Matsumin Valvecaster: Regulated 6.3V Heater Version

The finished pedal with an Electro-Harmonix 12AU7

The Valvecaster tube overdrive pedal design by someone called matsumin has been around a while and is a simple 12AU7 low-voltage tube circuit that saturates super easily, thus creating a nice valve overdrive crunch without too much effort or volume. It's a nice effect.

The best resources as of 2022 for the schematic are probably this one from Beavis Audio Research or here from Zynsonix, as well as this older thread on DIY Stompboxes. All good to look through, the Beavis Audio with probably the best schematic and explanation, as well as a nice variation with a mini 6111 tube.

I built one as my first ever pedal build—though I've made a half dozen audio amps in the past, so it's not new territory for me overall. It was fun and fairly straightforward, and the results are great. I can highly recommend this one.

I'll take you through my build and a couple of modifications to the circuit that I think are an improvement, as well as a new schematic that I hand drew out of pure impatience with learning a proper CAD application.


The Sound

We'll start with the fun part. How's the pedal sound? I made a short video after finishing it up demonstrating the effect and various levels of knobs to show it. I didn't show the tone high-cut dial in use, but you can imagine what it does.

Please excuse my lame commentary and even lamer guitar playing.

I think it rocks.


The Schematic & Modifications

Here's the schematic of my version:

I tried to learn a real CAD program to make a schematic before giving up and making one by hand instead. You're welcome.

I made two main changes to the original, which are worth considering for yours.

6.3V Regulated Heater Supply

The first and most significant change is using a voltage regulator IC to get 6ish volts for the heaters, and wiring them in parallel for the tube rather than in series.

Why? The original Valvecaster design put 9V directly to the tube in series, basically giving it only 75% of the voltage the heater for a 12AU7 expects. This would heat the tube up just enough to conduct, but likely puts it right on the edge of effective conductance, leading to additional distortion that, while maybe desirable for this application, isn't ideal in terms of a pure tube overdrive.

There are lots of reports of the Valvecaster sounding better when fed 12V, and I would bet the heater design is one of the main reasons, as opposed to the 9V (dropped more via the 220kΩ resistor) on the plate. Tubes conduct linearly down to very low plate voltages, but they don't do so hot (no pun intended) without the right heater voltage.

How do you do it? 12A*X tubes are interesting in that they generally have two options for heater wiring. You can either wire them as 12.6 volts in series through pins 4-5 (so each heater side effectively sees 6.3 volts as the resistor that it is), or 6.3 volts to both pin 4 and pin 5 with pin 9 hooked to ground for parallel operation, so each side of the heater gets its own 6.3V.

From Wikipedia, you can see pins 4 and 5 have pin 9 between them as a "center tap." This lets us wire the heaters as either 12.6V in series or 6.3V in parallel.

So, that's what we do here. To get the 6.3V, we can either use a simple 6V regulator IC like an LM7806 and get close enough, or use an adjustable regulator IC like an LM317 (of which I like to keep dozens on hand for their versatility).

For the LM317, you use two resistors to set the voltage, as shown. Use the values shown to get close to 6.3V, or you can calculate your own with whatever you have on hand.

If you want to use an LM7806, then just hook pin 1 to the 9V source, pin 2 to ground, and pin 3 is your 6V output. Easy as pie.

A heat sink is recommended, as the extra voltage "thrown away" is done so as heat. You can plaster the chip to the chassis, but be careful not to ground the package! Insulate it, for example by using thermally conductive but electrically insulating double-stick tape, as the exterior of the TO-220 IC is actually connected to the Output pin, not ground, and it is not cool to ground it (read below for how I found out).

Extra Smoothing Capacitors

Another small and probably inconsequential change are the two added capacitors for power supply filtering, C5 and C6. You can use whatever spare caps of at least 16ish volt rating here between 1 and 10 uF, and ideally put a small value 0.1 or 0.01uF ceramic cap across the + to ground as well to shunt high frequencies to ground.

These are fairly harmless and just filter your power line a bit more, which could be important if your power supply is of the cheap switching variety, and tends to be a good idea for DC powered heaters, since they can in theory cause noise on the supply line to end up in the signal.

A switch on a pedal?

The other main change to the circuit here is the main power switch. With most pedals, you can leave 'em plugged in without an issue, but for tubes it's nice to be able to power down the tube heaters when not in use. A simple switch of any kind at the main power input will let you do this, and if you use a direct pass-thru 3PDT footswitch, it'll still work even when the power is off.

Other Considerations

Final tips on parts here from my experience with hi-fi amps: for the capacitors in the signal path, C1, C2, and C3, I would highly recommend decent quality polyester film capacitors from i.e. Wima or Panasonic or another reputable maker. Don't use electrolytics or small ceramic capacitors here, or weird varieties like tantalum or mica; it will probably affect the sound negatively.

I used a couple Wima 0.1's in series (they measure to 0.048uF, wildly enough, but it doesn't have to be perfect) for C1 and C2, and a Panasonic polyester film 1uF for C3, and I like how it turned out. Your mileage may vary.

You can play with the type of C4 for the tone knob a bit, as you want a little oddity and personality there. I used a high quality multilayer ceramic cap, and I think it sounds pretty good.

The Build

After acquiring all the parts, I drilled out the chassis with the right holes and fit them roughly. I don't have a drill press, but I did have a Forstner bit the exact size of a 9-pin tube socket (not my first rodeo), and it worked well enough with my standard drill and a steady hand.

After that I sprayed the box with various spray paints in a splatter pattern, just kind of free-forming. This is how I do most of my electronics builds and they turn out pretty cool looking I think. I will likely clear coat it later as well.

Once the parts were all in, I proceeded to wire everything up.

I have regrets on the layout and messy point to point with so many bare leads, but it works well enough.

You can see my hacked together Wima box caps in weird places, and too many grounds in bare copper. Ah well.

The first time I powered it up, it didn't work. Had no idea why, but I tapped it a few times and it started working. Huh, that's weird (not what you want to say, usually).

I eventually discovered the issue was the regulator, taped with heat-conducting double stick tape to the chassis as a big heat sink, was touching the chassis. The mount on the regulator is (very oddly, but probably for a reason) the output voltage, so it was shorting it to ground. Ah. More tape and it was resolved.

Moral: don't mount the regulator to a grounded heat sink.

Once I got it all working, the first growl out of the overdrive was a surprise and delight. I messed with it for a good hour and had a blast.

A few more pics, posed with my cheap but mighty Monoprice Stage Right 5-watt 6V6 practice amp.

Thanks for reading!

Questions or ideas or comments? Continue the discussion on the reddit post in r/diypedals here.

Tristan Harward

Tristan Harward