Dumpster Dive Speaker Results In Tube Amplifier

[Michael Wiebusch] found the leftovers of a wrecked vintage tube radio in a pile of electronics junk. Unfortunately, he could not recover any vacuum tubes in it. And to his dismay, it didn’t even have the output transformer, which he figured would have been useful in a guitar amplifier project. The output transformer is not easy to come by nowadays, so he was hoping to at least score that item for his future build. All he could dig out from his dumpster find was a pair of speakers and he ended up building nice Output-Transformer-Less Tube Guitar Amplifier around them.

Valve output stages are generally high-impedance which means they cannot be directly interfaced to low impedance speakers. An impedance matching output transformer is thus used to interface the two. Back in the day when valves were still the mainstay of audio electronics, many cheap amplifier designs would skimp on the output transformer to save cost, and instead use high impedance speakers connected directly to the amplifier output.

[Michael] found a nice reference design of an OTL amplifier for a 620 ohm single speaker. He decided to use the same design but because these speakers were about 300 ohm each, he would have to wire his two speakers in series. At this point, he decided to make his build useful as a proper guitar amplifier by adding a preamplifier stage replicated from another design that he came across. A regular halogen lamp 12V transformer takes care of the heater power supply for all the tubes, and a second, smaller 12V transformer is wired backwards to provide the 300V needed for the plate supply.

The final result is pretty satisfactory, considering that it all started with just a pair of junked speakers. Check out the result in the video after the break.

14 thoughts on “Dumpster Dive Speaker Results In Tube Amplifier

  1. Nice save!
    I know that purists will cringe at the old parts being used for anything other than a total, “better than new” restoration, but I think it’s good to see old parts that would otherwise be landfill being put back into service no matter for what use. It’s also nice to see innovation to overcome the difficulty of sourcing antique components (nice 300v transformer hack!).

  2. Not to pick, but an old radio for a guitar amp? Perhaps for headphones. A small practice amp is generally about 15W and a real amp 2X+ times that. You might get some parts you could use for the preamp stage or to drive an old school reverb tank, but you are not going to make much of a guitar amp out of the guts of a little old radio.

  3. Radios like the one in TFA were often good for 10+ watts. It was not unusual for them to be turned up so the whole house could hear the broadcast. That’s quite enough for a guitar amp in a small venue, such as your own garage or a small church.

  4. The last time I saw a bastard high impedance speaker was back in the first gen of 22volt powered solid state germanium transistor stuff. This was either to get around a ohmage tax or just being dirt cheap.

  5. I did something similar to this when I was in college. I used a silicon diode voltage multiplier for plate voltage and a regular power transformer for the output transformer (just matched DC resistances on the windings). The output sound was interesting to say the least.

  6. Hint: you can use regular mains transformer as output transformer. I use mine (120/12 Vac) almost every day, hooked up to a radio with burned out output stage. TIP29 (oh the horror!) is driving the transformer mains side, and speaker is connected to the former low voltage side.

    It sounds very good, some people listening could not believe it until I opened the case.

    In all honesty some mains transformers sound better than others, but all of them work.

  7. That’s a sweet setup I love old tube stuff. Currently working on my final project for my EE degree, building a EL34 stereo SET amp. I’ve had the most fun with this project

  8. i had an interest in electronics starting at about age 3 years old. (but that is another story!) starting at age 5 (1961) until i was about 14, every year for my birthday my parents gave me a Broken tv for my birthday! sometimes i was able to fix those tv’s but there was not much motivation to fix those tv’s because we only could tune in one or two tv stations in those days and i could watch them on our family tv. what was great about those old tv’s was that there was always a separate mono audio amp chassis, and a stereo amp in the all in one home entertainment centers of the day! so most of those amps i sold to my friends for turntable amps, and i always had the best stereo in the neighborhood! in the early days those big wooden tv cabinets made great soapbox derby stye cars to roll down a big hill in too! (no electronics career for me even though i wanted one, i became involved with electrical power generation.)

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