12K5 Flea-Power Amp


On one of my business trips to USA I acquired a couple of 12K5 space charge power tetrodes as I was buying anything that is remotely interesting to me and cannot be found back home. I like the idea of very small power amp so these looked interesting with their minuscule 35mW output. Some time after that I saw a cute little output transformer in Radio Shack with 1K:8ohm ratio and max power handling of 50mW that could be used with 12K5.

When it comes to space charge tubes, one of the best sites to get the info is http://www.sophtamps.ca. Power amp is based on 12V all-tube amp found there. I didn’t have any 12U7 tubes, so I decided to make it a hybrid amp and do a JFET preamp. For preamp I used AMZ Mini Booster with few components changed.


Since it’s a tiny power amp it’s appropriate to squeeze it into a proportionally tiny box. I got a nice aluminum sheet box in a local store together with anodized blue cover. Preamp and power supply are fitted on separate boards. I used Measure Explorer 1″ square proto boards I got on eBay during one of one those trips to USA. These are the best quality perfboards I’ve ever tried. Tru-hole plating and each hole has two smaller holes next to it that share the same plating which makes connections simpler to do. Really nice for small circuits like this.

You will notice a couple of nice Dale resistors on the power supply board. Those are four 100ohm resistors wired in parallel to give (close enough to) 22ohm cathode resistor which I didn’t have at the moment. Amp is powered using 12VAC@0.83A wallwart which gives slightly over 13VAC under appox. 0.5A load so I added a 1.7ohm 1W resistor in series with the heaters. B+ is around 16-17V when rectified.


Click on a photo to see more details


You must be wondering how loud (or quiet) is 35mW. I can’t say it’ll make your ears bleed, but it’s comfortable level for room and especially for night practice. Using my 12″ Greenback, volume is a tad louder than typical conversation between few people without anyone getting really angry 🙂 . Also, it’s has some bluesy overdrive vibe to it, not very clean when volume is over half, but it never gets harsh. I was a bit concerned about it’s 300Hz-10kHz frequency response but it’s actually fine. It doesn’t seem to lack bass.

Useful links

12V all-tube Sopht amp

10 Responses to “12K5 Flea-Power Amp”
  1. Harleytodd says:

    Would this work for just powering headphones to listen to while I play my guitar? Love your site. Cool builds. Thanks for input and thoughts, I don’t want to run speakers, just plug my headphones in and hear myself play guitar while wife sleeps . Thanks

  2. Harleytodd says:

    I am an electronics novice, but want to build a little “amp” just to power headphones….would this work? Thoughts.? Thanks, your stuff is cool!

  3. Albuquerque says:

    Hey man, when the sound sample’s coming out???

  4. Andy says:

    I am looking to build a low power hybrid amp and this seems to fit the bill perfectly, do you have a circuit diagram for this hybrid amp that I could use to build my own 12K5 Flea Power Amp

  5. Blackcorvo says:

    Are you ever gonna release a sound sample of this puppy? I’d love to hear how it sounds!

    • Bancika says:

      yes, I’m finishing the cab, so I’ll be able to record something. Stay tuned 🙂

  6. B. Tesla says:

    Dear Bane,

    now you’ve gotta know when you build something that cute and pretty you owe us a sound demo, right?
    Amazing how tiny it looks, what is the size (X-Y-Z)?

    Kind regards,
    B. Tesla

Leave A Comment

  • About

    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.