Real 12AU7 Tube Distortion Box.... DIY for $15!

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Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by HideawayStudio » Tue Jun 16, 2009 10:11 pm

I've been tinkering with tube circuits over the last few weeks. Over the weekend I devised a really simple design for a tube distortion box. I had a go prototyping it and after a little tweaking it works a treat! The circuit is based on a 12AU7 double triode pulled from an old radio with an op amp driving the grid of the first tube stage, a pot to set the overdrive, and a voltage follower after the second stage with a pot setting the attenuation. It is a "starved plate" design allowing the whole thing to run on a safe 15 volt PSU. The tube is 100% in the audio path and is the bit providing the overdrive - not the op-amp which is simply amplifying and buffering the audio signal.

Assuming you can find an old tube the total parts cost is less than $15!!

If anyone is interested I will post up my schematic later this week.

Here is a piccy and a link to a rough demo using no more than a basic synth patch from a keyboard with FX switched off. The tube distortion is then washed through reverb and delay to sweeten it. The first few seconds of the demo show the synth running through the box with no overdrive which is then slowly brought in:

http://www.last.fm/music/D.A.Wilson/_/1 ... ?autostart

Hideaway Studio's $15 DIY 12AU7 Tube Distortion Box:
15_Dollar_Tube_Distortion_sm.JPG
EDIT: Yes that is hiss you can hear... this is not caused by the tube (which is remarkably quiet) - it's due to me using a cheap op-amp. It needs substituting with something better suited for audio applications.
Last edited by HideawayStudio on Wed Jun 17, 2009 6:05 pm, edited 2 times in total.

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Re: DIY Tube Distortion Box.... for $15!

Post by novielo » Tue Jun 16, 2009 10:28 pm

that's realy nice. did you tried to put a drum machine in it?

i'm looking forward to your schematic.
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Re: DIY Tube Distortion Box.... for $15!

Post by HideawayStudio » Tue Jun 16, 2009 10:31 pm

novielo wrote:that's realy nice. did you tried to put a drum machine in it?

i'm looking forward to your schematic.
Cheers... that's a good idea... Here's my Drum Station through it....

http://www.last.fm/music/D.A.Wilson/_/% ... ?autostart

This has highlighted an interesting issue/characteristic - the way it is setup at present it is definitely better suited for medium and high frequencies - too much energy at the bottom end causes it to totally blow out and as you can hear sometimes it almost sounds like it's trying to go unstable... cool! :) This could be used to effect though....

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Re: Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by HideawayStudio » Wed Jun 17, 2009 5:50 pm

I might have a go making another version running the tubes at around 100V just to see how the distortion/overdrive effect varies with increased anode voltage.

I'm getting the impression not so many tinker with studio electronics on this site - unless all the like minded are lurking in the wings? :) We're not all white lab coat donning nerds you know.... ok maybe I am.... and my fiancee.... oh and maybe all my friends... oh and maybe my colleagues.... urggh... possibly the cat too... :?

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Re: Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by nathanscribe » Wed Jun 17, 2009 9:32 pm

Looks interesting. I'd also like to see a schematic... drop me a PM if you fancy a nerd-to-nerd chat about this stuff (I seem to have become involved in making effects for folk lately...)

I'll listen to those demos on my proper speakers later.

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Re: DIY Tube Distortion Box.... for $15!

Post by novielo » Fri Jun 19, 2009 1:16 pm

HideawayStudio wrote:
novielo wrote: This has highlighted an interesting issue/characteristic - the way it is setup at present it is definitely better suited for medium and high frequencies - too much energy at the bottom end causes it to totally blow out and as you can hear sometimes it almost sounds like it's trying to go unstable... cool! :) This could be used to effect though....
i clicked the link before reading the rest of your post and that's exactly what i thought.
something like have the clean signal in center and running the tube distortion through a phaser and see what it does

sorry, i'm not a very good electronic geek at all. i can repair most of my stuff but i can't design anything. i'm more of a "i plug everything everywhere" guy.

but i realy found it stunning when people are doing things like this, anything from the matson mini modular to a diy tube distortion. :-)
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Re: Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by Clavier » Fri Jun 19, 2009 10:08 pm

I too am looking forward to seeing the schematic. I've grown increasingly fond of voicing circuits and tube rolling as of late. What did you use for the enclosure?
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Re: Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by HideawayStudio » Fri Jun 19, 2009 11:43 pm

OK - Here is the schematic - it's here in jpeg format due to file size limitations. I think the circuit is "blowing out" when passing bass heavy signals - I need to scope the circuit and locate the problem. One way to overcome this is to lower the value of the DC blocking caps from 2U2 to less than 1uF. It could simply be due to the anode voltage being too low - it still amazes me you can run tubes on 12 volts!!

Despite it's limitations I'm really liking the sound this thing produces. When it's wound right down so there is little or no distortion it sounds quite sweet. I must, however, replace the op-amp with a better one - the LM324 is evidently a little noisy! The good news is little to no hum - it's all hiss... It comes to something when a 1960s tube is considerably more quiet than a 2009 op-amp!!

Two other circuits I'd like to build are a voltage controlled gate for chopping effects - using a tube!.. and a tube filter - maybe even the resonant LC type.

The case is a die cast prototyping box mounted upside down so the electronics are mounted in the lid and the box fits over the top of the tube and pot shafts. The enclosure is probably made by Hammond. They are very inexpensive and easy to machine. The tube is a 12AU7 (or ECC82 if you are from England) made by Valvo and almost certainly from a 1960's radio.
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Re: Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by HideawayStudio » Mon Jun 22, 2009 9:41 pm

I've had another play with this thing tonight. I routed my ARP Omni-2 through it and again I really like the results - it this case it managed to turn a pretty tame sound into something that sounds like it came off a Daft Punk album! With some sounds the character of the sound can change really quite dramatically without sounding like completely OTT distortion. This thing must be doing me some good as it's encouraging me to be experimental again! What I've found is that as long as there isn't too much bottom end in the input signal it can sing really quite sweetly.

This is a demo using the Omni-2's viola preset where the overdrive level is very slowly wound up and then back down again. Notice how the chorus almost sounds like PWM as the signal becomes more and more distorted.

So typically of tubes the distortion never becomes unusably harsh. I can see myself using this box quite a lot - I just need to address the hiss which is due to me using a cheap op amp.

http://www.last.fm/music/D.A.Wilson/_/A ... ?autostart

For those who are sceptical - this thing was really cheap to build and you can't therefore be too critical! It's been good fun tinkering..... :D

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Re: Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by premieklovn » Mon Jun 22, 2009 10:23 pm

Very nice. Too bad I can't read schematics (yet). You should make a 5U modular version!

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Re: Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by Stab Frenzy » Tue Jun 23, 2009 6:26 am

HideawayStudio wrote:it still amazes me you can run tubes on 12 volts!!
Ha. Most of the budget tube preamps on the market are running the tube off 5-12V, that's why they sound so awful. :)
premieklovn wrote:Very nice. Too bad I can't read schematics (yet). You should make a 5U modular version!
Check out the CGS modules, there's a couple of tube VCAs there.

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Re: Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by HideawayStudio » Tue Jun 23, 2009 7:13 am

Stab Frenzy wrote:
HideawayStudio wrote:it still amazes me you can run tubes on 12 volts!!
Ha. Most of the budget tube preamps on the market are running the tube off 5-12V, that's why they sound so awful. :)
premieklovn wrote:Very nice. Too bad I can't read schematics (yet). You should make a 5U modular version!
Check out the CGS modules, there's a couple of tube VCAs there.
I've no real interest putting together a DIY tube preamp - there are a lot of off the shelf ones now, and as you imply, -some- of them even run on HT :) I personally think it's more acceptible to use lower voltages when using tubes for heavy colouration where non linearity and a higher noise floor are less of an issue. That said, I will have a go at making another overdrive unit with a 100V anode rail.

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Re: Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by madtheory » Tue Jun 23, 2009 9:28 am

Overdriven class A is a lovely sound :)

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Re: Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by piRoN » Tue Jun 23, 2009 11:32 am

Technically though, once you push up the overdrive it's no longer class A...
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Re: Real 12AU7 Tube Distortion Box.... DIY for $15!

Post by madtheory » Tue Jun 23, 2009 12:16 pm

OK, but it doesn't change class, does it?

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