Two Tube Pedal DIY Projects

[mshilarious]

If by chance you might have seen this thread:

http://www.homerecording.com/bbs/showthread.php?t=181098

You'll know that I've been playing with adding a full-voltage 12AX7 tube stage to a Peavey practice amp, for unknown reasons It was fun though.

As a result of a few unusual designs I had to incorporate to fit the existing Peavey PCB, I ended up with what I think is a pretty cool sounding tube stage. It has a range of tones from clean to dirty, and with the use of LEDs to limit and thus clip the signal, fuzztones as well, all controllable with a mere two knobs.

I thought the tube stage would be cool to peel off as a standalone DIY pedal project. I have drawn the schematics below, and I have a PCB design that I'll post tomorrow. I can also draw up a parts list if there is interest.

I've also been interested in doing something with a 6GM8 tube, which is a rather unknown tube that is the standard 9-pin tube size, but is designed to run at a very low plate voltage. I haven't tested this circuit yet, but I will build it in the next few days. I used the same basic design as my 12AX7, but with the much lower gains available from the 6GM8 I suspect it will have a more limited range of tones.

There are several advantages of the 6GM8 design: the big one is cost; without the high-voltage pieces, I estimate it would cost about $40-50 in parts, vs. up to $100 for the 12AX7.

The next is size--the 6GM8 PCB will be a mere 1.25" x 3", vs. 2" x 4" for the 12AX7. The 6GM8 can fit in nearly any pedal case you can dig up.

Because the power supply requirement is much lower, the 6GM8 would happily run off any 6V or 9V 0.5A AC adaptor you can muster, while the 12AX7 needs 120V AC power and will have an internal transformer.

You can also substitute another 12A_7 family tube and have a starved-plate design if you like.

Without further ado, in the next two posts I will show my drawings, feel free to comment as you like. These are now officially public domain, and cannot be copyrighted or patented.

Tune in tomorrow for the PCB design!

 

[invisiblenemies]

Why don't you stop monkeying around with these new posts & check your messages?

 

[mshilarious]

12AX7 schematic

One word of caution: Please do not attempt this if you are not familiar with the dangers of high voltages! Simply put, this pedal can KILL you!

The 6GM8 pedal, on the other hand, is very safe. Try that if you are less experienced.

 

[punkin]

Bring it!

I liked the amp mod btw...nice work!

 

[mshilarious]

D'Oh! I already noticed I left out something Hold on a minute . . .

 

[mshilarious]

Why don't you stop monkeying around with these new posts & check your messages?

Yikes! Thanks for the offer, but I don't have need for anything that big

 

[mshilarious]

12AX7 v1.1

OK, this time I mean it:

 

[invisiblenemies]

Yikes! Thanks for the offer, but I don't have need for anything that big

No problem. Good luck w/ the mods. Let's try to keep the electrocutions to a minimum.

 

[mshilarious]

6GM8 Pedal

The low-volt version:

 

[apl]

What's this sound like?

 

[mshilarious]

What's this sound like?

Man you are impatient! OK, OK, I know I said audio samples with the So-Cal last night. But my uncle visited, and then I spent the morning trying to finish the 12AX7 PCB!

Soon, I promise, soon . . .

 

[apl]

...audio samples with the So-Cal last night...

I mean with a Carvin. And with a starved 12AX7.

Never mind. Carvins always sound good no matter what.

 

[mshilarious]

I mean with a Carvin. And with a starved 12AX7.

Ain't got no Carvin. But let's back up--which pedal do you want to make? The one labeled 12AX7 here is not starved plate, see that +144V supply there? This is the real deal, at least for a preamp stage.

The 6GM8 pedal can be used with a 12AX7, that would be majorly starved plate but I don't know how that sounds because I haven't built it yet. So don't run off and build it until I do first

But lemme know which one interests you, because I will send you different parts

Anyway, I'm going to post the 12AX7 PCB first, as soon as I finish it, because that is a tested project.

Here are audio samples from the version of the 12AX7 built into my RAGE! Settings are post 2, all EQ on 10 (to take that out of the sound), no reverb, and pre setting on 3, 5, and 8, respectively.

This time I just flat out copped Harrison, instead of merely emulating his style:

12AX7 sounds

Grey Angel, if you are out there, this one is only a little over 1 MB

 

[apl]

I'm trying to find solid state 9V -> 200 or more.

 

[mshilarious]

I'm trying to find solid state 9V -> 200 or more.

Pedal board eh? If you're looking for a DC-DC converter, you don't need to go to 200V, 100V will do fine, just change the zeners to 24V. Also feed the 12V input straight from the 9V supply, although be aware the heaters in parallel draw 300mA, quite a lot really to drop 2.7V going into them for a single 1W resistor. This is why I left R18 in the 6GM8 pedal a little light at 5 ohms.

 

[mshilarious]

OK I *thought* the 12AX7 pedal circuit board would be really easy, since it was so much like the RAGE! mod. NOT! I had to do a total redesign to cram everything on there.

Here's the component layout:

 

[mshilarious]

And here is the copper layer. This is a 2" x 4" board, so this image is 260 dpi. The output from the PCB program looks nicer, so if you want, download the free Express PCB software from expresspcb.com & I'll send you the file.

I'll have lots of comments on construction and a parts list tomorrow. Two brief notes about the board before I head into the hot tub as my neck is pretty stiff: first, I call this the "short bus" version because all the holes are at least 1/16", since that's the smallest drill bit I have I can do a normal size pad version if anybody wants that. Second, I really had to stuff, so parts order will be paramount in a couple of places. Do the tube socket last!

apl has volunteered to be our guinea pig and build the circuit! I think. I hope. I don't wanna have to do it

 

[mshilarious]

12AX7 parts list

OK, this is the last post on the 12AX7, unless somebody needs help while building it.

First, I did make one change from the schematic to the layout; I swapped the two sides of the tube. Nothing you really need to worry about, unless you were tracing the circuit and trying to figure out what kind of crack I was smoking. It's all good

Parts:

Resistors: with a couple of exceptions, these are all common value 1/4W 5% carbon film resistors, available practically any supplier including Radio Shack. Please note the few resistors rated for 1W.

Capacitors:

C2, C3, and C8 are poly film box type, available from Digikey:

495-1118-ND
495-1103-ND

C1 is an ordinary ceramic.

C4 and C5 are metallized poly, 250V rated, I got mine from Parts Express:

027-400

Also from PartsX are C11 and C12, which are pricy:

020-656

for C10, C6, C7, and C9, I use the Nichicon UPW series, available from Digikey. 25V rating is good for all.

493-1811-ND
493-1832-ND

Pots: anything you like in audio taper. Pick out your favorite knobs too.

Other parts:

Tube, you're on your own PartsX sells the socket:

055-502

Power supply: 1N4003 is a common diode, as is the rectifier bridge. Both are available at Radio Shack as well as everywhere else.

Zeners from PartsX: 1N4753

Transformer: Digikey, 237-1256-ND

Transistors: Digikey, 2N5457FS-ND

Footswitch: Digikey, 432-1215-ND. This is a $13.59 part, but it's $10.45 each for 10, so lemme know if anybody wants to do a group order , cause I need to buy one for the 6GM8

LEDs: the LEDs on the board are the small rectangle kind, available from Radio Shack. On the PCB I left a "+12V out", that should be routed to the center lug on one pole of the footswitch, with one of the leads going to the R17 and D10, which are both off the PCB. This is the "Effect On" indicator. You could also add another resistor-LED combo straight off the +12V terminal as a "Power On" indicator.

Then you still need a box, fuse, fuse holder, and power switch

If you've gotten this far, I have a few leftovers for anybody who wants to try this! A 12AX7 tube I still haven't given away yet, the socket, the 0.1/250V caps, many of the other caps (not the PSU caps though), and the 1W resistors.

 

[mshilarious]

6GM8 Pedal

OK, here is the pedal I'm actually going to build. First, the layout:

Very minor modifications from the schematic I posted earlier. I rearranged the power supply so everything runs off the higher voltage--presumably 9V, except for the tube heaters. Should give a bit more headroom, and maybe more gain from the tubes I don't know because the 6GM8 datasheet doesn't provide specs for higher voltages, just 6.3V.

I'm not going to post a revised schematic until I build it and work out any other bugs.

Please note to fit this on a 1" x 3" board, nearly every single component has to stand up on the board, except for the caps, which as with the 12AX7 are poly film box types or electros (same part #s), other than the one 10pF ceramic which is also the same.

 

[mshilarious]

Copper layer is below! Again this is 1" x 3", 260 dpi.

Earlier I said that the 6GM8 pedal could also probably run a 12A_7 tube in extreme starved plate mode. That is not exactly true; the heater pins for the 6GM8 and 12A_7 tubes are slightly different. I will post another copper layer for a starved 12A_7 tube, you will have to decide which you want to build. I am not going to build the starved plate version, but I would suggest 12AU7 if you are going to try.