possible circuit for 12AU7 tube distortion. could anyone verify my schematic?

I am attempting to build a distortion circuit using 1 12AU7 tube as a diode. I know that the circuit is primitive but really its only to add in the extra tube sound to my amplifier. so my question is will this circuit work in its current design? if not what will make it work? 

Picture of possible circuit for 12AU7 tube distortion. could anyone verify my schematic?
12Au7 tube 1 design.png
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BobbyH281 year ago

man. not to sound like a negative Ned. the way that you are wanting to build this, you wont be using the 12au7 to is full potential. that being said the valve caster itself on 9v seems to fall short of that mark too. so run it on 12v.

Nope, not a thing is wired right, not even the Heater. That has to have a positive and negative to heat up, not be wired to the same point through a switch.

Also, tubes use 2 voltages, a heater voltage and a plate voltage, both of which are vastly different. Not even one voltage source shown here.

The Grid is what controls the tube's flow of electrons, so that is what your source should be connected to. The output would, I believe, come from the Plate. The Cathode (K) would be connected to ground.

Note, this is the simplest explanation of connections, there are a lot more parts needed to make a distortion/preamp like you want. I am not a tube guy, but I can use a search engine (just did to verify connections) to get ideas or direction. As a matter of fact, Nuts and Volts editor Bryan Bergeron is a musician in his spare time and he put together a tube preamp for his guitar that would probably be what you want. Let me look after work and see if I can find the issue for you.

fastcar123 (author)  Quercus austrina5 years ago
I'm sorry I realize that looking at my diagram over I neglected to label my parts. The black plug is supposed to represent a DC jack at 12v. I'll go back and label it when I get home.

I am however unaware that tube require 2 voltages. I only thought they need one DC current and one AC Signal input.

So what I need to do is put signal on plate 1 and then from cathode 1 to plate . From plate to to signal out and from cathode 2 to ground?

tubes are obviously far from my strong suit. But i am building my own guitar amp and i would like to have tube sound

I'm going to hand this one off to Re-design, since, as he stated, he has spent the last 2 years learning tubes. Better to listen to the voice of someone who is well versed rather than a "casual" hobbiest, at least as far as tubes go.

For solid state versions, try RedFreeCircuits. Check out the Music page. It has a FET Valve-like distortion that you might like to try. A lot easier and less expensive than tubes for experimentation.

I haven't found that article, but when I do, I'll try to get you a link to it.

fastcar123 (author)  Quercus austrina5 years ago
ok thank you. if you find the article you can post it on here
If you can get a hold of the March 2009 issue of Nuts and Volts magazine, look at pages 8 & 9 for the editorial I mentioned. It links to OatleyElectronics.com for their K270 sub-miniature tube preamp. I haven't found my paper copy yet, but the good folk at the N&V boards helped me find that issue. If you still want it, I'll make a scan when I find it.


In my defense.

I took that black thingy as a DC coaxial connector.

Oh, I took it as a pushbutton switch. Designators do wonders.

Its a big help, if you're copying a circuit, to cite it here first. This is what you are trying to use.

and IT uses no HT or heater drive. Its a very, very unconventional valve circuit.

What happens when you try it exactly ? I suspect your problem is that you need a proper 9V DC supply from a wallwart, and not a battery. This thing is going to take several watts to run it properly.

Here are some notes on the circuit I've studied.

fastcar123 (author)  steveastrouk5 years ago
Its ran off of a 9v wallwart connected to a toggle switch. I'm not sure how many watts it outputs but my guess is about 3. I'll have to do the math later when im actually working on it. When I try running it my heater doesn't light up and I have no output signal at all. I've built it 4 times with similar results. Im going to read these note and post again on here
Your heaters SHOULD light up. if they don't, they won't emit electrons
fastcar123 (author)  steveastrouk5 years ago
I think I should stop with the valve caster circuit. Clearly its not working for me. If I build this alternate circuit I will need to have 2controls of some kind.
Get your valves glowing FIRST. You don't even have to build the rest of the circuit.
fastcar123 (author)  steveastrouk5 years ago
ok once I get the heater to glow and sound to come through. What's the next step? Will you help me with that?
sure. I think the original circuit will work, if you can wire it. Valves are TOUGH to bust.
fastcar123 (author)  steveastrouk5 years ago
What do you mean exactly that valves are tough? Tough to build, break or modify?
Tough to break. Except by dropping on concrete.
fastcar123 (author)  steveastrouk5 years ago
also what if I connect it to the output of the amplifier to the speaker. I know that sounds strange but it sounds like when I play guitar that it needs to be amplified more. almost as if the volume on thwe guitar is turned almost all the way down
Do you mean connect it to the INPUT to an amplifier ? Yes, the circuit is just designed to inject distortion to the signal, not gain.
fastcar123 (author)  steveastrouk5 years ago
I need more amplification in the preamp stage before the tube. How can I do that?
Use a preamp - ANY preamp. A valve one would be in keeping, but an amp would need a proper HT supply. A semiconducting amp would run quite happily on a 12V supply, you could easily make one from a LM386 or even a 741 opamp.

Remember, the effect of adding a preamp might affect the output, I don't know.
fastcar123 (author)  steveastrouk5 years ago
Can you elaborate further on that?
Which bit ?
fastcar123 (author)  steveastrouk5 years ago
How exactly should I wire this up? I have a preamp and a separate amplifier board . So do I need to connect this from the preamp to a secondary amp to the main amp?
Preamp > valve caster > power amp.
fastcar123 (author)  steveastrouk5 years ago
I don't need a 1 watt power amp after the preamp to the the valve caster?
Without some numbers I can't comment much more.
If you're messing around with audio circuits, you really need to get a scope and a signal genny together - you can do a lot of audio work with scopes running off your sound card.

Stick a sine wave of known amplitude in the input, measure the output of the valve caster. Deduce from that what gain you need.

fastcar123 (author)  steveastrouk5 years ago
I don't have any fancy equipment unfortunately although a scope would help tremendously
The soundcard scopes will help you greatly, look for "winscope"
fastcar123 (author)  steveastrouk5 years ago
And then what?
"Stick a sine wave of known amplitude in the input, measure the output of the valve caster. Deduce from that what gain you need. "

I don't think the valvecaster has any positive gain
fastcar123 (author)  steveastrouk5 years ago
take a look at my new discussion. I have gotten the valve caster to work. don't ask how I did it I just did. my problem is I have a horrid fuzz the nearly drowns out the sound and I need to know how to fix it. I am working on getting the oscilloscope but haven't got the money right now
The windows ones are free !

Don't forget to award the Best answer for getting you this far.
fastcar123 (author)  steveastrouk5 years ago
Does it work on laptops with windows 8?
Dunno. I suspect so, because it runs on XP
fastcar123 (author)  steveastrouk5 years ago
It should work
You don't want a power amp anyway, you need a preamp. You need VOLTAGE gain, not POWER.
fastcar123 (author)  steveastrouk5 years ago
Ok I'll just take it from the pre amp to the caster then to the amp
fastcar123 (author)  steveastrouk5 years ago
could I burn up the heaters with 15V?
Yes, easily.
fastcar123 (author)  steveastrouk5 years ago
How can I get more signal through it? And how can I kill the buzz. There's a horrible buzz that makes the amp unusable.
The buzz is probably either a ground loop, or a cruddy supply. Put a 12V regulator on the heaters and try again
fastcar123 (author)  steveastrouk5 years ago
My preamp has a regulator on it (its for 15v) but ill still put int on the power
fastcar123 (author)  steveastrouk5 years ago
This experiment seems like it doesn't work on my end
Re-design5 years ago
Here are my suggestions.  And just so you know where they come from, I've spent the last two years learning how tubes work and how to restore tube amps and radio equipment.

The 12AU7 is a double diode.  It's like having two separate tubes.  They don't interact with each other unless something is wrong.  Your design is trying to combine all the elements into one tube.

Your blue ground wire is okay but I'd wire it sleeve to sleeve.

Your input tip should go to 2K which is the cathode that is heated by the filament.  This heated cathode will give off electrons that come from your guitar.  In a diode the electrons only go one way.  So you will get full clipping.  In solid state boxes they use diodes or circuits that don't give full clipping so the effect is softer.

Next connect the output tip to 2G which is the grid.  The grid is then closest electrode to the cathode and will pick up more electrons than the plate.  There is no amplification so you want to get all of the electrons that you can get.

Wire the heaters to 12 volts either clean A/C or DC. 

The plate is not used when using the tube as a diode.  This is a similar use as when tubes were used as detectors in radios.

I would not wire any controls until I got the thing working.

After I got it working I would try to add a control that varies the balance between clean and full on so that you could control the amount of distortion produced.

You could also add the other half of the tube wired as an amp but that will require adding at least 50 volts and some resistors and capacitors that will have to be calculated  using MATH.

But as I said before get it working as simply as possible first then revise and add controls etc.

I'm with you heaters are 12v or split 6v using pin_9.
Some tubes were injured by a lower heater volt supply.
And Tube LIFE gets cut by the third or fourth power of
excessive heater voltage from months to days before dead tubing.

Never ever heard of running a gas tube on less then its forward
Plate-Cathode voltage drop.  The 12AU7 is designed to run up
to 165 VDC and 100 VDC plate for class A amplification using the
full 5000 tube transconductance.
I guess modernistic design for Guitar screams and Moog synth
soundings will eventually try to work on electron gas squirting
photonick trick derivations #$%#@!! 

No, with my limited knowledge of valves, I am surprised it works, but its not dissimilar of course to using a FET in the same mode.
Rodger that  ....  When FETs appeared I had a sigh of relief going from
bipolar semiconductors that use current to the FET gate voltage controlled ones without the gas tube forward drop and old grid voltage control..
fastcar123 (author)  iceng5 years ago
Wait so do I use 12VDC to power the heaters or 110 with a DC converter?
110 with a wall wart like the pic is good to use.

fastcar123 (author)  Re-design5 years ago
my heaters are lit and warm so I am on the right track
fastcar123 (author)  Re-design5 years ago
I was able to get it to work in an extremely rare case with its current wiring. I believe there isn't enough power either from the guitar, the transformer or both. I could increase the heater voltage from 12 to 15 volts using another wall wart that I have currently powering the preamplifier, but I don't know if that will help.
I meant to say the the 12AU7 is a double TRIODE but wired this way is use as a DIODE.
fastcar123 (author)  Re-design5 years ago
sorry for the strange format. the nupdated schematic is up top. I will need to figure out how to put in variables into the circuit
fastcar123 (author)  Re-design5 years ago
lets try this one. keep in mind that this is my first tube circuit and im trying to come up with an alternative to Matsumin's Valve Caster circuit (with which ive had no success with)
iceng5 years ago
I don't see a plate supply voltage source.

The only correct wiring is the heater.