Introduction: The ValveLiTzer: Low-voltage Tube Booster

About: Go sit in the Faraday cage and think about what you've done...
Here's a little tube booster project for guitarists. It colors the sound with some tube distortion (although it's more an overdrive than a distortion pedal), a little compression, and it boosts the signal, too. It's a "dirty boost," with the flavor of tubes, and can really spice up an amp (and it does add punch.)

Now With More Gain!
Updated schematic added, see the last page...

Plus, it's low-voltage--no more than 13V, so it's perfectly safe for "tube neophytes" to build. No high-voltage dangers with this one. It can even be powered with a 9V battery (but read the step on "Powering Options.")

With only a few inexpensive parts and a simple circuit, this should be an easy first-time tube project!

I didn't use a video cam mic, so the "youtube" audio is halfway-decent quality. But the mp3 file (look below, beneath the pictures) is much's same audio track.

Step 1: Background

Vacuum tubes have an interesting characteristic called "starved cathode" operation, which results in a good deal of distortion when the tubes are run at very low voltages. Matsumin's Valve Caster tube booster was my introduction to low-voltage tube projects. These voltages, in fact, are so low that many old-skewl electronic techs would tell you that the tubes shouldn't even work... But they do (some do, anyway.) Ignoring the normal plate voltages, if run at 9V the filament voltages are so low that the heater filaments shouldn't even function (but they do.)

Matsumin's project uses 12AU7 tubes, and is a very worthy build. This build, the ValveLiTzer, uses a slightly more oddball tube: the 12FQ8. Why use a weird tube? Because I have about 25 of 'em, and no guitar amps or stompboxes use them. So why not build something?

But the 12FQ8 isn't a typical audio tube. It's a twin-triode, but with 4 plates, and a single shared cathode. Would it even work as an audio amplifier? Only one way to find out...

Why the name ValveLiTzer? These tubes came from the tone generator in a defunct WurliTzer organ.

There are a few web comments (re: are 12FQ8's appropriate for guitar amp use?) but no one to my knowledge actually has to date. Certainly more complex applications are possible.

See they next page for info on procuring the tube (unless you find an old WurliTzer...)

Step 2: Parts

See step #4, The Build, for a definitive list.

But here's a quick rundown of parts:

-- a metal case
-- one 9-pin miniature tube socket (normal size for a preamp tube like a 12AX7, etc.)
-- one 12FQ8 tube
-- (2) 1/4 in. mono phono jacks
-- (1) Audio-taper POT (500K)
-- (1) Linear-taper POT (50K)
-- a few mylar capacitors (mylar for signal, ceramic or other for bypass.) Or polypro, polyester, etc., for the signal is fine, too.
-- several 1/4 watt resistors
-- one footswitch, ON/ON variety

-- one power supply (battery or 9V-13V supply)
-- a jack for the power input, or a battery clip

Obtaining 12FQ8 Tubes

There's been some negativity about using this tube. Although these are not common, they aren't hard to find or really expensive. has them currently for $5.50 per tube. They are usually easy to find on Ebay, too.

Step 3: Design

Nothing terribly unique about the design. It's fairly standardized preamp type of circuit, but uses an oddball tube at very low voltages.

Due to the low voltages, the plate resistors are quite high compared to normal values.

Pot #1 (P1) is a simple voltage-divider volume control. An audio-taper pot is best for the volume.

Pot #2 (P2) sets the bias, and effects the overall character of the output. Playing with it changes the gain, and the compression level, too. A linear-taper pot works well for the bias.

The 0.1uF bypass cap (C3) is a small, conservative value. Anything from 0.1uF to 10uF might be substituted--larger values will boost the bass, and the volume of the effect... I used a small cap (tantalum? :-P ) here since the value is small, but an electrolytic can be substituted if larger values are desired. A 10uF was tried initially, but was too farty / bassy. 1uF might be a good choice, also.

In theory, the 12FQ8 might be capable of some very funky effects. However, I doubt these could be achieved easily at low voltages.

Step 4: The Build

Here's a suggested layout / wiring diagram.

Most of the components can be soldered directly to the 9-pin tube socket. It really simplifies things. It's also very compact, if that matters.

I integrated a 3.5mm jack for power, as an external supply works best for me. There's no on/off switch, but the make/break type of phono jack can always be substituted and used as a switch (on when plug inserted, off when removed.)

The bypass switch just routes the signal around the circuit. The tube continues to draw current even when switched out. This bad for batteries, but the tube needs 8-10 seconds to warm up, so it's the only practical choice (and typical of most stompboxes.)

It's a little difficult to find this type of footswitch. It's an ON/ON SPST switch. Any switch made specifically for guitar FX boxes will do. It's not quite a "true bypass," as it doesn't switch out the input resistive pad...

Step 5: Updated Wiring Diagram (V0.4)

Here's an updated version of the ValveliZter with the "gain" POT, and a true bypass switch.

There's a PDF included, as before.

If you see a problem, let me know... ;-)

Step 6: Add a Stompbox Case...

I have a box of old bulk film cans, and this project is small enough to fit easily. Like any "stompbox," this should be built in a shielded metal box.

Pilot holes were drilled and then enlarged with a handy "step" drill bit.

One tip: to integrate easily with other stomp boxes, the input should be on the right, output on the left (mine is opposite, oops.)

If batteries are the preferred power source, then a larger box will be preferable.

There are several ways to protect the tube from damage. One great way is to install draw pull or handles on each side. I haven't incorporated that into the design. The can is little small for handles. But if you're gigging with this F/X, the bare tube is a bad idea.

Step 7: Powering Options

This unit will work with a DC voltage between 9 - 13V. A higher voltage will FRY the tube filaments (this is a standard 12.6V filament tube.) Max voltage for tube filaments is the rated voltage, +-10%. So 13.86V is the absolute maximum voltage before the filament burns out (and I wouldn't run any tube filament that high, regardless of the specs.)

Tube veterans will of course understand that higher voltages can be applied to the tubes if the filament and the plate voltage are separate. If so, the cap voltage ratings must be adjusted to handle any higher voltage, also.

I've been using a regulated 13V power supply, and the unit is very quiet. If an unregulated wallwart is used, expect a lot of noise... A variable regulated supply would be just the thing.

To be honest, the effect is a little more tubey @ 9V, although the boost effect is less.

9V batteries don't last long, however. The unit draws about 135mA @ 9V. I wouldn't expect a 9V battery to last much more than an hour with that current draw.

NiMH AA rechargables would work well. 7 or 8 NiMH cells should do nicely.

If a regulated 9V to 13V supply isn't available, it can be built easily. An LM317 regulator is perfect for this task. Since it's output voltage is adjustable, it's better than a fixed regulator--as noted above, the source voltage changes the effect somewhat...

Or for that matter, a car or golf cart battery would work well...

Step 8: Possible Mods

This circuit is really a first step. I wanted to emulate the feel of a bluesy tube amp, not make a "fuzz," per say. There's a very noticeable bump in gain, and a nice 70's rock type of sound, as-is.

But you might want something else... If the over-the-top fuzz is your thing, it can be modded to "clip" the signal even more.

More punch and distortion:

-- Increase R4, and R3 also.
-- Greater capacitance on the coupling resistor C1 and C2. I originally had C2 @ 0.068uF, but there was a loss of clarity (not a big deal if max distortion is the goal.) Just increasing each to 0.02 will have a noticeable effect.
-- Add an FET booster circuit to the front end.
-- Larger cathode-bypass capacitor (C3). NOTE: this has been confirmed--see the end pages for updates, included a higher-gain mod with a larger bypass cap...

Less gain and distortion, more pure tube sound:

-- Drop R3 to 220K

More Adjustability:

-- R4 could be replaced with a 2M POT.
-- Or, replace both R4 and R3 with a single 2M or 3M POT.
-- A tone control or two could be added.

Other ideas...

This would make a nice front-end for a LM386 miniamp, too...

Step 9: What's Next?

As I have more than 25 of these tubes, what's the next step?

Obvious, no? The ValveLiTzer II !

Yep, two tubes, four stages of tube preamp goodness...

Step 10: New Gain Control, Version 0.4

Here's an update to the project, which adds quite a bit of controllable gain. The previous version was shooting at a "pure" tube sound, but I've had requests for more power !

Upping the bypass cap alone (C3) really adds gain. I've specified it as 100uF, but any value from 22uF upward will add gain. A POT (P3) has been added to adjust the additional gain.

C3 can be upped without adding the POT, of course.

The older "Bias" POT is still in place. The two together can be adjusted to suit...

C3      100uF, polarized electrolytic, (16V minimum) 
P3      50K audio taper potentiometer