Low-Voltage Tube Amp is Great for Beginners

If you ever wanted to build your own tube amplifier but you were intimidated by working with high voltages, [Marcel]’s low-voltage tube amp design might spark your interest. The design operates with a B+ (plate) voltage of only 40v, making it less intimidating and dangerous than many other amps that operate over 300V. It’s also incredibly easy to build—the whole design uses only 11 components.

The amplifier is designed around the ECL82 tube, which includes both a triode and a pentode in one package. The ECL82 is practically an amplifier in a tube: it was designed for low-cost electronics like record players that needed to be as simple as possible. The triode in the ECL82 is used as a pre-amplifier for the incoming signal. The pentode is controlled with the pre-amplified signal and acts as a power amplifier.

[Marcel]’s amplifier also uses a PY88 tube rectifier instead of semiconductor diodes, making it an entirely silicon-free design. Although [Marcel] hasn’t posted up detailed build instructions yet, his simple schematic should be all you need to get started. If you want some more background information about tube amps but you don’t know where to start, check out our post on basic tube amp design from earlier this year.

38 thoughts on “Low-Voltage Tube Amp is Great for Beginners

    1. Why? For the same reason you blink an LED with a microcontroller or print “Hello World” on any desktop computer!

      Also, 1 watt tube amps make *perfect* guitar practice amps. 1 watt driven into that yummy tube distortion is just about exactly the volume limit for practicing in an apartment or a house you are sharing with others.

  1. One small issue with the schematic. No spec given for the output transformer. Yes, I could run the calculations, and figure out what would work, but I bet I am the exception in your audience.

    Given that he is running on low voltage, and single ended, transformer choice is a bit far from typical.

      1. I’ve got a 100V line transformer with these characteristics:

        Power: 10 / 7,5 / 5 / 2,5 / 1,25 W
        Impedance (primary): 1000 / 1330 / 2000 / 4000 / 8000 ohms
        Output (secondary): 4 / 8 ohms

        Could it really work in a single ended design like this?? I thought this kind of transformers would go into saturation so easily with the current going through the primary that would make them useless. But if your experience shows the contrary maybe I’ll give it a try. In that case, which would be the best impedance to choose on the primary?

    1. I used an output transformer from an old tube radio, but i dont have its specifications. I measured 600Ohms and 1.2Ohms on its windings but I dont know how much turns it is.

      Of course it would be good to use an appropiate audio transformer with the calculated values, but tube amplifier audio transformers are quite expensive, at least for this little circuit.
      If you want to try it out, you can use for example a normal 230V / 12V transformer connecting the primary winding(230V) to the tube output and the secondary winding(12V) to the speaker. Dont expect no hifi-quality, but thats not the point, it just should work :)

  2. The grid-leak biasing scheme is not what I found recommended by tube gurus. Positive half-waves of grid signals could easily become non-linearly distorted due to grid-cathode circuit breaking into conduction. It would be nice to use a resistor in the cathode appropriately sized so its voltage drop would exceed the peak pentode driving voltage. This way grid always remains negative with respect to cathode. The power output might suffer a bit.

  3. As noted in the article, running the PY88 heater at 6 volts means that the rectifier will give a very poor output. While the PY88 may be plentiful (in some parts of the world) and is a perfectly good rectifier normally, substituting a rectifier that uses a 6 volt (or 6.3 volt) heater would be better. As noted above,some idea regarding the output transformer would help. There are other tweaks that could be done too to improve the design, but the simplicity of this one would encourage newcomers to experiment.

    1. You are comletely right. I made a small mistake in my description, the PY88 filaments gets the 30V it needs from the anode voltage (trough a resistor) so it heats like it should. Thank you for correcting this!

  4. During the built, I found out that the amplifier is much louder if I touch the output transformer (the iron shell) with my hand or if I connect it to the big power transformer’s iron.
    I wonder If someone knows why this happens and could expllain this??
    Thanks in advance.

    1. Maybe just something grounding properly that wasnt before? The shell or the power transformer is grounded i’m sure. Not positive if the output transformer shell is supposed to be grounded or not

  5. Why indeed
    I suppose I could build a headphone amp with a pair of 572Bs but why???

    This project is pointless and shows that just because you can, that often isn’t reason enough to do something.

    1. Because you can is the best reason to do anything, if you want to. And for that matter, 95% of what we do is ‘pointless’ if you look at the functional purpose of it. Better performance and practicality are fine goals to have, but they’re not EVERYONE’S goals.

    2. Hey man reasoning like that are pushing this conversation into a philosophical area. Speaking of wich the entire mankind is useless because learning is simply not a valid reason (according to youe reasoning). What’s the point of living in complete predictability and never try something just for getting a feel of it? I tell you now: For humanistic reasons and pedagogic reasons. What? Yes to learn about humanity and to experience first hand some material issue that has neen known to man before but might not have a direct practical purpose today. Why are people replicating stone tools today? To learn about humans in the past.

  6. For years I’ve built amps with tubes, discrete transistors and chips of several types. Tube amps are by far the least efficient, but I don’t care. They’re simple circuits that get the job done with the fewest total components (count the transistors inside a power opamp chip some time). And a tube amp can look cool as well. They’re the lawn mowers of the electronics world: impossible to perfect, but also very difficult to get wrong.

    1. “Very difficult to get wrong” often depends on the tube. Like semiconductor amplifiers, some tubes are rather more willing to turn themselves into oscillators than stable amplifiers. For this reason tube amplifier stages sometimes employ series resistors in the grid circuits to help prevent spurious oscillations, especially for power tubes. Quite a few tube builders learn this fact after the first project didn’t work as planned…

      1. High transconductance (over ~7500 uS) tubes do like to feed back and oscillate. This is usually manageable with careful lead dress and stopper resistors. I’ve been using some Russian mil spec video pentodes rated at around 20,000uS (6J52P)and they behave fine even with long dangly breadboard leads as long as the grids are stoppered at the socket and the cathode, signal grid and anode wires are kept apart.
        Low transconductance tubes like the ubiquitous 12AU7, 6SN7 and so on are pretty immune to misuse.

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