A Stereo Tube Amp For Less Than $5

Many of us have aspirations of owning a tube amp. Regardless of the debate on whether or not tube audio is nicer to listen to, or even if you can hear the difference at all, they’re gorgeous to look at. However, the price of buying one to find out if it floats your boat is often too high to justify a purchase.

A motor transformer

[The Post Apocalyptic Inventor] has built a stereo tube amplifier in the style of the Fallout video games. The idea came when he realised that the TK 125 tape recorder manufactured by Grundig was still using tube audio in the late 60s. What’s more, they frequently sell on eBay for 1-10€ in Germany. [TPAI] was able to salvage the main power amplifier from one of these models, and restore it so that it could be re-purposed and see use once more.

The teardown of the original cassette recorder yields some interesting parts. Firstly, an integrated motor transformer — an induction motor whose stator acts as the magnetic core of the transformer responsible for the tube electronics. There’s also an integrated capacitor which contains three separate electrolytics. The video after the break is well worth a watch (we always find [TPAI]’s videos entertaining).

A new chassis is created out of a steel base plate and aluminium angle, and some neat frames for the motor transformers are made from scrap copper wire bent and soldered together. It looks great, though there’s always the option to use a cake tin instead.

If you’re interested in the design of tube amps, we’ve covered heaps of cool builds: from this low-voltage design to this tiny guitar amp, or even ones using tubes which are flat.

18 thoughts on “A Stereo Tube Amp For Less Than $5

  1. ” Firstly, an integrated motor transformer — an induction motor whose stator acts as the magnetic core of the transformer responsible for the tube electronics. ”

    That in itself is interesting. When a hack meant something.

    1. [warning] one of my buttons has been pushed. “When a hack meant something”. Not sure what you mean,. in the event you are suggesting that”motor winding” was the result of hacking, you are devaluing the engineers that design the products that the DIY community modify. Was a time hack was a pejorative term used for poor and often dangerous modifications and workmanship. I have done my share of hacks to be able to get home(figuratively and literately), but it never occurred to me that work was worthy of being put on a pedestal.

      1. Then, Doug, you are a man’s man, as they say. You gott’er done. You got the bacon AND, made it all the way home, no matter what. Some guys can’t. Some gals, can. Good on ya, as they say waaay down south… which I ain’t. A very few cannot even change a flat tire, despite not being well off nor deprived of oxygen somewhere along the way. Yes, you are great, annnnd humble, as so many here. Carry on!

  2. Two and three section electrolytic capacitors were the norm in the 50’s and 60’s, nothing unusual there. They didn’t phase out until the end of the tube era.

  3. I guess it is funny how stuff that was low end junk when one was growing up is now exciting for kiddies. Motor/transformer combos were common is a lot of things that did not require much power back in that era. I guess if you want a low spec low power amp and tubes excite you, have at it.

    1. Also the watts per channel for the old equipment is usually ridiculously beefy, which really explains most of why it sounded better. Nowadays things are engineered on thinner margins, amplifiers are typically driven close to capacity for normal listening. Turns out it helps a lot to have the ceiling far above what you’re actually using.

      Find an old amplifier with a toroid transformer that is basically half of a dumbbell, perhaps around 300 watts per channel, and enjoy.

      1. I have to echo Mr Lawson – the old tube hi-fi amps were often fairly modest in output by today’s standards, but that was ok because the speakers they usually drove were usually very efficient – bass-reflex, horn-loaded, coaxial, etc. And often very big. Altec A7s, anyone?

        I remember a mono Heathkit tube power amp that weighed like 30+ pounds (mostly from the power and output transformers) but could only do maybe 50 w into 8 ohms.

      2. This is wrong in a couple of ways. First off, there was a lot of tube stuff that was low spec and did not sound good. The thing is most of it has been disposed of by now. When you deal with antiques you are tempted to say they built things better back than because the build quality of the specimens you see are better. What you don’t see is all of the junk that was manufactured “back than” that was junk that fell apart. The junk gets binned and the better stuff gets handed down.

        As far as the size. Even if you don’t count modern switching amplifiers, and just count transistors, the physical size and weight difference is very significant. Both required power transformers but the tube amps also required output transformers. Now with switching power supplies and class D amplifiers the power to weight ratios are downright crazy. Back when I did pro audio we biamped and we had amp boxes for the bass cabinets. Each amp box had 3 350 WPC amps in it as well as forced air cooling, and with one guy on each side they were ball busters to lift if you had to lift them. Now you can get 1400W RMS in a 4U rackmount unit and one person in reasonable physical condition can carry two of them.

    1. According to the original spec somewhere around 2-3W..
      But as most of you know a 3W tube amp is not to compare with 3W transistor amp. I was almost freakin out how much sound came out of that 4W tube amp Radio I rapaired a few years ago…

      1. Tom, a watt is a watt.

        You may say that rubes overdrive with less distortion than transistors or the more typical argument, that the distortion characteristics of tube based amplifiers sounds better than the the distortion characteristics of solid state amplifiers, but as far as power at any given amount of distortion, a watt is a watt. Solid state amplifiers typically have better pre clipping distortion figures. Often better by an order of magnitude.

  4. Interesting bit of work. However I feel the final product shouldn’t be used to determine becoming a tubes are best audiophile, or not.Consumer goods for the masses, contracted to a price point, is nothing new. I’m in that age group they may the last to grow up when all all audio equipment used tubes. Some of my older cousins had battery powered portable radios that used tubes.

  5. The first tube stage and maybe the second are for tape head pre gain. Not needed or wanted (noise) for line level inputs in the digital age. Multi-caps no problem, but that motor trick… ugh. Still it shouldn’t be hard to find a transformer to run it, it won’t need oiling.

  6. Indeed.
    Have a very nice old antique GDO here which is cooling its heels because the electrolytic (dual section one) is kaput and I can’t locate a cheap replacement.
    I thought about subbing in a pair of audiophile $$$ non polar capacitors but lack any this large.
    Also its valve is nearly dead alas, majorly gassy.
    (interesting factoid: these are now illegal to send by post due to barium in the getters, go figure!)

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