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 βrealβ 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:
DIY Guitar Freak 
July 29, 2010 at 17:14
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!
July 29, 2010 at 21:29
Thanks! For the output transformer I used a Hammond 125A and for the power transformer I used a toroidal transformer with 185V 0.04A and 6.3V 1.4A secondary. Here is a link to the PT I used.
As an important advice: You should be aware of the lethal voltages in those tube amps. In this amp for example they can go up to 260V DC!
If you have any further question, please feel free to ask.
April 9, 2015 at 22:44
Hi, nice amp, great tones, where can I get the toroidal transformer?
Thanks
Ed
July 29, 2010 at 21:31
oops, here is the link: http://www.musikding.de/product_info.php/language/en/info/p943_Ringkerntrafo-230V—-185V–6-3V.html
July 30, 2010 at 04:03
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!
July 30, 2010 at 10:42
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!
August 11, 2010 at 18:26
[…] The final pedal in action (through the 12AX7, ECC99 MiniAmp): […]
September 21, 2010 at 02:50
howzabout some inside pics?
September 21, 2010 at 07:56
you’re right, that would be interesting for you guys π … I added an inside pic.
October 7, 2010 at 10:25
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.
Fabien.
October 8, 2010 at 20:43
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. π But I would recommend you to simply follow the basic layout of my amp and the wiring will come on its own.
December 21, 2010 at 05:20
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?
December 21, 2010 at 16:01
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 βnext stepβ, 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 (http://www.freewebs.com/valvewizard/) 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. π 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 βadvancedβ 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. π You could use my pictures as a rough guide for the layout.
Oh and I’ll think about making a components list…
June 15, 2011 at 19:23
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!
~Martin
June 16, 2011 at 10:41
I’ll send you an e-mail…
August 14, 2011 at 21:46
How can I use preamp from this amp and power amp from version 2 which has little more headroom?
August 14, 2011 at 23:19
You need a phase inverter for the MiniAmp 2.0 poweramp, so you’d need an extra triode… what are you aiming for?
August 15, 2011 at 09:46
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?
August 15, 2011 at 15:13
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?
August 15, 2011 at 15:25
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?
August 15, 2011 at 16:30
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.
August 25, 2011 at 21:08
Do you think should standby switch be necesary because of solid state rectifier?
August 25, 2011 at 22:11
No, I don’t think so, that’s why it doesn’t has one. π
October 21, 2011 at 11:10
Hallo! Hammerprojekt, den werde ich auch bauen. Der Schaltplan enthΓ€lt allerdings keine Angaben, welche Spannungsfestigkeit die Kondensatoren, bzw. welche Leistungsfestigkeit die WiderstΓ€nde haben mΓΌssen. Kannst Du da noch Infos geben?
October 21, 2011 at 11:42
FΓΌr die WiderstΓ€nde kannst du 1/2W nehmen, nur der Widerstand R15 sollte 1W haben. C7 und C8 sollten um die 200V verkraften kΓΆnnen, genauso wie die Koppelkondensatoren C1 und C6, fΓΌr die restlichen Kondensatoren reichen 16V oder hΓΆher.
November 10, 2011 at 19:58
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!
November 10, 2011 at 21:51
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.
November 11, 2011 at 16:54
thanks a lot!!
November 16, 2011 at 19:26
Hi,
ich bin total begeistert von dem Projekt und hab’s jetzt auch nachgebaut. Vielen Dank fΓΌr die Anleitung. Allerdings macht der VerstΓ€rker bei mir keinen Mucks π¦
Lautsprecher und Kabel sind i.O., d.h. es muss an meiner Schaltung liegen. Die RΓΆhren glΓΌhen auch ganz toll. Kann ich somit darauf schlieΓen, dass sie funktionieren? Wenn nein, wie kΓΆnnte ich die sonst testen?
Hast du irgendwelche Messpunkte, an denen ich überprüfen kânnte, ob der Rest der Schaltung ordnungsgemÀà arbeitet? Und was wird eigentlich mit dem Trimmpoti eingestellt? Ist es richtig, dass ich Erdung (Schutzleiter) und Masse der Schaltung verbinde?
Liebe GrΓΌΓe und mach weiter so.
November 17, 2011 at 16:53
Ich schreibe dir mal eine Mail…
November 17, 2011 at 21:51
Hi,
der Amp hat fantastisches Sound, Super!!
Kann man ein Tube Rectifier verwenden? Welches Tube und Transformator wΓ€re geeignet? Oder bringt es ΓΌberhaupt ein Tube Rectifier zu verwenden? Wie Hoch genau die Spannung A und B?
Ich werde anfangen mit dem Projekt.
Liebe GrΓΌΓe
November 19, 2011 at 18:20
Man kann natΓΌrlich auch eine beliebige RΓΆhrengleichrichtung benutzen, man mΓΌsste nur darauf achten das der Transformator auch die passende Heizspannung und genug Stromreserven hat. Allerdings lohnt sich das kaum den Einfluss einer RΓΆhrengleichrichtung 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 VerstΓ€rker im Moment nicht bei mir habe. Ich werde sie nochmal messen wenn ich ihn wieder habe, sie mΓΌssten irgendwo zwischen 200V und 250V liegen.
GruΓ
February 3, 2012 at 18:28
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!
February 3, 2012 at 20:09
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.
February 16, 2012 at 20:14
thanks for help! I will try it… π
February 3, 2012 at 18:29
Sorry, email adress is: J.kohout2006@seznam.cz π
February 21, 2012 at 21:55
Hi! Great project! Respect!
Can you share the part list of the amp here? …or in e-mail?
borzmajom@gmail.com
Thank you so much!
t
February 26, 2012 at 11:32
Hi, sorry I have no parts list for this project. But it’s pretty much just everything in the schematic + enclosure and some screws. π
May 3, 2012 at 04:33
With the ECC99 tubes, what is the bias setting that you used?
May 19, 2012 at 14:01
I think it was somewhere in the same region as the second MiniAmps 150Ohm.
January 30, 2013 at 01:24
Jan 2013 – came across your site Nov 2012 – & great sound clips. Got exact transformer & used Russian 6N2P/ECC99 and custom transformer from Ampmaker.com. 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!
January 30, 2013 at 20:38
Thanks, I’m glad you like it! π
February 6, 2013 at 22:46
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
February 10, 2013 at 16:08
You just have to set the bias differently via P2.
February 9, 2013 at 02:12
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.
Thanks.
February 10, 2013 at 15:56
Hi I added a subscribtion button. π
April 4, 2013 at 02:46
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.
thanks
April 21, 2013 at 23:04
Sry, I don’t know this ebay seller and his schematic.
September 1, 2014 at 08:01
Hey, I am Chuck Harrell and sorry if I missed your request for amp details. I do boutique builds of 1/2 watters. contact me at ceharizona@gmail.com
January 20, 2014 at 17:23
Very good write-up. I definitely appreciate this website.
Thanks!
November 30, 2014 at 21:52
Do you have a parts list for this? I am thinking about putting one in a cigar box with a speaker
March 27, 2015 at 08:20
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?
April 13, 2015 at 19:43
20v on a valve circuit isn’t a problem
that make less than 10%, change nothing
July 29, 2015 at 11:25
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?
July 29, 2015 at 18:26
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.
February 26, 2016 at 11:16
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 ?
February 26, 2016 at 17:14
Never mind, I checked the MiniAmp 2.0 project, and I`ll follow the new schematic.
February 29, 2016 at 16:47
Hi, I came again, this time I would like to ask, this amp, what is the role of P2?
February 29, 2016 at 19:55
Hi, it is used for biasing the output tubes.
April 21, 2016 at 10:49
Very interesting project, I surelly build one.
Just one question what ‘s the power of the power transformer?
May 7, 2016 at 03:54
Hello man ,nice work .What diodes are there ?
May 7, 2016 at 07:20
Hi, I used 1N4007 diodes.
June 13, 2016 at 17:25
Great Work π
June 14, 2017 at 19:58
check it out
March 28, 2018 at 00:01
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.
January 21, 2019 at 23:19
On OT why did you connect 4 ohm switch to 5-4 to get 27k primary rather than connect to 5-6 to get 22K primary
July 1, 2019 at 20:11
TUBE CHINESE PORN
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October 30, 2019 at 04:31
Have you tried a 12at7 or a 12au7 I think if you try a 12at7 maybe 12au7 for more head room tone and it would be a clean power boots and then you can used a overdrive pedal like boss SD-1 and see how that goes π§π€¨π€π
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