers were and still are expensive has a lot to do with materials. To build a transformer with enough oomph to run everything takes a lot of iron and copper, the latter of which is notoriously expensive these days. There’s also the problem of market demand; with most modern electronics favoring switched-mode power supplies, there’s just not a huge market for these big lunkers anymore, making for a supply and demand equation that’s not in the hobbyist’s favor.

Rather than shelling out $70 or more for something like a Hammond 269EX, [Terry]’s suggestion is to modify an isolation transformer, specifically the Triad N-68X. The transformer has a primary designed for either 120 or 230 volts, and a secondary that delivers 115 volts. Turn that around, though, and you can get 230 volts out from the typical North American mains supply — good enough for the plate supply on the little amp shown. That leaves the problem of powering the heaters for the tubes, which is usually the job of a second 6- or 12-volt winding on a power transformer. Luckily, the surplus market has a lot of little 6.3-volt transformers available on the cheap, so that shouldn’t be a problem.

We have to say that the amp [Terry] put these transformers to work in sounds pretty amazing — not a hint of hum. Good work, we say, but we hope he has a plan in case the vacuum tube shortage gets any worse.

Thanks to [Stephen Walters] for the tip.

31 thoughts on “Flipped Transformer Powers Budget-Friendly Vacuum Tube Amp

  1. Good description. :-) Another option I’ve used is to pick a transformer with dual 120/120 vac primaries, and use one as the primary, and the other as a HV secondary. Its defined secondary (6 vac, 12 vac or whatever) can power the filaments.

    120 vac rectified and filtered gives you about 160 vdc. If that’s not enough, you can connect it to a doubler to get around 320 vdc.

    A point on “rare” tubes: Though 6v was the most common filament voltage, many tubes were built with other voltages. The ones with “oddball” filament voltages are often cheaper and easier to find. I’m building a 1-tube radio right now with a 23Z9 Compactron (dual triode + pentode).

    1. Most transformers with “dual primaries” are actually made with a single winding that is center tapped, so in your use case, it would be just the same as rectifying wall power, with no isolation whatsoever, and also a bit less efficient.

      1. Only the smallest, cheapest, lowest-quality and low power transformers use center-tapped primaries. Most are actually two windings, brought out to four terminals. Wired in parallel for 120V, series for 240V. It’s a much more efficient use of copper and winding aperture size than leaving one winding fallow during 120V operation. It’s even easier to wind.

    1. You must wire them in phase! Otherwise, it’s like paralleling two batteries with the polarities reversed (*bang*)!

      They must also have *identical* turns ratios. Otherwise they can overheat from circulating currents. The best way is to use two identical transformers. Or, power both primaries, and measure the current flowing between the two secondaries with no other load (it should be ~zero).

      1. Some tricks I’ve used before:

        – Give each transformer it’s own rectifier, and parallel them on the DC side. No risk of circulating currents.
        – In 230V land with dual primary transformers: Wire both transformers for 115V and connect the primaries of both transformers in series giving you 230V again. Then connect the secondaries in parallel. Automatic balancing :)

        1. What if you delayed the phase of one transformer, similar to how a single phase AC motor has a run capacitor to create pseudo two-phase AC, so the rectified peaks wouldn’t happen at the same time?

          Would it make for less ripple on the DC output?

          1. If you’d like to use an extra capacitor and magnetic part to get smoother DC, there’s a couple of ways that come to mind before trying to phase shift the wave. The first one is to just use series inductance and parallel capacitance the naive way, paying little attention to the particulars and allowing the inductor to saturate if you draw enough current. Which basically means, you may find you don’t really want to bother with a choke that’s big enough to actually tolerate all that, but it’s been done anyway.

            However the cool way is to specifically filter out the 120hz component of the ripple by using the right capacitance to tune a series inductor. “Resonant choke power supply” is a decent keyword phrase. To correct myself, you’re probably tuning using the inductance since you’re buying a fixed-value capacitor but you can adjust the number of turns a bit on the inductor. I believe it does actually work out so that a big motor’s run capacitor is in a range where a suitable inductor could be created for smoothing rectified single phase mains. It’ll need less of one than you’d need if you didn’t do this.
            The output of this one would look something like this waveform in shape, though you can of course smooth it more than this.

            I have never gotten around to it, but I thought about making a CC/CV power supply with almost all magnetics, except for the bridge rectifier and any capacitors. https://hackaday.com/2023/10/09/learning-about-ferroresonant-transformers-while-fixing-a-1970s-power-supply/#comment-6689960

          2. I’m not sure if my comment will get thru the filter, but in case it does not there’s other ways to use a spare magnetic core (or especially a transformer) and a capacitor to better smooth rectified AC to DC. I’m not sure you’d have a great time trying to get a good phase shift going under all circumstances, while being better than the usual methods. I tried in the other comment to link something looking at the “Resonant Choke Power Supply”.

            P.S. I say especially a transformer, because you can resonate it by attaching the capacitor to a secondary you’ve wound for the purpose, without touching the primary. At least, it works in the simulations; it may be a compromised way to do things in the real world but should still be beneficial. You do still need a beefy core even though doing it this way reduces how much you need compared to simpler solutions.

      1. Toroids are hard to wind. Conventional transformers with E-I laminations are much easier to rewind for other voltages. The hardest part is taking the core apart. If you find one with the right primary voltage and a split bobbin, it’s fairly easy to rewind the secondary for just about any voltage you want.

        1. I mean, separate is convenient, admittedly, especially if you have bobbins. You can also use other shapes like the pot’s that are a bit rounder and better shielded rather than purely EI, but either way. I haven’t actually confirmed that the cheap toroids have high enough permeability in the audio range that you don’t need very many turns. The ones I got before were IIRC yellow, a powdered iron with a fairly low permeability. They worked when I wanted to make a crude magnetic amplifier for 1.5MHz or so but aren’t the same at all as what I’m thinking of. There’s uncommon ferrites and fancy alloys that go way higher in permeability although their other characteristics can suffer. Of course, laminated steel is common and will let you run more power before saturating than other core materials, but still. I just threw toroids out as the simple high-performing shape that has no airgap, even if you’re not going to wind a lot of turns by hand.

          I agree if you can just pop a new winding on something with an existing mains primary and some leftover space, that’ll be pretty easy for your power. But even then a lot of EI transformers have been not only glued, but glued with alternating side for the E and the I on each lamination so that it’s too hard to reasonably disassemble.

    1. If you’re not aiming for hi-fi sound, you can use a power transformer as an audio output transformer. Audio transformer windings are usually specified by impedance (in ohms). Take the square root of the ohms to get the turns ratio.

      Suppose you need an audio output transformer with a 3K ohm primary and 8 ohm secondary. Sqrt(3000)=54.7 and Sqrt(8)=2.8, so the turns ratio is 54.7/2.8 = 19.5. So use a transformer with a 120v primary and 6v secondary (turns ratio 120/6 = 20). That’s close enough. :-)

      1. I was just going to post something similar about using a low-voltage power transformer as an output transformer. It’s been done and it works. Just make sure it’s appropriately sized to avoid saturation, overheating, and distortion.

        1. Like I said originally, it’s been done too but 20 Hz to 20kHz is a long way from the design frequency of a power transformer and so there’s room to improve.

          Here’s a page that when I glance over it, it looks reasonably good.

          FWIW I kind of agree that winding your own would be a real pain, it’s just that I sort of figure that’s what someone who wants budget and old-style hifi would consider worthwhile.

          1. Specifically the 8th section briefly mentions both using regular mains transformers and a toroidal one. It makes me think that if someone doesn’t want to use a proper audio transformer, they may well like to consider buying a couple cheap chinese variacs since you could adjust them on the fly and it would be interesting to hear the result.

  2. I use to use two transformers. Tie the secondary into the second transformer secondary and you’ll get around 120V isolated. Also you can use the secondaries that were tapped into as filament windings. If a higher voltage is needed then a voltage doubler or charge pump can be used.

  3. Another possibility in a pinch is two 120V-to-lower-voltage transformers back to back. For example, if you have two identical 12V centre-tapped transformers, wire the outside leads of one to the outside and inside of the other. Instant voltage doubling. You lose a bit of efficiency, but suitable transformers can be had cheaply from some of the old non-switching wall-warts that used a centre-tap and two diodes to achieve full-wave rectification.

  4. Another option would be to kit-bash an old microwave oven.
    The transformers have separate windings for 120V input and the 2KV output. Take a cold chisel, hammer it through the windings and remove them that way. Easy to wind your own secondary then.

    They are good for continuous operation at a 300-500 Watts.

    Do not attempt to play with the high voltage unless you know what you are doing. There have been several fatalities in the news. It will hurt bad and you will hurt all the time that you are dying.

    1. Microwave transformers have a magnetic shunt between the primary and secondary. It reduces the coupling between primary and secondary, which has the effect of creating a current-limiting series inductance. It limits the output current, and worsens the voltage regulation. If you don’t remove the shunt, performance will be really poor as a conventional transformer.

      1. Luckily, the shunt is literally just a couple chunks of core material that people sometimes don’t notice falling out when removing the winding(s). They look like shims although they have a more important purpose.

    1. That’s kind of a thing with audio transformers to avoid coupling of hum and audio feedback, but should be no problem with shielded transformers. Also, spacing audio transformers away from power transformers is good practice, as is mounting power
      transformers below chassis, especially beneficial with steel chassis. There is some good info in the ARRL Handbooks on construction practices, especially in the older editions for those building tube equipment.

  5. Here’s another technique that I picked up from the Rick Tone Trem-o-Drive. This has the advantage of supplying the B+ AND the heaters. Take two 120V-12V transformers and connect the secondaries together. This gives you 120V isolated AND 12V for the heaters. You can do a 120V-6V if you’re using 6V heaters. If you want 240V, you can connect the secondaries of 120-12V and a 120V-6V as the second transformer. This will give you 12V at the interface between the two and 240V on the primaries of the 120V-6V transformer.

    I think the original Rick Tone site has disappeared, but the schematics are still on the internet.

    1. Another good option is something like what the peeps from the Brazilian forum Handmades did with their Camine HB-1, HB-2, and other such projects, where they use a 30v+30v transformer to get 60vAC, and run that through a voltage doubler to get 145v DC. It also powers the filaments from the 30vAC center tap (where they’re wired in series).

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.