Tube distrotion stompbox with expression pedal?

[TheHairesy]

Hey guys, I'm new here and this is my first post.

I'm an audio engineering student at SAE and I have to finish my final project in a few months. For my project I came up with an idea for a distortion pedal with an expression pedal built in so you could swell from a clean tone to a distorted tone. Now I have no experience whatsoever in electronics except for a pedal building workshop I joined, so what I'm planning is maybe a bit too ambitious, but nevertheless I want to try. (we had some lessons covering tubes and other electronics but never any practical lessons on it. I know how to solder anyway.)

I want to use 2 12ax7's if possible, there's a store pretty close to my house that sells them and pretty much any other kind of electronic components. Now I read a few threads on this site saying that most pedal using tubes don't have them running on the correct voltage, so I would like to know how to actually get 200-300 volts running through the pedal, or at least get the high gain sound I'm looking for.

I've never really seen a distortion with an expression pedal built in so for that I would have to make my own schematic and find a way to combine it with another if that's even a possibility.

So I wanted to ask you people some advice and maybe some good schematics to base my design off of. If it helps, the controlls I want to build in to this thing are: on/true bypass, 3 band EQ, gain and the expression pedal for the dry/wet controll.

I hope I gave enough information to get some advice. I hope to hear something soon!

peace!

[rankot]

Take a look at this thread, for example:

http://www.diystompboxes.com/smfforum/index.php?topic=93936.0

[TheHairesy]

Thanks! However the link to the dropbox file 404'd so I can't get the building instructions.

[GibsonGM]

Can you follow schematics, build from one?  If so, I have the info as I built the "GTFO" 1 or 2 years back. I use it all the time, it's a great preamp!

PM me for build info.   Welcome to the forum     

[GibsonGM]

I want to see your ideas for an expression pedal control for this.  I've always intended to do the same but haven't  yet.  Sort of like the mechanical version of what they do on Digitech pedals and such - that was always very useful on stage!   Vactrols or something.   I've been using a 1M pot for V3 input, that might offer a place to try something!   But that would probably be difficult to do well. 

So, maybe a more simple concept...monkey with a cathode bypass cap on stage 3, use an LDR/LED (diy vactrol) to vary resistance, and set the cap large enough to act like a gain control (1u?)....that would be a lot easier, it's not a pot (which is very very hard with vactrols, so they say).   Wah shell, use a pot in it to vary and LED's brightness on an LDR.   

[TheHairesy]

I met up with my uncle today who knows alot more about electronics than me. And we came up with the idea to replace the gain pot with the expression pedal. I probably will have to replace the electronics in the expression pedal to match those in the distortion. Or might there be a way to add a dry wet controll at the end of the chain? Cause I think if I assign the pedal to the gain pot it will also affect the volume when using it.

[GibsonGM]

So, are you going to try to run a "line" out to the pot inside the pedal, then 'return' it to the grid of the tube, like a remote pot?   It MIGHT work, but I think you'd pick up tons of noise.  Really hard to run audio over any appreciable, unshielded*  distance like that and expect it to come back without having picked up tons of garbage noise, EVEN if you use shielded cable.  Many have tried; many have failed.

* I am using "shielded" here as in, the wire should be inside a grounded metal enclosure to avoid acting like an antenna...running signal out to a remote pot, it's not...no protection, really...

So you could simulate a pot with LDR's, and control LEDs from a distance....but....
Pots are hard, because they are basically 2 resistors in series, where each change in PROPORTION to the other;  using LEDs and LDRs, or a couple store-bought vactrols, would be really difficult due to not being able to get that proportion right, thus your control could...suck and be weird....you may come close this way, but would need some complicated circuitry to dim one/brighten the other the SAME...balance is probably not attainable.

One thing that people HAVE done to solve this is to use a digital pot.  There are threads on here about it, and net searches will help, too.    You send a control voltage from an exp. pedal to a digital pot, which does the work for you....now, I don't know if this will work with the higher voltages present in a tube preamp, signal voltage could be like 60V or even more!   So you'd have to check that.     This leads to...

Solution:  You COULD build a more traditional Distortion Pedal, rather than a tube-based 'pedal' (I think of GTFO as a preamp, tho they all are in truth).   Run on 9V, use your exp. pedal to control THAT, get an "A+", LOL....adapting the tech to match high tube amp voltages might really be more like 2 or 3 projects in one.  Proof of concept, discuss that it can be expanded to other 'parameters' in other units....boom.

THEN build a GTFO and rock out!     

[printer2]

Thanks! However the link to the dropbox file 404'd so I can't get the building instructions.

The last page of the thread has schematics and layouts but make sure you have your antivirus up to date, it tried to infect me when I clicked on the schematic images.

[TheHairesy]

I had another Idea about connecting the expression pedal. I orderer a DIY DI box kit, so it allows me to split the signal at the input, then run 1 of the signals through the distorion circuit and the other one straight to the output. right before the output I would add a dry/wet controll so I could mix the 2 signals together.

[vigilante397]

Welcome to the forum

So were you thinking then of using the expression pedal as the blend control between your clean and dirty signals? Because that would be neat. If it were me I would want to add a clean boost to the clean side you can match the volume of the dirty side. Personally I think it would be really cool to do a pair of 12AX7's then use one half of tube #1 as a "clean channel" then the other three stages could be for the "gain channel," which runs parallel until the blend pot, which is on an expression pedal.

Well damn, now I want to build one

[TheHairesy]

I have a third 12ax7 I bought as a spare so I guess I could use that as clean channel. How would you suggest I do that? Also I'm a bit stumped on how to make the dry/wet controll, wiring and schematic wise. It would work in a similair fashion to a crossfader I guess?

[iainpunk]

Yeah, a dry wet control is basically a crossfader. Its quite simple to implement though. You put signal from the clean channel into pin 1 and the siganl from 'dirt' chanel into pin 3 of the potentiometer. Pin 2 is the output of the control. I highly recommend putting a buffer direct after this control since the impedance changes over the range of the pot (being the highest in the middle).

For the clean channel, id use a cathode follower running into a passive tone stack and into another cathode follower, this gives versatility to better add to the distorted tone. If for instance the distortion cuts a lot of bottom end, you can add them back in with the clean channel, for instance.

Godspeed

[TheHairesy]

Hey guys, so a bunch of things happened in my life that aren't what you call good but life is starting to look pretty normal again. I sat down with my dean and I got my deadline extended to September 2nd.

So to start off where I left, the last  thing I got around to doing was building the internal power supply unit since the ones I ordererd never arrived.
Now when I wanted to see if it worked I plugged 12V into my circuit and then started measuring what the PSU did, but instead f just ginving a steady 300V like it should the numbers on my multimeter just kept jumping around never reaching a high voltage. now I wonder what the problem is, I truly hope I didn't fry any of the components.

any thoughts on this?

[vigilante397]

What kind of power supply are you using? Can we see your schematic? Your layout? Some pictures?