DIY 12AX7, ECC99 MiniAmp

Here is one of my latest projects, a 12AX7, ECC99 MiniAmp. My idea was to build a tube amp head for home practice and recording just as small as possible, but with the usual amp design. That means I wanted to stay with the usual PT and wanted to avoid a small SMPS like in the Zvex Nano or a Back-To-Back solution, simply because I wasn’t that experienced with tube amps (till then: 2 tube amp-kits built, some tube amps modified) and wanted to avoid an unstable, experimental power supply design in my first build from scratch. So the MiniAmp started as a AX84 FireFly without the cascode boost-stage with one 12AX7 in the preamp and a 12AU7 in the poweramp in a self-split push-pull configuration. After researching the parts (transformers etc.) I decided to use a Hammond-style BB-type enclosure. You will find the unmodified FireFly schematic here on AX84.

And here are some pictures and a video with soundsamples I recorded right after I fired it up:

So from that point I began to modify this circuit a little bit more to my taste. I read in the AX84 forum that an ECC99 instead of the 12AU7 in the output-stage would provide a better bass response and turn this little amp from a toy to a “real” amp. So I put in one of these, adjusted the bias and wow it really helped in the bass department and gave it a little bit more headroom, too. Then I started to modify the preamp to shape the overall tone and overdrive characteristics to my taste. I added a fixed grid resistor on the second triode stage, biased this stage colder and added a cathode bypass cap. I also made the cathode bypass cap on the first stage switchable to get some influence on the tone, this way you can get more bass response in the cleaner lower volume settings and less bass for the louder overdriven sounds so the overall tone doesn’t get muddy. The amp puts out between 0.5 to 1 Watt clean and is just perfect for bedroom practicing and recording. Here you can find my modified schematic.

Some pictures and a video-demo of the modified 12AX7. ECC99 MiniAmp:

Here is a picture from the inside. It’s more like a 3-dimensional wiring, the picture doesn’t really get it:


65 Responses to “DIY 12AX7, ECC99 MiniAmp”

  1. Man, this is such a cool project! I found your site while looking for wiring schematics for a 1-watt tube amp, and now I am completely determined to build one of these.

    I have been building 1/2 watt solid-state guitar amps into cigar boxes, and I wanted to graduate to something bigger for a while. What type of transformers did you use in this project?

    Keep up the cool work, and if you have any advice please send it my way!

  2. mgwhit Says:

    Fantastic amp! Looking at your schematic, though, I can’t tell what C6 is supposed to connect to on V1b. I’m assuming the plate (like the Firefly), but it really looks like the grid (which is a trick I’ve never seen before). would you mind clarifying? Thanks!

    • Thanks for your hint! You are absolutely right! It’s a drawing mistake, of course C6 should be connected to the anode. I corrected the schematic… Again, thank you!

  3. […] The final pedal in action (through the 12AX7, ECC99 MiniAmp): […]

  4. larry stephens Says:

    howzabout some inside pics?

    • you’re right, that would be interesting for you guys 🙂 … I added an inside pic.

      • Hi,

        It seems that you’ve done a very good work !
        It appears to be so small and so efficient too !

        I’m interested in making an amp like yours, and it would be fine for me to have more pics of the inside, such as how is done wiring and so on.
        Could you put some more inside pics ?

        Thanks in advance.

      • Hi,
        I’ll see what I can do, it’s a little bit difficult to make good pictures of such small things with a crappy camera. 😀 But I would recommend you to simply follow the basic layout of my amp and the wiring will come on its own.

  5. Somehow amp building is still Chinese for me. Where can I get the basics? BTW can I get the names of components? Is it possible to build from scratch?

    • I don’t know how much you know about electronics and soldering. If you know nothing I think it would be better if you would start with a simple effects pedal that works only with 9V (like a booster or fuzz pedal) to learn the basics about building electronics stuff and soldering. You can find detailed information about that all over the internet.
      If you’ve already done that and just want to go to the “next step”, amp building, then the easiest way would be an amp kit with detailed instructions to get an idea of how an amp works, amp design and layout. At least that’s the way I did it…
      If you want to learn how a tube amp works, what all those components are for and how the components influence the sound, I can recommend you the website and book of the valve wizard, Merlin Blencowe ( and studying some amp forums on the internet, like the AX84 Forum for example.
      The main step from effects building to amp building is the handling of lethal voltages (even when the amp is turned off!). You have to be aware of these high voltages and handle them with care and caution!
      Now to this amp:
      Yes, you can build this one from scratch, I did it, so everybody can do it. 🙂 BUT if you never built an amp before, it could be difficult for you to get the idea of what components are necessary (suitable transformer, screws, washers etc.) and where to put them on the enclosure or how to get the layout noise free for example. In addition this amp is a very tight fit that could ask for some “advanced” soldering skills if you’re not willing to go with a bigger chassis. But if you think you can handle all this and you got some basic soldering skills, feel free to try this amp as your first amp project. 🙂 You could use my pictures as a rough guide for the layout.
      Oh and I’ll think about making a components list…

  6. Martin Says:

    Hi there, cool little amp, thanks for sharing this. I’m about to build a stereo version using the power amp section only as an afterburner for my Solid Metal preamp (by Damage Control) to power a 4X12 cab. I’d like to discuss some details of the circuit to be narrowed down. Is there a chance you drop me your e-mail address? Thanks a lot in advance!

  7. How can I use preamp from this amp and power amp from version 2 which has little more headroom?

  8. I don’t need so much gain that 2.0 has(I play mostly clean to early blues crunch), but the 2.0 has more Watts than this one, so more headroom. Why is phase inverter needed?

    • You need the PI to get the ECC99s triodes to work as a real push-pull power amp to get more output out of it. These Miniamps both produce really nice clean tones at bedroom levels. If you want the Miniamp 2.0 output, I’d advise you to build the 2.0, it covers the smaller Minamps tone and gives you even more with the TMB-tonestack. Do you have experiences with low wattage tube amplifiers by means of loudness? Perhaps you could try out some low wattage amps in a music store near you to decide what wattage you’ll need to get your sweet spot at the right volume/loudness?

      • I am familliar with the loudness of these amps. I built el84 based 5W amp but it is too loud cranked. So you say the 2.0 version can get that nice creamy sound as this 1.0 version does? Because preamp sections are little drfferent. What advantages does feedback loop with 18k(R24) resistor bring?

      • Yes, I’m pretty sure that you can dial in that tone. The feedback loop boosts the highs and attenuates the bass a little bit, it’s a feature I lend from the vox night train preamp. It seems to add some top end for some really nice sparkling clean tones, even with humbuckers.

  9. Do you think should standby switch be necesary because of solid state rectifier?

  10. No, I don’t think so, that’s why it doesn’t has one. 🙂

  11. Izzy Madhatter Says:

    Hallo! Hammerprojekt, den werde ich auch bauen. Der Schaltplan enthält allerdings keine Angaben, welche Spannungsfestigkeit die Kondensatoren, bzw. welche Leistungsfestigkeit die Widerstände haben müssen. Kannst Du da noch Infos geben?

    • Für die Widerstände kannst du 1/2W nehmen, nur der Widerstand R15 sollte 1W haben. C7 und C8 sollten um die 200V verkraften können, genauso wie die Koppelkondensatoren C1 und C6, für die restlichen Kondensatoren reichen 16V oder höher.

  12. hi. i’d like to know if is it possible to use a fixed res instead the P2. if yes, which value do you think coul be best? 150r as in miniamp 2?
    thanks a lot!

    • I can’t remember what I finally used and I don’t have the amp right now so I can’t check, but I think it was somewhere in the same region as the second MiniAmps 150Ohm.

  13. thanks a lot!!

  14. Hi,
    ich bin total begeistert von dem Projekt und hab’s jetzt auch nachgebaut. Vielen Dank für die Anleitung. Allerdings macht der Verstärker bei mir keinen Mucks 😦
    Lautsprecher und Kabel sind i.O., d.h. es muss an meiner Schaltung liegen. Die Röhren glühen auch ganz toll. Kann ich somit darauf schließen, dass sie funktionieren? Wenn nein, wie könnte ich die sonst testen?
    Hast du irgendwelche Messpunkte, an denen ich überprüfen könnte, ob der Rest der Schaltung ordnungsgemäß arbeitet? Und was wird eigentlich mit dem Trimmpoti eingestellt? Ist es richtig, dass ich Erdung (Schutzleiter) und Masse der Schaltung verbinde?
    Liebe Grüße und mach weiter so.

  15. Hi,
    der Amp hat fantastisches Sound, Super!!
    Kann man ein Tube Rectifier verwenden? Welches Tube und Transformator wäre geeignet? Oder bringt es überhaupt ein Tube Rectifier zu verwenden? Wie Hoch genau die Spannung A und B?

    Ich werde anfangen mit dem Projekt.

    Liebe Grüße

    • Man kann natürlich auch eine beliebige Röhrengleichrichtung benutzen, man müsste nur darauf achten das der Transformator auch die passende Heizspannung und genug Stromreserven hat. Allerdings lohnt sich das kaum den Einfluss einer Röhrengleichrichtung auf den Ton bzw. die Dynamik kann man auch einfach mittels eines zu den Gleichrichter-Dioden in Serie geschalteten Widerstands “simulieren”. Die Spannungen kann ich im Moment nicht genau nachmessen, da ich den Verstärker im Moment nicht bei mir habe. Ich werde sie nochmal messen wenn ich ihn wieder habe, sie müssten irgendwo zwischen 200V und 250V liegen.


  16. Dude, this is really cool project! i have some questions… What do you think, can i use ecc85 instead of ecc99? And What is anode voltage (B+)? Is it ok if i my tranformer delivers 290V? Isnt it too much? Please reply! J.kohout2006@seznam,cz

    Thanks a lot!

    • You can use an ECC85, just the bias would be set different. You could use a 290V AC transformer, too. But you have to watch the triodes maximum tolerable anode voltages and possibly rebias all triode stages. Both changes will affect the tone of the amp in some way.

  17. Sorry, email adress is: 🙂

  18. Hi! Great project! Respect!
    Can you share the part list of the amp here? …or in e-mail?

    Thank you so much!

    • Hi, sorry I have no parts list for this project. But it’s pretty much just everything in the schematic + enclosure and some screws. 🙂

  19. John H Says:

    With the ECC99 tubes, what is the bias setting that you used?

  20. Mick Purchase. Says:

    Jan 2013 – came across your site Nov 2012 – & great sound clips. Got exact transformer & used Russian 6N2P/ECC99 and custom transformer from 99 biased with 2x 470R in parallel – into 8R Celestion 10″. Also 470R volume pot. What an incredible sound, probably best sounding amp I’ve got. and plenty loud enough for indoors! Overdrive very smooth and impressive, like big amp writ small. The preamp is nice and not fizzy. Great design.
    Thanks so much for sharing this – all power to your soldering iron!

  21. Great project. I’m a new builder and I have a 12bh7 that I want to put on this thing. What would I need to change?
    Thank you

  22. I’ll right away grab your rss as I can’t to find your email
    subscription link or newsletter service. Do you have
    any? Please permit me understand in order that I may just subscribe.

  23. Mark E DeSade Says:

    Love your work. Some of the pix are reminiscent of a guy that sells on ebay videosync1 or something close, thats where I learned of the toroidal PT. He used to post the schematics on ebay with the amp but stopped doing that. If you happen to know of what i speak
    I am looking for the schematic for his 4 pot hammond chassis
    schematic. I think his name is Chuck Harrell but not for sure. He promises to e-mail but not.


  24. Very good write-up. I definitely appreciate this website.

  25. Do you have a parts list for this? I am thinking about putting one in a cigar box with a speaker

  26. Really great stuff here! I want to build your mini-amp and am currently stocking up the components. The “185V transformer” is not available at the moment, so I am considering one with 200V secondary. I read your comment about the sag-resistor, what would be a good (starting) value for this to bring down the B+ with about 20V? Is it right to use the following? The current draw of the two tubes would be in the direction of only 10mA (?). Then dividing 20V by 0.01A would give 2kOhm resistor. Power will be 20V x 0.01A = 0.2 Watt. So a 2k resistor of 1 Watt of higher would do the job? Does this make sense?

  27. Hi! nice build I like it a lot.
    I want to build one like this, but I don’t know what speaker I should connect to. I know it has to be a 4 or 8 Ohm speaker, but what about the power of the speaker? The 1W ones just look ridiculous, could you recomend me someone?

    • You could use any speaker that can handle 2 watt or higher. 2 watt because the output overdriven will get a bit higher than the 1 watt measured clean. Guitar amp speakers are preferable, because they have the suitable frequency response.

  28. Hello. I am planning to build your wonderful amp. Could you please post a list of the components you used ? Also I noticed that the power transformer is no longer available on Musikding. Do you have an idea where I can find one or if I can use a different one ?

  29. chongwen wang Says:

    Hi, I came again, this time I would like to ask, this amp, what is the role of P2?

  30. Christian Siega Vignut Says:

    Very interesting project, I surelly build one.
    Just one question what ‘s the power of the power transformer?

  31. Georgos Says:

    Hello man ,nice work .What diodes are there ?

  32. Great Work 🙂

  33. Regarding the bass response of the Firefly: it has plenty of bass if you use a bigger OT. I have built one with a Hammond 125E that was salvaged from my 18 watter.

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