A Tiny Tube Amp For Not A Lot

At the extreme budget end of tube audio lie single-tube amplifiers usually using very cheap small-signal pentodes. They’ve appeared here before in various guises, and a fitting addition to those previous projects comes from [Kris Slyka]. It’s a classic circuit with a transformer output, and it provides enough amplification to drive a pair of headphones or even a speaker at low levels.

The fairly conventional circuit of the tube amplifier.

Most tube enthusiasts will instantly recognize the anode follower circuit with a transformer in the anode feed through which the output is taken. The tube works in Class A, which means that it’s in its least efficient mode but the one with the least distortion. The transformer itself isn’t an audio part, but a small mains transformer taken from a scrap wall wart. It serves not only for isolation, but also to transform the high impedance output from the tube into a low impedance suitable for driving a headphone or speaker.

The HT voltage is a relatively low 24 V, but it still manages to drive headphones acceptably. Speaker levels require a pre-amp, but even then it’s likely that this circuit is pushing the tube beyond what it’s capable of with a speaker. The more it operates towards the edge of its performance envelope the more distortion it will generate and the worse a sound it will produce. This isn’t such a problem in a guitar application as here, but hi-fi enthusiasts may find it to be too much. It would be interesting to subject it as a headphone amplifier to a series of audio tests to evaluate the effect of a mains transformer over a dedicated audio one.

Last year we took a very in-depth look at the commonly-available Chinese kit pre-amps that use a similar anode-follower circuit but without the transformer. We’ve also seen a similar amp that uses an op-amp as an impedance converter, as well as a novel take on the idea whose unusual biasing allows it to run from only 3.3 volts. These circuits can be so cheap to get started with that we’d suggest anyone give them a try.

49 thoughts on “A Tiny Tube Amp For Not A Lot

    1. You’re transforming high impedance to low, so the mains winding (big volts, little current) connects to the valve (pins 1&2 on the schematic) and the output winding (little volts, big current) connects to the speaker (pins 3&4).

  1. Kris could replace the inductor driver in his tuning fork clock with a microphone and speaker using this.

    If he doesn’t I will! I think you’d need a cap across the transformer primary (to make a tuned amp) selecting 440Hz or there shouts otherwise the feedback will be broadband and useless for the clock even if the tuning fork rung itself to pieces.

    1. Hah, I’ve thought about doing a vaccum tube based version of the fork oscillator. But then I’d also be tempted to do the rest of the clock using tubes, and that’s a very slippery slope :p

  2. That kind of amp could be installed even in a toilet bowl and it would keep running after nuclear explosion. Such a shame that tube-based designs were obsoleted by silicon.

  3. I’ve just been refurbing a 1960s Dansette portable record player (v.popular here in the UK in its day) and was surprised to find a single valve amp to go from stylus levels to speaker levels. Probably a specially wound transformer rather than a mains transformer though, and not exactly “hi fi”.

    1. The cartridge was usually not dynamic (coil) but crystal (piezoelectric), with comparatively very high output. One valve was standard for cheap record players.

    2. Dansette weren’t laggards in adopting transistors elsewhere, had a MW/LW Dansette tabletop radio that had early germanium transistors in it, 3 in the RF section, 6 in the amp IIRC. That had a nice warm tone for listening.

      It’s probably an input impedance matching thing, really high impedance pickup combined with lower amplification qualities of the early germanium.

  4. I know lots of tube amps use end-stage transformers… But isn’t this detrimental to audio quality? I beleive that transformers do attenuate bass and subbass bands, which are relatively important in modern music. Not sure about high end of audio spectrum.

    1. The wallwart transformer is designed for AC frequency 50-60Hz, so there should still be quite a bit of bass. It takes a lot of power driving bass which this amp doesn’t have, so chances are that the transformer won’t be saturating at the lower frequencies either.

    2. Yes, but good transformers can have an impressive range. The flaws can also be benefits if you happen to like the sound. Good transformers are expensive, though.
      In the 1950s, Philips already recognized this flaw and designed a series of radios and amplifiers without output transformers. They used special loudspeakers with an impedance of 800 ohm. They were fairly succesful, but in the end they switched back to using transformers again.

    3. Any tube used in home audio will pass at most a few tens of milliamps. Even the fairly large 6L6 delivers only 66 mA. A single class A 6L6 connected to an 8 ohm speaker couldn’t deliver more than a 4.4 milliwatt sinewave. A transformer is necessary to boost the current from the high-impedance tube output.
      Transformers can be designed to pass low audio frequencies. The more the power and the lower the frequency the bigger the transformer has to be: we’re talking about 5 pounds or more for serious hifi. To get high audio frequencies through the same transformer requires good design, materials, and construction. Take a look at some McIntosh photos to see how massive they can be.

    1. It’s not obsolete when aesthetics are a priority. Electronics can be art.

      On the other hand lets valves are not obsolete if resilience to EMP, static, and high power are needed. I.e. if you put an transmitter in an empty field and lightening strikes nearby you will either need swap out transistors constantly or use a valve…

      Depending on the weather this is a serious concern and can mean you are having to service and swap out transistor arrays multiple times per year vs a single valve for years.

    2. When will people ever stop complaining what a stranger thousands of miles away likes to do with their own money and free time and acknowledge that it’s alright to have different interests than their own?

    3. So, largely supplanted by newer and cheaper technologies = obsolete?

      To my mind, if a product is still being produced (tubes, vinyl, cassettes) to meet a market demand, then it != obsolete.

      Why not just let people use what they enjoy?

  5. A small toroidal power transformer might be a better choice than the EI type used here. They can easily be wound with more wire to change the impedance ratios, and they often work over a wider bandwidth than EI power transformers. Toroidal power transformers have been used in amateur electrostatic loudspeakers for many years to avoid the cost and difficulty of obtaining audio specific transformers.

  6. The problem when considering obsolescence is the concept of disposability which then creates the mentality of everything and everyone becoming disposable and/or obsolete at some point. It somehow devalues everything and everyone once their usefulness runs out and we become strictly users.

  7. Tubes are still used in recording studios and music instrument amplification. They are not used as ubiquitously as they once were, obviously, and for some applications have plainly become obsolete (computers, for example). But they still produce audio in a way that is very pleasing to the ear. Ears are typically how audio is perceived, so that may explain their continued use.

    Re: PA systems– you need to go to a decent theater with a Meyer, EAW, or similar quality system and a good engineer at the board. Just like there’s plenty of crappy car stereos there’s plenty of crappy PA’s, but you can experience some amazing sound in the right circumstances.

    1. I use very high Quality class A oil cooled tube Amps with Lundahl transformers for my grandchildren aged 8 months !Transistors is not my favourite amplification, To much low level distortion!

  8. Threw around some impedance calculations, to see what the plate sees with different secondary loads.
    8Ω sec = 3.2KΩ pri.
    16Ω sec = 6.4KΩ pri.
    24Ω sec = 9.6KΩ pri. We’re starting to get into the 10KΩ region, and above. Many dynamic phones start around this impedance.
    600Ω sec = 240KΩ sec, or 0.24MΩ if you prefer. Some phones really do go this high.

    I’ll let someone else chart this against the tube’s curves. There is an optimum load for this amplifier.

  9. It’d be nice to stick some volume control(s) in the circuit, so you’re not just relying upon the guitar volume knob, or an in-line attenuator on your headphones. Seems like a combo tube, one which has a triode and pentode in the same envelope, might be a more useful choice, though I don’t think any that I know of would operate at 24 V. Still, nice to see people experimenting with single-tube setups!

  10. Good thing tubes still are used… Anybody ever thought about how big and how complicated it would be to use a solid state final power amplifier of a commercial broadcasting station, 20000 Watts or more?…. Wolfman Jack’s radio station XERF in Mexico was 250000 watts of power….that would be a massive set of really big transistors…

    1. Hot switchable LDMOS pallet amplifiers have been the norm for much of the high power broadcasting transmitters.
      Today a modern single LDMOS device at 50v will put out an effortless 1kW with as little as 1W input exciter power.
      No use in running huge iron to provide 7000V plate voltage for hollowstate vacuum tube device with limited lifespan, when ya can run a solidstate LDMOS device.
      For playing guitar? No Doubt TUBES RULE !
      The vacuum tube amplifier is every bit as much the tone and part of the instrument as is the guitar. Just as a 200 plus yr old Stradivarius Violin is the premier instrument of the worlds greatest classical musicians…. The RCA 6L6GC Blackplate or the Telefunken EL34 is component of choice for classic rock musician.
      Tubes allow you to feel the music in a way that no modern digital class D modern modeling amplifier can ever deliver today.

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