Introduction: The ValveLiTzer: Low-voltage Tube Booster

Picture of The ValveLiTzer: Low-voltage Tube Booster
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

Picture of 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

Picture of 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

Picture of 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

Picture of 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)

Picture of 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...

Picture of 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

Picture of 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

Picture of 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?

Picture of 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

Picture of 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


ShadowN2 (author)2017-09-19

Hi there,

This looks fantastic, perfect for a first project with Valve Amps. I have been looking for a low voltage pre amp that will have little gain (less Voltage) but that will colour the sound.

Will this work in a Hi Fi setup? I am going for a cleaner sound with little to no distortion, is this possible? Would this work in a Hi Fi setup? If possible, what changes could I make?

Kind Regards,


asfetcu (author)2017-05-02

how wire 3PDT bypass switch ?

oldsoulsound (author)2016-12-30

Would using a Voo Doo Labs ISO5 9V power supply work in this circuit?

gmoon (author)oldsoulsound2017-01-09

It should be OK. (sorry for the delay)

liimu (author)2015-11-03

First, nice little project this. I made one, v0.4. But I have some issues. When the Valvelitzer is on, the pots on my guitar gets skratchy (only) when I turn them. Second, the gain pot does not work at all. Bias pot - very (very) slightly. But the volume pot works, and I get a darkish gain-boost. Would like the boost to get a little brighter, though.

The tube is a NOS GE, and the fillaments light up a little. I have tried three of these tubes, but the gain pot does not work. Tried both with HB and SC guitars. Triple checked the wiring, and everything seems ok. Have you experienced some of my 'symptoms' before?

As the sound of the pedal:

It does not give a huge boost in sound, but it works very well stacked with a cleaner brighter pedal after it. A clean-boost of some sort. I have a Lovepedal Englishman in clean boost setting that vorks well. The Englishman is a bright pedal, and the Valvelitzer ads warmth and grit to the sound, but still cleanish on full volume.

I also have a Tube Driver from Tube Works on my board. The valvelitzer before that pedal brings the Tube Driver (with low gain on the TD) into fuzzy/warmish/broken-speaker - kind of sound, but all controllable. Sounds very cool on brighter/lighter-sounding single coils.

With medium gain on pedals after the Valvelitzer, the noise level is on the higher side, as when I stack the Valvelitzer with the Tube Driver. Noise level with the Englishman in clean boost setting is exeptable. Defenetly sounds moore 'tubey'.

As power voltage concerns, it sounds better with 9v. 12v makes the Valvelitzer moore 'farty', and the basic caracter of the pedal is lower. As the constructor aimed for; it outputs warm tube sound. I was supprised. The pedal is a little noisy, but it is well worth the effort. Cred to gmoon.

If I only could get that gain pot to work..

gmoon (author)liimu2015-11-04

Cool, thanks for the feedback!

One thing about the gain pot -- it doesn't do much at one extreme of the bias setting. I'd expect it to be as the bias POT nears zero resistance, 'cause on the low end of bias the gain POT would do...well, nothing (cathode would already be grounded).

I've had the thought of revisiting the circuit, maybe picking a fixed value for the bias so the results would be more predictable.

I don't know about scratchy POTs on the guitar, other than adding a coupling capacitor in front of the effect. In case some DC is leaking from somewhere (shouldn't really happen straight from the guitar, tho).

Anargopunk (author)gmoon2016-08-22


another way for a gain POT is connecting pin 3 to the tube's pin 9, POT pin 1 to ground and a 100uf cap between POT pins 1 and 2 (see image, srry for the use of MS Paint), this works verry good for me, u used it in a transistor/tube hybrid pedal, but i used a trim pot, and another pot for changeable gain

Francois1313 (author)2016-03-08

i got 12ba6,12 be6 and 12av6 tubes .would those be good for that build???

gmoon (author)Francois13132016-03-10

One of those is a pentode, one a heptode and the third is a triode / diode. None will work with this circuit, but all are worth experimenting with...

Anargopunk (author)gmoon2016-08-22

use the pentode and the triode for the two gain stages, the pentode will sound heavy, since the pentode has a far higher mu (gain) factor than the triodes in this schematic

holla2040 (author)2016-07-09

OK, this project is too cool. I love your film can upcycle, love it! I just picked up a Wurlitzer for $17 and now have a ton of 12fq8s.

gmoon (author)holla20402016-07-11

Awesome, thanks. Make something cool!

holla2040 (author)gmoon2016-07-11

Been looking into univibes.

Francois1313 (author)2016-03-08

what would be the wiring if i use a 3pdt swith and a green led ligthÉÉÉ

gmoon (author)Francois13132016-03-10

Just wire it like any standard "true bypass" pedal. Make sure you have the correct resistor for the LED, as this one is 12 to 13VDC.

Francois1313 (author)2016-03-08

what would be the wiring if i use a 3pdt swith and a green led ligthÉÉÉ

Francois1313 (author)2016-03-08

i got 12ba6,12 be6 and 12av6 tubes .would those be good for that build???

Nate Palsa (author)2015-05-30

Such a cool project! I've been looking for a small 12vdc unit to run in front of a class-D amp for a while and this one looks like it'll work wonderfully! I would probably opt for the low gain mod as I am just trying to add a little tube coloration to a small boombox. Therefore, opting out of the volume pot since the chip amp will handle that. The bias pot seems like a nice feature to retain however.

What would you recommend to use this with a stereo input signal? Is the 12FQ8 capable of that or do I need to use another tube?


gmoon (author)Nate Palsa2015-05-31

Thanks! I'd probably recommend you go ahead and have an adjustable gain and a volume control, but via trim pots that you can set internally and then leave as-is. Or set via fixed resistive dividers (breadboard it for the correct values). This approach will give you the best signal-to-noise.

To retain a stereo output, one of these tube preamps per channel (two separate preamps) will be needed.

If the Ible doesn't make it clear, definitely use a regulated supply... Good luck!

PaulK8 (author)2015-03-17

Sorry if this is a silly question, but would this pedal work well with my solid state amp? Or does it need to run through a tube amp, like a tube screamer?

gmoon (author)PaulK82015-03-17

YMMV. It will work with a solid state amp. As to whether you like how it sounds with a particular amp or not, that's subjective.

I'd guess that it wouldn't add much hair to the sound of SS amps. A little, maybe. A boost over unity, yes. Some tubeishness (whatever that is), yeah. But the Tube Screamer analogy is a good one, probably.

PaulK8 (author)gmoon2015-03-17

Ok, not what I thought you were gonna say hahaha, I thought you were gonna say that the pedal is meant for SS amps, to be like a tube amp emulator if you will.

If I were going to make one of these It would be for loads of fuzzy fuzz. Check out a band called Conan on youtube, that sort of super fuzz. I guess it would prob work for that more so than for the hard driven bluesy sort of sound.

gmoon (author)PaulK82015-03-18

This probably isn't the pedal for metal. I listened to Conan for a bit, and that's a more modern "scooped" metal sound, more common for post thrash and speed metal.

I'm not even sure what pedal would get that with a SS amp. You'll have to experiment. Distortion is a strange beast. Often times what sounds good at bedroom levels solo, sounds like crap at high volume with bass and drums.

My own taste in metal runs toward doom, and back to early Sabbath, etc., records...

chris.patrick.9461 (author)2015-01-27

On step 4, you refer to an SPDT switch as an "ON/ON SPST switch". By definition, that is not an SPST switch. I feel like that could trip up some less experienced makers.

gmoon (author)chris.patrick.94612015-01-28

That's probably correct, although SPDT doesn't describe the switch either, as there isn't a "dual" position for the actuator--it's a footswitch, and you can't know the current state of the switch visually.

Maybe "Latching double throw" would be best...

chris.patrick.9461 (author)gmoon2015-01-30

Why would those characteristics make it not count as a Single-Pole, Double-Throw switch? Great instructable btw; I'm building one now.

gmoon (author)chris.patrick.94612015-01-31

It does describe it electrically, but still doesn't adequately describe the switch used, 'cause it's latching, not double-throw.

Thanks! I think I need to revisit this one and play with the circuit a little for fun...

bmostert (author)2014-10-24

Hello! Just built the 0.4 version, this is my first stompbox project and it was a lot of fun! :) I got about half of it working, I need some help getting the rest of it to work as im really new to electronics. My switch is dead on the off position and on the on position (or maybe its the bipass?) Only the 500K audio pot works. Even when I touch the leads on the bias and gain pots I get nothing (no noise). Id really appreciate any help.

gmoon (author)bmostert2014-10-27

Hmmm. The gain pot shouldn't work in bypass mode at all, but if the signal is going through the tube (your tests suggest is isn't) something is wired wrong.

You worked off the wiring diagram? First off, does the filament glow? Check and recheck the wiring...

CarterBond (author)2014-08-17

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

than89 (author)2014-07-04

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)than892014-07-07

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.

robot797 (author)2013-12-01

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

cmpmuller (author)2013-11-20

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)cmpmuller2013-11-21

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.

fastcar123 (author)2012-11-24

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)fastcar1232012-11-24

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...

fastcar123 (author)gmoon2012-11-24

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.

fastcar123 (author)gmoon2012-11-24

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

fastcar123 (author)2012-11-24

fastcar123 (author)2012-11-24

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

fastcar123 (author)2012-11-24

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

fastcar123 (author)2011-12-22

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)fastcar1232011-12-22

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:

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

--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...)

fastcar123 (author)gmoon2011-12-22

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

gmoon (author)fastcar1232011-12-22

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. ;-)

fastcar123 (author)gmoon2011-12-22

can you send the link for that. for some reason my computer wont open it properly

gmoon (author)fastcar1232011-12-23

It's a PDF. Try a "right button" / "Save Link As".

The link itself:

Here's a scrn capture of the pinout. And the other tubes in that family (12ax7, 12at7, etc) share the same pinout.

fastcar123 (author)gmoon2011-12-23

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)fastcar1232011-12-24

It's a start. The "codes" on each pin are a help. The tube is split internally in to the 1st and 2nd valves.


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.

(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.

About This Instructable




Bio: Go sit in the Faraday cage and think about what you've done...
More by gmoon:Isolation transformer upgrade for old guitar ampsThe ValveLiTzer: Low-voltage Tube BoosterGuitar Tube Amp
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