The Milli-Amp

Details

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.

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!

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.

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

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!

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!

Files

milliamp.png

Wiring Layout

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

Preview

MilliAmp.pdf

Circuit Diagram

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

Preview

Components

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

Project Logs

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New video!

Kris Slyka • 03/30/2021 at 17:17 • 0 comments

There is now a YouTube video where I show this thing off! Go check it out!

Discussions

ERIK M. wrote 09/26/2024 at 00:17

how can we know what cab impedance we will need?

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bwlyonsguitars wrote 02/11/2023 at 22:55

I built one of these follwoing the schematic and the picture as well. I have NO sound. What should be the output voltage from the output transformer?

Do the input and output jacks need to be grounded to the chassis?

And, does increasing the output voltage of the output transformer have any effect upon the output volume?

I think this is a great little amp and I'd like to see it working.

Thanks!

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Keri Szafir wrote 01/04/2023 at 20:42

A lovely teeny tiny amp that can go right on your pedalboard! :)

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roger lightbown wrote 12/20/2022 at 15:13

I rebuilt it on a breadboard to make component changes easier. I'm getting some output now, but it's extremely quiet. Like I say, I'm using a preamp but you'd hardly know! Is there an easy way to increase the power output? I've tried varying the value of the 100k bias resistor, but it really doesn't seem to make much difference. Even went as low as 100 ohms. There's no output without it in the circuit so I'm assuming it is necessary. Maybe I just need a more powerful preamp? Or maybe my output transformer (20:1) needs to be a lower ratio? I'm still getting my head around tube amps so apologies if these are basic questions.

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roger lightbown wrote 12/20/2022 at 02:19

I love this amp, looks perfect for what I need! I've built it but zero sound. I assume there should be a reasonably high voltage going into the output transformer, but I'm getting nothing. What voltage should I be seeing? I've checked and checked the circuit and all seems ok. I'm feeding a tube preamp into it, so input signal should be ok. The heater seems to be working ok. Anything to try? I could maybe try a smaller bias resistor in place of the 100k?

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Alex wrote 07/14/2022 at 19:16

Hi, I'm the guy who put 120V on your amp. This a further development of your idea into a complete guitar amp, Thanks for your design. I put a fender style all tube preamp before the 6j1.

https://www.diyaudio.com/community/threads/very-little-wattage-guitar-tube-amp-opinion.388211/

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jxu labo wrote 07/06/2022 at 02:30

It's a very cool project.

Your idea changed my guitar amp world.
Dear Kris Slyka, from my best friend in Japan.

6J1 Milli Champ
https://sites.google.com/view/je7jxu

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justin Jaehnke wrote 12/30/2021 at 20:19

I am very new to this could you give your most detailed descriptions of components, particularly the the power supply and transformer this is something I would really like to try getting into this and it seems like pretty affordable components. Thank you for your time any info is much appreciated

Justin

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Laszlo wrote 11/24/2021 at 20:03

Very cool. Have you ever thought about using a bigger tube, like a 6v6 or el34? Would it be possible? I want something for my multifx, so it should take line level and has enough clean headroom.

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Kris Slyka wrote 11/25/2021 at 11:50

I've thought about it, but I'm not sure how well it would work. Aside from needing a lot of heater current which might not be great for a pedal they're rated for a much higher voltage, something like 350-400V as opposed to the 120V of the 6J1. Buuuut… they also have a high transconductance, so it might work? I don't have any lying around to test with, but it would totally be worth trying I think.

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jjnicola wrote 03/11/2021 at 06:33

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

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

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

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

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

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

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

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

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

Hah, thanks!

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The Milli-Amp

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