Tube Amplifier Uses Low Voltage, Sips Battery

Much like vinyl records, tube amplifiers are still prized for their perceived sound qualities, even though both technologies have been largely replaced otherwise. The major drawback to designing around vacuum tubes, if you can find them at all, is often driving them with the large voltages they often require to heat them to the proper temperatures. There are a small handful of old tubes that need an impressively low voltage to work, though, and [J.G.] has put a few of them to work in this battery-powered audio tube amplifier.

The key to the build is the Russian-made 2SH27L battery tubes which are originally designed in Germany for high-frequency applications but can be made to work for audio amplification in a pinch. The power amplifier section also makes use of 2P29L tubes, which have similar characteristics as far as power draw is concerned. Normally, vacuum tubes rely on a resistive heater to eject electrons from a conductive surface, which can involve large amounts of power, but both of these types of tubes are designed to achieve this effect with only 2.2 volts provided to the heaters.

[J.G.] is powering this amplifier with a battery outputting 5V via a USB connection, and driving a fairly standard set of speakers borrowed from a computer. While there aren’t any audio files for us to hear, it certainly looks impressive. And, as it is getting harder and harder to find vacuum tubes nowadays, if you’re determined to build your own amplifier anyway take a look at this one which uses vacuum tubes built from scratch.

23 thoughts on “Tube Amplifier Uses Low Voltage, Sips Battery

  1. The project is empty, there are no files or instructions. No photos, diagrams or videos.
    Is it enough to send an anecdote to be published on HaD? I must be missing something, could someone put a link to the hack?

    1. On par with in general, i’m affraid. Its more of a reddit post with way too much empty space around the tiny bits of info. And again the writeup here uses more words than the original article.

      But to be at least on topic: what the heck are the very bright blue lights above the faintly visible VU meters?

  2. “large voltages they often require to heat them” – This is a plain wrong! Low voltage are required to heat them, usually 6.3 Volts. Large voltage are required for the anodes to draw electronics off the cathode and cause power amplification.

    1. Exactly. Filament voltages are either 6.3 or 12.6 as denoted by the first digits of the tube number in most cases. The high voltage is the “plate voltage”, and for whatever reason I always remember 400 volts for this, but it varies. But I hate tubes and thumb my nose at them at every opportunity.

  3. This is the second Time this week that hackaday really disappoints me. Three obvious errors in the first paragraph, I’m not reading the rest.
    I don’t know what is going on, but please go back to your previous standard of quality.

    1. What’s your point? These are datasheet for an old tube and a link to a completely unrelated project using a 240V power supply.

      Still doesn’t make the article less full of mistakes, nonsenses and nothingness. The only correct content of the article is: “someone somewhere allegedly made an amp with a low voltage tube”.

      And I really doubt they are in any way using the tube to amplify anything at 5V without rising this voltage somewhere. But I admit I could be wrong, not an expert.

  4. > The major drawback to designing around vacuum tubes, if you can find them at all…

    If you can’t find vacuum tubes, you’re not looking very hard. There are literally piles of them at every vintage electronics expo I’ve visited, and most common consumer-grade receiver tubes can be had for less than $5 each, e.g. any of the All-American Five (12BA6, 12BE6, 12AU6, 50C5, & 35W4). Hell, eBay is full of them, as are several vacuum tube seller sites online.

    > …is often driving them with the large voltages they often require to heat them to the proper temperatures.

    Not even close, my dude. As others have pointed out, you’ll usually only need about 6 or 12 volts for each tube heater, at about 300 or 150 mA, respectively. That voltage and current can go MUCH lower OR higher, though.

    But it’s evident that the writer has never used or designed with vacuum tubes before. That in itself isn’t a sin, but to write something so wrong on a site dedicated to technology, hacking, and electronics of all vintages, seems particularly troubling. Especially when the information is basic knowledge among those of us who DO work with tubes.

    One more note on vacuum tube heaters. The heater exists not so much to heat the tube (as a whole), but the cathode, because that’s what causes it to emit electrons. Look up “thermionic emission” if you’re confused. Indirect heating is a thing, but I’m already far into the weeds.

    > There are a small handful of old tubes that need an impressively low voltage to work…

    There are more than a small handful. Most of those with heater voltages around one volt were designed for use in battery-powered applications, like portable radios and hearing aids. But they found their way into all sorts of electronics, like my Victoreen CD V-715 Model 1B survey meter, which has a 5886 subminiature electrometer pentode. That little tube is smaller than my pinky finger, and its heater is rated 1.25 V at 10 mA. That’s easy for the single D-size battery in the meter to power.

    But that’s only one of several dozen in that physical size, and there are more still in most other forms/bases. Most of those for portable applications have heaters rated 2 V or less, and use well under 100 mA.

    The big exception is power rectifiers, and similar tubes like (industrial) thyratrons. Those have filament voltages that are quite low too, but they can draw several amperes of current. These are designed to be used with a mains-connected power transformer that has a low-voltage, high-current winding, specifically for this purpose.

    I think the author of this article meant to say “high anode or plate voltage,” and he would have been correct, for most receiver vacuum tubes. However, that’s not always the case. Using the 5886 again as an example, that tube has a plate voltage limit of just 22.5 volts! The plate voltage depends upon several design decisions, which specific type of vacuum tube you’re using, and what you’re trying to accomplish in a specific application.

    Perhaps the biggest sin is the one already pointed out by others: the linked project has no photos (other than the one of some fugly blue LEDs at the top of this article), design files, notes, nothing anyone can actually use.

    Why was this written and published? It’s a mystery.

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