Low Voltage vacuum tube Saturation Amplifier

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PermO

Well-known member
Joined
Mar 16, 2011
Messages
792
Location
Hilversum, Netherlands
Saturation Amp.jpg
Hello,

This is my first original design, and I would like to share it with you people.
What started as a guitar distortion pedal which I did not like, ended up a stereo bus processor which I do like.
It is based on one 12au7 per channel running at 12.6V, the heater voltage, so the whole thing runs at 12.6V from a 2.5A rated SMPS PSU.
There's a FET on the output stage to force the output impedance down so it integrates with an unbalanced bus insert on my mixer.
It can run pretty clean, I have been tweaking the circuit a lot, to the point that I can play albums trough it at a nominal level, without being offended.
Pushing the drive results in harmonic distortion and slight compression, there's a sweetspot depending on programm material, overdoing it will result in nasty clipping.
It's not for mastering, cool results can be had on drumbus, synths, parallel on bass... etc. The less harmonic complex the material the harder you can push it.

Low Voltage Vacuum Tube Harmonic Saturation Amplifier.jpg
It can be build cheap, run off a wallwart, and produce some cool sounds.
Wiring guide;
Wiring Guide correction.jpg
So, yeah.. there it is...


Cheers !
 

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Bussfunker : )
I like the stompbox aesthetics very much. What colour is the faceplate? Some kind of beige/yellow ? Same as the schematic ?
Thank you very much for sharing.
 
Bussfunker : )
I like the stompbox aesthetics very much. What colour is the faceplate? Some kind of beige/yellow ? Same as the schematic ?
Thank you very much for sharing.
Thanks !

I used a camouflage Khaki spraycan from a army dumpstore.
Decals are printed waterslide, covered with clearcoat.
I'd say it's a light olive tint. RAL6011 comes pretty close.

The stompbox switches can be found cheap in China and they feel nice and sturdy, I don't run any signal trought it, it just switches the bypass relays on the backplane of the unit.

I'll see if I can post some clips this weekend.
 
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Nice!
I've used something in the same vein, basically a passive 10k monitor controller with a low voltage 6201 gain stage tacked on after and it always sounded good to me. Was only a single stage with an inverted output though. Always wanted to change that. Have always been curious about Merlin's stuff on this too . Thanks for sharing.
 
Looks cool,

I was thinking of doing something similar.

Do you have any sound files you could share?
 



Here's two loops, Just using the bypass switch on the unit (I think I need to adress those clicks), it's off, on, off, on.
On the synth organ it's pretty much cryshing it
On the synthbass it's medium +/- 10dB push... normally I would run that parallel.

I tried to show overall transparency by running a small clip from an album but the AI won't let me upload that.

I think it could do nice things on vocals also...


...and a bit of a drumloop, same concept, off, on, off, on...
 
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Thanks !

I like what it does for such a simple build, there's defenitely more in it than those three clips.

A friend of mine borrowed my first mono prototype and also ordered some parts to build one, he really enjoyed it on synths using it more in a overdrive fashion. That one did not have the FET and needed a high Z input stage.

There's probably more stuff that can be done within the 12.6V concept, I'd be interested to see more takes on this
 
Perhaps you're already aware, but back in the sixties (I was repairing radio and TV, including car radios), there was a line of vacuum tubes designed intentionally to run on 12 V power only. They were called "space charge" tubes and actually worked very well. They were part of the "last gasp" of tubes as they disappeared from car radios. One of these radio designs usually had several of these space-charge tubes and a germanium power transistor as the power output stage. It, of course, eliminated the vibrator and high-voltage power supply used in all previous car radios. I've attached an example - the 12AE6 was a triode (with detector diode as well) for the standard AM detector and first audio amp stage in the design - it replaced the 12AV6 that had the same function in conventional high-voltage designs. But I realize "quality" in the conventional sense is not what you're after here - it's all about distortion.
 

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Yes, I was reading an article on space charge tubes and the autor mentioned a good quality 12au7 comes "close" to that, it won't run as intended but as long as you don't demand too much gain it will run rather clean.
So this got me experimenting. I use modern production Tung Sol tubes.
The most challenging part was to find a balance in the circuit to make it able to run 'clean' on a line level insert, and push the "drive" level from there and compensate at the output gain so overall levels will remain even.

There's a usefull sweetspot with this circuit, it can handle a full mix if not pushed, that was my reference point.

I'm quite happy with the result
 
In the schematics, the 1uf's connect to pins 1 and 2. In the component placement diagram they connect to pins 1 and 3. Do I misread?
 
In the schematics, the 1uf's connect to pins 1 and 2. In the component placement diagram they connect to pins 1 and 3. Do I misread?

Oeps, yes, thanks for spotting that !

now I don't know if the fault is in the schematic or the wiring diagram...

I still have my paper drawings, I'll check after my dinner.
 

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