Introduction: The Valve Caster 2.0: Tube Boost and Overdrive

This is a little tube boost pedal for your guitar amp. It is a two mode effect that adds some tube sound to your guitar. The first mode is a boost. It adds some overdrive and tube compression along with boosting the signal. The second mode adds a second tube that provides much more overdrive.  The project is based on Matsumin's Valve Caster. I learned about the Valve Caster from gmoon's ValveLiTzer.

Step 1: Some Background

This project uses a very interesting mode of operation of the vacuum tube, "starved cathode". Instead of running the tubes at a few hundred volts on the plate like you are supposed to, it runs them at 9-12V. This starved cathode mode gives all of the distortion and sound of vacuum tubes but at a fraction of the danger.

I first heard about the Valve Caster from gmoon's ValveLiTzer project. He recreated the Valve Caster but used the funky 12FQ8. I had 12au7s laying around so I recreated the original Valve Caster instead. I was impressed with the results. It gave a nice amount of "natural tube distortion" and some overdrive. Having a second 12au7, I built a second Valve Caster circuit and ran the output of the first into it to see if I could get more overdrive. It worked but not as well as I had hoped.  So I did some research into tube overdrive and gain characteristics and figured out how to get large amounts of gain from the second tube. This is how the Valve Caster 2.0 was born.

Step 2: The Parts

There is some freedom in what you can do with this build, but the most basic required parts are as follows:
  • (2) 12au7 vacuum tubes (I have tried using 12ax7s and they do not work very well at 9-12v)
  • (5) mylar capacitors. values do not matter too much. I used (3) 47nf and (2) 22nf capacitors.
  • An assortment of resistors including 100k, 220k, 330k-680k, and 1M ohm.
  • An audio taper potentiometer between 10k and 100k ohms. Audio taper, or logarithmic taper, is different from linear and it works better with volume control
  • 2  9 pin tube sockets
  • 2  DPDT or 3PDT stomp switches
Some other things you might want to find include:
  • A metal box to enclose it in
  • A few linear potentiometers for other controls
  • 2  1/4" audio cable jacks
  • A 10nf capacitor for tone control
  • A 9-12v power supply

Step 3: The Circuit

This is the circuit. The first stage is very similar to matsumin's circuit except that I switched the 220k and 100k plate resistors around. The second stage has a high value resistor on the first plate to increase the gain of the triode. The 100k resistor on the other plate amplifies the volume again so that it isn't extremely quiet. The gain of a triode is directly related to the value of the plate resistor (among a bunch of other factors) because a larger resistor limits the current. Limited current increases the voltage drop when the tube when the tube is conducting, this increases the gain. I put 330k-680k on the schematic because anything in that range gives really good gain. You could of course go higher than 680k but I choose not to because it was too much distortion for my liking.

There are three pots shown, the volume pot on the right which says 10k-50k audio. It should be an audio pot for the best volume control. I used a 10k pot but you can really use any The other two pots are gain pots. They control how much gain each stage of the circuit has. I only included the gain pot on the first stage, but I would put the second one in if I could make a second pedal. I used a 10k pot for the gain control but it doesn't provide noticeable difference from high to low. A 50k pot would work much better.

I found that the values of the capacitors doesn't have too much of an effect on the circuit, neither does the values of the grid resistors. You have some freedom for values there without much worry of something not working.

If you would like to add a tone pot, put a 10nf capacitor and a 50k--100k pot in from the output of the second bypass switch to ground. The capacitor should be after the 1uf capacitor but before the volume control.

Step 4: Building the Box

If you can and would prefer this, you can wire some of the circuit up before you start building the enclosure. It may be easier to do so.

To start building the stomp box, you will have to choose which controls you are most important to you and how many you want to or can fit on the enclosure. I could only fit two comfortably so I choose the first gain control and the volume control as the two to put on it. Next is to figure out where everything is going on the box and to make sure that everything will fit where you want it without problems.

Next its time to start drilling out the holes to mount everything. I recommend placing the tub sockets in the same direction so that the pins are identical. This makes wiring it much easier. Also you may want to find a way to protect the tubes, especially if you are going to use this at a gig. I left my tubes unprotected because I will probably only use this "in the studio" (aka practicing).

When placing the input and output jacks, remember that all effects pedals have input on the right and output on the left, in the case that you want to use this with other effects.

Step 5: Wiring

The beauty of this circuit is that it is quite simple. Most of the circuit can be built right on the tube sockets. It keeps things simple and compact.

The biggest thing to watch out for when wiring the circuit is interference. External interference is kept low if a metal box is used, but internal interference can cause oscillations (ringing, squealing, clicking). Keep the signal wires to and from the switches as short as possible and keep them away from other wires if at all possible. I had a bad case of ringing going on when I first tried mine out. When the high gain mode was on and the volume was a bit over 1/4 it started a really annoying high pitched ringing! I poked around and traced it to some overlapping signal wires from inverted gain stages (tubes invert the signal and two inverters connected together creates and oscillator).

The switches are wired so that one selects or deselects the entire tube circuit, allowing you to engage or disengage it, and the other selects the second tube stage only so that you can choose between the boost or overdrive. I used DPDT switches. These allow you to make a full bypass around the circuit. The downside to them are that you cannot add an indicator LED to know which mode it is in. To have indicator LEDs, you would need 3PDT switches. 

Step 6: Power Supply

The beauty of this device is that it can be powered from batteries or a power supply and neither needs to be special in any way. It can be run on a 9v battery if that's all you have. Just be warned that it draws about 300 mA so it will burn through batteries quickly. If you use a power supply, the more regulated, the better. I've tried two different wall-warts, one was a cheap 9v switch mode, it produced a ton of noise and was unsuitable for this. The second which I use now is the good 9v wall-wart for guitar effects pedals. There is no noise and it is properly regulated. Any power supply that can supply at least 300 mA and is electrically quiet. The voltage range should be from 9-13v. Going beyond 13v risks burning out the filaments prematurely, as the filaments are rated for 12.6v.

Step 7: Operation

The first thing to do when running the pedal is to first plug in your guitar and amp without adding power to the pedal. Then cycle through the "on/off" switch, it selects or bypasses the entire circuit. If you wired everything correctly, you should get your clean signal during one state of the switch and when you switch it, you shouldn't hear anything.

If that works, add power and wait for the tubes to warm up then engage the pedal and rock out!


BenH226 (author)2017-08-07

Great effect for a first pedal. *I made it*

This boosts your signal really well. I had my amp on 1 and the Volume control on the pedal about 1 or 2 and boy was it loud!

I used a 500k pot plus a 470k resistor in series at the first plate of the second tube as a Distortion control. however it doesn't seem to affect it much.

I have 3x 12au7 tubes, all second hand and I exchanged and swapped them around. 1 was really weak in the 1st stage but my 2nd was massive on the distortion. When I swapped them around it evened them out a bit.

So it's obviously related to quality and age of your tubes your using. I wish I had some other 12a*7 tubes to try in the circuit for different results.

It might be time to start looking on E-bay for some others!

asfetcu (author)2017-04-25

okay. Can u please show one more image where is the final effect opened. Because i cant find out how to wire the jacks

BenH226 (author)asfetcu2017-08-07

evan.stoddard (author)2014-03-08

I have a couple of 12AX7s laying around instead of AU7s... I don't know a lot about tubes but will those work?

sconner1 (author)evan.stoddard2014-09-10

12ax7 does the same thing as 12au7 just with a little more gain. They are interchangable.

RankoT (author)sconner12017-01-09

They are not interchangable. 12AX7 needs more than 100V of voltage to work, while 12AU7 must work with 9-12V.

sconner1 (author)RankoT2017-01-09

The filament of both the 12ax7 and 12au7 use 12v. You can starve the plate voltage of any of this group: 12ax7, 12at7, 12ay7, 12au7. They have different gain factors 12ax7 having the most with 100 and the others are 60, 40, and 17 in that order. The 12ax7 can handle 330v on the plate but can also use less "starving" it. That causes it to distort, which is the purpose of this circuit. They all should operate but they will sound different, I predict the 12ax7 should distort earliest out of these tubes because it's starving the most in this application.

sconner1 (author)sconner12017-01-09

From what the author and others have said the 12ax7 may just be too starved to get any meaningful result.

kriksis (author)2015-01-27


What would happen if at stage 1. I would use 12AX7 and 12AU7 for 2. stage? Would it somehow combine the amplification of AU with gain of AX?


Teslaling (author)kriksis2015-01-27

I actually did a lot of experimentation with the 12ax7 and I have found that the 12ax7 is not good for low voltage like the 12au7. I don't know why but it does not amplify nearly as well, to the point where it almost nearly attenuates signal. You could, of course, try it for yourself but do not be surprised if it does not work.

Best of luck.

sconner1 (author)Teslaling2017-01-09

Perhaps this is because of the severe starvation the ax7 will feel here. One might adjust some values in the circuit to get more appealing results with it. Maybe it's just too far out of its range to get the desired output.

kriksis (author)Teslaling2015-01-27


You mean, 12ax7 does not work with this specific build or they don't work as gain preamp tube as general? Because I think that there are several Vox preamps using exactly ax ones. I just have couple of AXes and AUes. I just thought combining them could be an option as I've heard, the pinout and filament current are the same for both of these.

Wish you all the best and thanks for the fast reply.

Teslaling (author)kriksis2015-01-28

Yes they will not work with this build. They are common in preamps but remember that tube preamps are designed to run off of proper plate voltage which is usually 100-300V for these tubes. This build uses an extremely low voltage of 9-12V which shouldn't normally work with these tubes. The 12au7s work but the 12ax7s do not for some reason.

kriksis (author)Teslaling2015-01-29

Ok, then I'll use au ones. Thanks and good luck with future projects. :)

WoodMNCo (author)2016-04-19

This is a great project. Do you have (or anyone who's made this) a finished shot of the wiring? Trying to get my head around any suggested mods, i.e 3pdt, power supply with a the 9v wall wart, led. Thanks!

pjuelsgaard (author)2015-07-26

If anyone's making one of these, could you be so kind as to make a step-by-step (or solder-by-solder) video or picture-series, as it would be tremendous help for a rookie pedalmaker

pjuelsgaard (author)2015-06-23

Hey, I was wondering. I read that the 12AX7 didn't work, but if I wanted to try to up the gain factor a bit would the 12AV7 or 12AY7 be compatible?
I am completely new at this, learning along the way, so any sort of info is appreciated

chrisp2037 (author)2015-03-10

So I want to build one of these but I want to incorporate some of my own ideas but know nothing about the audio circuits. Not completely illiterate, have an AS in electronics engineering and worked on f18s for a bit but thats the extent of my knowledge. I want to incorporate a di and a compressor on the output of the pedal ie modify it for connecting directly to a pa system in a live setting do any of the veteran builders have any recommendations for circuitry as well as reading.

kriksis (author)2015-01-30

Hi again,

This might be a silly question, but I am fairly new with tube schematics.

So, if I look at first stage, then the left picture symbolizes the first section of tube? And the right one, - second section. Is there any difference between those sections? Or as long the same pattern is used for second tube it does not matter?


nirep (author)2014-03-10

Is there any way to do it with transistors?

sconner1 (author)nirep2014-09-10

Look up fuzz circuits like clones of the Big Muff. They do a similar thing, one transistor overdrives the next.

evan.stoddard (author)nirep2014-03-26

The whole point of this instructable is to make a pedal with tubes... not transistors...

GoettervaterZeus (author)2013-06-07

This is amazing!
If I would want to build this with a stereo input, is there an easier way from simply doubling the schematics?


12AX7s are dual triodes which makes them well suited to stereo applications.
But, I would change the routing of this circuit.
This circuit has the first grid and plate driving the second of the same tube in series.
Nothing wrong with that.
But, for matching left to right, I would route Left signal to the "A" grid of the first tube and then the "A" plate to "A" grid of the second tube. Likewise, Right signal to the "B" grid of the first tube and the "B" plate of the first tube drives the "B" grid of the second tube.
This will mask variations between tubes, since each tube will handle both left and right signals, both signals will get the same coloration of each tube equally.

ian.brewster.52 (author)sconner12014-09-09

like a push pull but using a single signal often found in the larger output valves?

have you a circuit diagram?

SniperDan (author)2014-08-12

Is it possible to get either a few better pictures or a simplified wiring diagram, for us less confident or experienced with electronics? Including where pots can be used and what they do in each situation?

SniperDan (author)SniperDan2014-08-12

E.g a pic, somewhat like this ( would be incredible helpful to me and probably others

isamxx19 (author)2014-07-22

Ok i would like to make my own design of this but first of all how are you powering the heaters in the tubes? I thought they need to be powered with ac and the plates are powered with dc? Also how would putting the tone control earlier in the gain stage effect things? Im planning on making a 4 or 5 stage gain circuit with maybe the same biasing idea as you where the first triode of the tube is resisted more and then its reamplified in the next triode.

jmacfarlane (author)2014-07-17

I may be missing something here, but in the description for the wiring diagram you say there are 2 gain pots and a volume pot, but I only see the gain pots in the diagram itself?

очеренков (author)2014-07-12

My version + all sound modes))

fuzzybacchus (author)2014-07-07

any thoughts about using this schematic to power a cigar box amp? I have a solid state cigar box amp that I fried :-( but now I have the knobs and speakers and everything... would the signal strength coming out of the petal be enough to power a MOD6 speaker? what would I need to do (anything?) to convert the line out signal from this petal into something usable by the speaker (would I need to build/buy an output transformer?)

Teslaling (author)fuzzybacchus2014-07-08

You would need a power amplifier circuit to be able to drive speakers that big. These tubes have a maximum plate power of about 1 watt but in this circuit, the power if far smaller than even that. You would need to run the signal out of this into a power amp stage which would include one or two power tubes and an output transformer.

TheMilksteak (author)2014-04-10

Hey sorry, I wired this all up right except for the bypass switches. I guess they were never my strong suit. How did you guy about wiring the dpdt's in? Thanks in advance.

firegoat73 (author)2014-03-06

I definitely like what you did here, so much so that I've either bought or ordered the parts to make this already. waiting on the tubes and switches to show up.

but here's my question, if I wanted the first stage to be cleaner, to get more of that clean tube fatness that you don't get out of a solid state amp, how would I go about that? this will be my first foray into tube electronics and this project doesn't seem too overwhelming to start with.

timber89 (author)2014-03-01

This seems awesome!
I'm still new with electronics and I have a question about the schematic.
Why does it look like there are 4 tubes in the diagram? Does anybody have a link to where I can learn about reading tube schematics?

capes83 (author)timber892014-03-02

Hi Timber, if you look at the numbers on the tubes on the diagrams, you'll see it's just one tube shown twice, making it easier to see the connections. You might want to check, and also have a look on diystompboxes. Enjoy!!

capes83 (author)2014-03-02

Hi there, built this, absolutely love it. Got a question though..

If I wanted to add an analogue mA meter in so I could see the effect of turning the gain, how would I go about it? I've tried putting it in series with the gain to ground, and a few other ways, but can't get a reading! Would love to know if it's possible..


Svenjammin (author)2014-02-01

I want to have this built in to a guitar i'm making, any suggestions?

usafbrat (author)2013-12-12

How much did this cost in total? I'm thinking about making this for an uncle!

Teslaling (author)usafbrat2013-12-12

The biggest cost factor is from where you source your tubes. I got a good deal on ebay for a pair for about $4 with shipping. The rest of the components will run you $5-10 depending on what you have and from where you buy. The box that I used was from Sparkfun and is $4.95 so it should only be $20 give or take.

Good Luck!

sconner1 (author)2013-10-23

I like the look of the box.
It's like an old radio chassis.

Teslaling (author)sconner12013-12-12

Thanks! When I first saw the box I had a similar though and saw that it was the perfect size for a guitar effects pedal which inspired me to make this

myonus (author)2013-02-06

Let me say first of all, 12ax7 dont require alot to function correctly. Only about 700-850 volts. This is why (1) those big tube amps are so dangerous to tinker with (2) Every single true tube pedal is nothing more than somthing that looks cool (3) And why you really can't tell its on except for a little lower tone and finally.........(4) the transformers required to push that kind of voltage are huge and heavy. Ever look in a tube amp? I spent months of research and development to make a small tube preamp that could do what my 100w Marshall does when I turn it to 11. It was all a waste, of time, research, and cost of trial and error parts. Obviously the tubes can light up, but they will never, and I mean never reach thier heat efficiency to do what they do below 700volts. Also, if you ever decide to tinker with this kind of power, please be careful. I have nocked my arm back a couple of times. Its not fun. Ephiphone makes a 5 watt tube amp head (99 bucks) Use that, then pull the power down with a resistor to line level and go back into a power amp to drive your 4x12 cabinet. This is an awsome little project you did, and my hats are off to ya for sure, I just wish I hadnt spent some much effort myself. Good Luck, and stay alive.

robot797 (author)myonus2013-12-01

dude you mad
tube ams only have 200-400V

sconner1 (author)myonus2013-10-23

1) 12AX7 can run on as little as 12V on the plate and the obvious 12V filament.
That's why-
2) Tube pedals can work, and work well. They will distort easily with a low plate voltage which is the job of an distortion pedal.
3) You cant hear the fuzz of the second tube stage being overdriven by the first?
4) A tube doesn't need a transformer to drive another high impedance stage like another tube or the amp input (probably a tube), just coupling caps.
This circuit is basically a preamp stage not a power amp. Power amp stages are the areas that have 250-500V plates. The output transformer takes the high impedance of the tube and matches it to the low impedance of the speaker coil. They're big and heavy to transfer (with losses) the power at the output (100W). The difference between this circuit and your Marshall at 11 is that your Marshall is overextending the excursion limits of the speaker cone itself in the physical realm. They're similar in concept but different in physics. One's electrical, the other is mechanical.

ElBeefcake (author)myonus2013-10-10

The miss information in this post is bugging me so hard that I had to create an account.

(1) The 12ax7's maximum anode voltage is 300V, though they're mostly run around 250V.
(2) No
(3) No
(4) The reason those transformers in a big 100W Marshall are so huge is because they have to power the output tubes. The preamp tubes, like the 12ax7, use a lot less current than the power tubes. One could easily fit a transformer inside of a largish pedal that would be sufficient to power a couple of 12ax7 or other preamp tubes.

x-tian (author)2013-01-15

I don't really understand how a 12ax7 doesn't work very well at 12v. It's designed for 12v applications. Maybe use different resistor ratings?

sconner1 (author)x-tian2013-10-23

The 12 in 12AX7 is the filament (heater) voltage.
The plate voltage can be anything up to 300V.
At 300V the plate will have the highest efficiency. (The highest output with the least distortion.) But in a distortion pedal that's the goal.
So to get more distortion we lower the plate voltage until it cant faithfully amplify the input signal, then use another gain stage to make that distorted signal louder, or (optionally) to overdrive the next gain stage to get even more distortion, and so on until the output is as distorted as you like.

Teslaling (author)x-tian2013-01-15

The gain of the 12ax7 is much higher than the 12au7 so what ends up happening is that the 12ax7 produces a very high gain but at low volume. The output of a 12ax7 is extremely quiet which defeats part of the purpose of the pedal.

Also keep in mind that the filament works at 12v, but the plate voltage, which does the amplification, is normally supposed to be 100-200v.

poofrabbit (author)2012-11-29

Congratulations on being a finalist in the DIY Audio Contest!! Good luck to you!

About This Instructable




Bio: I like to make things that move, sense, calculate, compute, blink, and make noise. I like making things that create high voltages, electrical arcs, and ... More »
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