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.

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.

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

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.

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.

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. 

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.

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!