The Valve Caster 2.0: Tube Boost and Overdrive





Introduction: The Valve Caster 2.0: Tube Boost and Overdrive

About: 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 can light fluorescent bulbs at a distance. I like to d...
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!

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    Hi, I’m hoping you might help me. I’m making a pedal like yours. I’m using the following to build my vero board... your build you said you switched the 100k and 220k resistors in the first stage (r2a and r3a I believe in the vero plan). Should I consider reversing the resistors as they are shown in the vero diagram. I can NOT read schematics to cross compare your schematic to the vero diagram so I’m totally lost.

    I’m also considering changing the r4a and r4b resistors to 510k as I don’t have the 470ks. Do you think this will be ok? Are these the resistors you said had a good range ...up to 680k?



    Have you tried a 3 stage/tube version?

    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!

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

    1 reply

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

    4 replies

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

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

    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.

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


    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?


    5 replies

    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.

    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.


    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.

    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.

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

    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!

    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

    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

    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.

    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?