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The Milli-Amp

A tiny low voltage tube guitar amp!

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The milli-amp is a tiny, 24V powered, less than one watt guitar amplifier built from scrap I had lying around. Besides the tube and an output transformer repurposed from an old 12V wall wart it only uses a hand full resistors and capacitors.

This project started as a way to find a use for a bunch of really cheap 6J1 pentodes I got on Aliexpress for around $1 each.
I started out with just building a small class A preamp and saw that it performed surprisingly well at low voltages and began to wonder if it would maybe be able to develop enough power to drive a pair of headphones.

Simple Class A Preamp
Simple Class A Preamp

I knew that for this to be even remotely possible I'd need an output transformer to match the relatively high voltage and high impedance output of the tube to the low voltage, low impedance signal needed by a pair of headphones.
Using a nice audio transformer seemed like kinda of a waste though, so instead I hacksawed open a bunch of old 12V wall warts. With a turns ratio of 20:1 I figured they'd be close enough. Turns out, they were! I wasn't just able to drive my headphones, I was able to drive them at a level almost too loud to listen to comfortably. Success!

Sacrificing some old wall warts
Sacrificing some old wall warts

All this success made me wonder if maybe it would even be able to drive the 12" speaker in my guitar combo. Some quick connections with some alligator leads later I was ready to be let down. And I was.
It was audible, but barely louder than the sound my electric guitar made by itself. Which wasn't too surprising considering this amplifier lacked any kind of preamp and fed the guitar signal straight into what was basically a "power" amplifier.

Yep, that's all there is to it!
Yep, that's all there is to it!

But like most problems in life, this one could be solved with more guitar pedals, so I hooked up an external tube preamp pedal and cranked the output volume on it to drive the "power" amp harder. And it worked! It was way louder than it had any right to be. And I finally had a guitar amp that I could turn up all the way without annoying the neighbors.

Metalworking on carpeted floors is a bad idea
Metalworking on carpeted floors is a bad idea

Now suddenly convinced that this might actually be something slightly useful I decided to give it it's own enclosure so it could sit proudly on top of my regular guitar amp. I added in a 120Ω resistor for the heater so I could run it off of the same 24V power supply that powered the rest of the amp, which is kinda wasteful but keeps the whole thing really simple. All the connections were made point to point, because that's how all good tube guitar amps are made.

Highest quality point to point wiring here!
Highest quality point to point wiring here!

This thing was a ton of fun to build and I actually play it more than my old combo now. And I think you should also build one, it's fun!

milliamp.png

Wiring Layout

Portable Network Graphics (PNG) - 115.59 kB - 03/03/2021 at 10:38

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MilliAmp.pdf

Circuit Diagram

Adobe Portable Document Format - 21.36 kB - 03/02/2021 at 11:51

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  • 1 × 6J1 Pentode 6J1 Pentode with matching socket
  • 1 × 240V to 12V (20:1) transformer Audio output transformer
  • 1 × 100nF Capacitor Input capacitor
  • 1 × 220µF 36V Electrolytic Capacitor Power supply filtering capacitor
  • 1 × 100kΩ Resistor Biasing resistor

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Discussions

jjnicola wrote 03/11/2021 at 06:33 point

hey! Nice!!! I built an hybrid with 2x 6j1 and recycled parts of an old Aiwa hifi (transformer and power amp ). The 2x 6j1 are used as pre amp and I am very happy with its sound.

At the beginning https://youtu.be/-BcxR0hRDx4

and this is how it sounds at the moment https://youtu.be/cbETRppO3F4

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salvim wrote 03/08/2021 at 18:16 point

I'd love to hear some sound sample...

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Kris Slyka wrote 03/08/2021 at 19:14 point

I'm currently working on a little youtube video with some audio samples!

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pauloscabeni wrote 03/08/2021 at 17:56 point

That's great! Is it possible to build this amplifier and a simple valve preamp that shares the same power supply? I was thinking about making a mini guitar amp head for home use, without the need of any external pedal to do the preamp funcion. Congratulations for this project, it's very inspiring.

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Kris Slyka wrote 03/08/2021 at 19:16 point

Yeah, that should be simple enough! You could add another 6J1 or even a 12AU7 for a more "classic" triode preamp sound. I have a video on my youtube channel on how to build one of those! https://www.youtube.com/watch?v=TllEAf0Hgnc

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pauloscabeni wrote 03/09/2021 at 00:30 point

Now I understand why your name sounds familiar to me: I was already subscribed to your YouTube channel :) Great channel, love the compressor pedal video. Thanks for the answer here.

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Dan Maloney wrote 03/03/2021 at 00:59 point

I hate to say it like this, but that's adorable!

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Kris Slyka wrote 03/03/2021 at 08:38 point

Hah, thanks!

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