Instructables
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 better...it's same audio track.
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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...)
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CarterBond2 months ago

I think I shall try this. I have Red and Orange metal project box, it should go nicely in it.

than893 months ago

Hey everyone, hope all is well!! Just built this bad boy (2pot version).. It seems to work fine, but the bias pot doesn't do anything but stopping the sound when i turn it all the way up (the valve looks brighter too). It also takes a few seconds for the sound to disappear, few seconds for it to appear again when I turn it back down... Any suggestions? I'm clueless.. Thanks a lot

gmoon (author)  than893 months ago

Not sure what's going on here...

The bias pot won't appear to do much if the gain is low--which you can of course hard-wire so it's low gain all the time. I'd suggest looking at the wiring of both pots to be sure it's correct.

robot79710 months ago
after lisening to this verry verry carfully
it sound like the base is a little bit to much distorted
it sounds nice
heck it even sounds like a old style amp
but a little distorted
cmpmuller11 months ago
Hi ! I've built this according to the V.04, and it works great..But :-) in a chain, I have a true bypass pedal after it, and unless it is on, or unless I have another buffered pedal after it in the chain, I lose a lot of clarity and output with the valvelitzer engaged. Do you know the input impedance ? (I saw that the output impedance is around 30-60K, any way to lower that? ) I am running a tillman preamp first in the chain, then 2 more true bypass pedals and a buffered one. Somehow the impedance chain seems to be messed up this way.
gmoon (author)  cmpmuller11 months ago
Hey, cmpmuller.

Yeah, I'd guess the output impedance is something like 60K, based on similar values for triode stages (even though this is an oddball tube). But a typical triode gain stage in a fender has something like 40K output impedance. So it's a tube thing--unless there's a cathode follower after the gain stage to drop the impedance, it's gonna need a high input impedance after to maintain the high end.

If you're OK with buffering, you could add an emitter follower (transistor) or an opamp after the tube to drop the output impedance.

Another thing to try would be a "brighness switch", which would add a small cap from C2 to the wiper of the volume pot (connected by a switch so it could be switched out). Actually from the top leg of the volume POT to the wiper. Something in the 100 to 250 pF range. That would boost the highs without lowering the output impedance, at least when the volume control isn't at the max.
fastcar1231 year ago
January 1st 2012. For some reason it won't let me copy any links right now.

The only difference is I deleted the jack (including the resistor on the input) and p1. I also grounded everything to one place on pin 5. The sound works when the circuit is off but when it's on I get no sound output
gmoon (author)  fastcar1231 year ago
Did it work before you deleted the volume control? Or were you still trying to get it functional? But you were trying to build something similar to this, right? But with a different tube I think...

As far as the Valvecaster, you should probably go to one of the forums posts where it's covered in depth. Lots of examples and pics there. They can offer more help than me. Remember, I've never even built one...
I was really wanting to get this to work with just one tube but It looks like if I'm going to build the valve caster I'm going to have to order a new set of components.
I never had a volume control for several reasons but mostly just tto simplify it. I want to build a circuit similar to yours with a different tube yes.

I'll go there but I'm far from tube savvy I just know how to solder a circuit together
fastcar1231 year ago
http://www.beavisaudio.com/projects/ValveCaster/
fastcar1231 year ago
If you could tell me the values for all of the components for the pcb- less 1 tube design on bevis audio then I could build that one instead
fastcar1231 year ago
If you could tell me the values for all of the components for the pcb- less 1 tube design on bevis audio then I could build that one instead
fastcar1232 years ago
Would it be possible to build this exact circuit useing the "Valve Casters" 12UA7 tube?
if not what is the differance between the 2 circuits that make the the tube not interchangeable?
gmoon (author)  fastcar1232 years ago
You could make something very similar for the 12AU7, but not exactly. And there's no way to guarantee the results because the 12FQ8 is a different tube, with different internal geometry.

There are some similarities--both tubes can use pins 4 & 5 for 12V heaters. No changes there.

Both tubes contain two valves; the 12AU7 contains two normal triodes, the 12FQ8 is something else (but I'm still using it as two triodes). Still, each tube's pinouts are different enough to require changes:

12FQ8
--Dual plates for each valve (four total).
--A single, shared cathode for both valves (one total).

12AU7
--One plate per valve (two total).
--One cathode per valve (two total).

To convert a 12AU7 to something similar:

Since the 'litzer simply ties both plates together on each valve, then the single plates of 12AU7 should substitute for the dual-plates wired together. It will simply require one plate per valve rather than two. One pin, rather than two.

Likewise, tie the two cathodes of the 12AU7 directly together. This will simulate the single, shared cathode. So the single cathode pin becomes two pins.

None of this will necessarily give you the same sound, though. Consider it an experiment (much like the ValveLiTzer was). Frankly, if you compare the two projects, they aren't that different (low voltage, dual triode amplifiers). Once you convert this project to use a standard triode tube, the schematic probably won't look that different from the ValveCaster...

(I'm not going to look up and compare the pin numbers of each tube; you can figure that out yourself from the datasheets...)
I notice the Valve Caster project calls for 2 tube in what looks like a sereis circuit, however VallveLitizer calls for 1. so in order to combine the 2 plates I would need something like this?
I have basicly no experiance with any sort of tube system so i am probably wrong with this diagram but i do want to try it but unfortunatly i wasnt able to get a hold of any 12FQ8's
Updated-wiring-diagram-V04.1.jpg
gmoon (author)  fastcar1232 years ago
Both the tubes are "dual"--two valves in one glass envelope. Both projects use two stages of amplification--as you say, in series.

How you draw a tube in a schematic is up to the designer. Often the two valves inside a single tube are drawn far apart on the schematic. It's tougher to do that with the 12FQ8, because of the single cathode--hey, it's a strange tube! So it might be harder to see there are two stages in the ValveLiTzer, but they are there...

Here's a datasheet for the 12au7. Compare the drawing with the 12FQ8. Your schematic--and your wiring diagram (the image you edited is a wiring diagram, not a schematic), MUST use the pinouts of the 12AU7, 'cause that's the tube you're using. So start from scratch, the pins aren't ordered quite like the 12FQ8. There is some overlap of the pins between the two, though.

On the 12au7 datasheet, you can see the two cathodes are pins 3 and 8. Those need to be wired together.

So--can you find the two plates on the 12au7 drawing? Their (2) pins will replace the four pins on the 12FQ8.

(I want you to figure some of this out yourself. ;-)
can you send the link for that. for some reason my computer wont open it properly
gmoon (author)  fastcar1232 years ago
It's a PDF. Try a "right button" / "Save Link As".

The link itself:
http://frank.pocnet.net/sheets/127/1/12AU7.pdf

Here's a scrn capture of the pinout. And the other tubes in that family (12ax7, 12at7, etc) share the same pinout.
12AU7.jpg
ok let me try to decode this

1- annode pate 1guitar input? {hot})
2- no idea
3- cathode of plate 1 which should go to pin 6?
4- VDC +
5- Common cathode (ground)
6- annode of plate 2
7- no idea
8- cathode of plate 2 (output to guitar jack {hot})
9- no idea

so im not entirely sure about this but i might be somewhat getting the hang of it. let me know what you think about it?
gmoon (author)  fastcar1232 years ago
It's a start. The "codes" on each pin are a help. The tube is split internally in to the 1st and 2nd valves.

12au7

1) 2P -- 2nd Plate
2) 2G -- 2nd Grid
3) 2K -- 2nd Cathode
4) H -- Heater
5) H -- Heater
6) 1P -- 1st Plate
7) 1G -- 1st Grid
8) 1K -- 1st Cathode
9) HT -- Heater Tap (center of heater filament, for 6V operation)

The grids in a tube are the "control"--voltage change here varies the amount of electrons that fly from the cathode to the plate. The grid is like the base of a BJ transistor, or the gate of a mosfet. So a grid would generally be the "input" of a circuit.

12FQ8
(numbering "sides" (1st & 2nd) in the same order as the 12au7)

1) 2nd Plate A
2) 2nd Grid
3) 2nd Plate B
4) Heater
5) Heater
6) 1st Plate B
7) 1st Grid
8) 1st Plate A
9) Cathode

There's no HT (heater tap) on the 12FQ8. That's OK, we don't need it.

So keep the heater wiring, as-is. Substitute the single plates for the dual plates (use one instead of two pins per stage). Connect the cathodes together.

Notice that the grids are the same pins on each. And that at least one of the plates on each side of the 12FQ8 coincides with a plate on the 12AU7.

The cathode pins are completely different.
12AU7.jpg12FQ8.jpg
so if my understanding is correct the the wireing diagram for the 12AU7 tube should look something like this?
12AU7 tube Valve guitar design.jpg
gmoon (author)  fastcar1232 years ago
Getting close.

R1 and R2 are swapped. 47K to the grid, 1M to the GND.

You're missing a plate load resistor on stage two (R4 on the original schematic, 1Meg). So there should be a 1M resistor between the plate and V+, pins 6 and 4.

Pin 9 (HT) shouldn't be connected. Don't use it at all.

Beyond that, I don't see problems (doesn't mean they aren't there, I just don't see any ;-). Good luck--there are no guarantees here, but in theory you should get output...
alright. so ive removed the 9th pin compleatly, put in the resistor between the plates and swaped R1 and R2
how does this look to you?
12AU7 tube Valve guitar design.jpg
^ right here ^
gmoon (author)  fastcar1232 years ago
Looks OK to me. I might have missed something, though.

But so long as you keep the voltage below 12V you shouldn't be able to fry the tube, so mis-connections can be corrected.

(Just don't use or touch Pin 9 with power, or you CAN fry the filament--that's the filament center, so it becomes two 6V filaments rather than one 12V.)
ok ill keep you updated when i start the project
fastcar1231 year ago
I built the circuit according to my circuit I drew up earlier minus the volume potentiometer but when it is on I get no output. Any suggestions?
gmoon (author)  fastcar1231 year ago
Hey--I missed some of your posts, sorry. Do you have the circuit schematic posted anywhere?

Without seeing it, my first guess is that you might be missing some coupling capacitors between stages. Subsequent stages might be overwhelmed by DC offset without coupling caps.

Other than that, can you test the different stages (tube, SS, etc) separately? Did everything work before you added the tube?
fastcar1231 year ago
I plan to build this circuit for an amplifier internally on the preamp stage. if the tube isn't touching anything would it be able to take a bit of wear and tear? (jostled around, rides in the car in different sitting orientations, being dropped, etc.)
gmoon (author)  fastcar1231 year ago
Within reason, tubes are probably more robust than most people think. I'd be leery of dropping the effect, but if you use a tube socket equipped with a shield, that helps.

Some folks use draw pull handles to make tube guards, to prevent the tubes from being stomped. You can even build the effect with the tube mounted inside the box; just have some decent ventilation holes...
Actually I thought that this project would be perfect for an amplifier I'm building. I would just mount the tube internally on some sort or metal raised platform as for ventilation, what would decent ventilation for a wooden box that's about the size of a normal amplifier head?
gmoon (author)  fastcar1231 year ago
For a wooden head cab, I'd make sure the tube socket was mounted on something metal, of decent size to sink away some heat, and then have ventilation holes above and below the tube. The holes wouldn't need to be huge, just big enough so air flows out from the top, and in on the bottom.

In a normal sized amp head, you should have plenty of room to mount the other components away from any hot spots.

Peavey had a whole line of amp (called PAG, "Parallel Axis Geometry") including the Triumph and the Bravo where all the tubes mounted inside the chassis. They needed a fan to keep it cool, but IMO it worked very well (I own a Bravo that was converted to a head).
upon opening my amp head I realized that when I made it I didn't plan for there to be any extra room needed so I have wires and PC boards scattered about inside it. so I wondered what would happen if I mounted it on the side inside the head rather than on the base or if I mounted it on the inside of the top piece. does the orientation effect lifespan, sound, heat variance, and the ability to change the tube out if needed
I plan to mount it on a square u-shaped piece of stainless steel that is raised above the wood about 2 inches
The amp head is actually already built at this point but I put a reverb pedal in it that I would rather trade out for tube overdrive. Could I just mount a fan on the back and have it connected to the same 9v walwart power source?
I can mount it on the opposite side of the transformers
Just getting started building some solid-state amps -- but this looks way cool. I'm ordering parts and want to get a decent regulated 12v adaptor. Does anyone have any data on average current draw ? Thanks
gmoon (author)  SleepySamSlim1 year ago
Should be approx 180mA @ 12V...
jgafford2 years ago
Finished making V0.4 earlier today, still got a lot of hum but it's tolerable. I'll put up some pics in a few days.
I'd like to label the pots, but not sure what the correct terms would be. Gain, bias, volume, tone..etc?
gmoon (author)  jgafford2 years ago
Cool, hope you like it.

It's pretty much Gain, Volume and Bias (which is very subtle, but interacts with the gain).

If you don't have a quiet power supply, you might add the Huminator from Beavis Audio, as Jon suggests below...
jgafford gmoon2 years ago
I'm already using a regulated power supply set to 12v, but the huminator sounds like a good idea. And I was more asking which pot did which function.
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