Simple point to point tube distortion ideas

[Chrisfromiowa]

Hello,
I got a few parts in one of the members excellent grab bags that included a small preamp tube. I've seen a few very simple tube distortion units and am trying to figure out a good recipe for a point to point build. Any directions I should or shouldn't go? Thanks C

[merlinb]

Knowing what tube would help...

[Chrisfromiowa]

Yes, I guess it would, I thought I'd included it...  12AX7EH. Thanks Merlinb

[GibsonGM]

Oh, that one?  Man, hard to find!!  What can we make with THAT thing?

No, just kidding!  That is a very useful tube; the most common preamp tube in the world.  It is a dual triode, which is similar to say, 2 JFETs in one package.  So you can make a cascaded gain stage booster/overdrive type thang.

Look up "Valvecaster"!   Great one to start with.

[teemuk]

A tube with lower internal plate impedance and higher current amplifying capacity, such as 12AU7, would perform better in "starved plate" circuits than a 12AX7 but if goal is basically to generate distortion then I wouldn't worry too much what tube you actually stick to the socket. Getting that achieved shoudln'y be much of a problem. If it works too well just misbias it intentionally and call it a day.

[EBK]

I'm in the same situation, but I'll ask a slightly different question.  What if I'm looking for something mostly in the sweet and rich, slightly outside of clean, sound area with one of these tubes?  Possibilities?

[Chrisfromiowa]

I found this Valvecater build with a couple of mods, there's even a point to point version (my 16 year old daughter is obsessed with that idea ever since we replaced a bad cap on an old pedal with no PCB). So is 12volts absolutely necessary for a tube/valve pedal or just this variant? Thanks
https://cdn.instructables.com/ORIG/FXM/9B6H/IK1FLK5B/FXM9B6HIK1FLK5B.pdf

[EBK]

After some research, I think I might try to build a Westbury W-20 (at 266V).  Schematic from R.G.:
http://www.geofex.com/FX_images/westbury.gif

[thermionix]

So is 12volts absolutely necessary for a tube/valve pedal or just this variant?

The heaters (filaments) in the 12A_7 tubes are designed to run on 12.6v (series), or 6.3v (parallel).  So you need one of those voltages (or close) in your circuit.  People sometimes run the plates (anodes) on 12v along with the heaters to keep the power supply simple.


(parentheses)

[pinkjimiphoton]

imho if ya want any kinda possibility of tone, go with the highest voltage ya can. you need to run the heaters with 6.3 or 12.6v but the higher the voltage to the plates, the more headroom and less garbage noise ya can get. if all ya want is distortion, 12v is fine i guess, but i found on my builds i couldn't find a happy medium below about 48v on the plates, and even that was kinda gritchy sounding, but less noise and sputter and HISSSSSSSSS when it was cranked up.
if ya experiment with higher voltages, remember to over rate the voltage handling of your caps by at least 50%, if ya use low volt stompbox stuff it can cause some interesting results usually including letting the magick smoke outta some caps. smells terrible and quite a surprise when they pop, just how much crud is packed in there

[EBK]

This is a kind of curse we're getting ourselves into, isn't it?
Just like "dabbling" in modular synth circuits or building a simple fuzz circuit so you can then build the "next" better one ad infinitum.   

[pinkjimiphoton]

we're all screwed

[Djentronio]

Isn't there something about copious amounts of feedback for tubes because they're quite non-linear? Couldn't the sound we want be achieved by just removing feedback? More gain, more distortion.


http://www.theaudioarchive.com/TAA_Resources_Tubes_versus_Solid_State.htm

Nevermind. I guess transistors are highly nonlinear without "Complex circuits and considerable negative feedback required for low distortion".

I would've thought tubes are like FETs in terms of non-linearity, but maybe that's only outside of 'small signal' range.

[thermionix]

I think the very first circuit I tried to build for guitar was something much like what OP has in mind, one triode of a 12AX7 cascading into the other triode to make distortion.  This was over 20 years ago so I don't remember what I did well enough to know what I did wrong.  It oscillated, like a bad tremolo.  Maybe I didn't decouple properly, I don't know.

[GibsonGM]

I think the very first circuit I tried to build for guitar was something much like what OP has in mind, one triode of a 12AX7 cascading into the other triode to make distortion.  This was over 20 years ago so I don't remember what I did well enough to know what I did wrong.  It oscillated, like a bad tremolo.  Maybe I didn't decouple properly, I don't know.

LOL, that can be one fun result of playing with gain stages!   Power supply issue?

For the OP:  a triode gain stage is just that...a small amplifying stage (small at low voltages - at high voltages, where things like 12AX7s are designed to operate, really...you can be amplifying quite a big more).      If you run one gain stage into another, you are multiplying the amount of gain you apply to the original signal...so, a gain of 10x in the 1st stage, 10x in the 2nd = 100x gain, theoretically.  Til you reach the limits of the power supply, at least.   You can control how much gain you feed the 2nd stage with a pot or resistor divider so that you're not running b@lls to the wall all the time....tweaking the amount of signal going to each successive stage is part of 'voicing' these things properly.    Eventually, you'll want 4 or more stages   

The 'magic' of tube circuits is in the way you tailor the frequency response of each stage, as well as in between stages...where you operate on the load line (and yes, tubes ARE quite linear until you hit the 'bounds' of the power supply/biasing, often on purpose), and what you use for a supply voltage.   One look at a load line will show you that you get best performance when you use a higher voltage - but you CAN run on 12V with some pretty good results, to get up and running in the tube game.   Then later, you make the nice 300V tube preamp.

Merlin has a lot of info on his "Valve Wizard" site, I recommend you go read some!  (Google him)   But for now, a valvecaster type circuit will get you going, safely, without the high voltage risks until you learn some more.  And will probably get you more curious, which is a good thing!

[thermionix]

Power supply issue?

Maybe.  What I do remember about the project...I had purchased an old military tube commo *something* box from a mil-surp shop.  It had an appropriate PT, with HT and filament secondaries.  I don't remember the rectifier situation, but I'm sure I used whatever was in there, pretty sure I even tried using the original filter caps, tube socket, and maybe some coupling caps.  I was pretty clueless for sure.  I thought it would be cool to have a "real tube distortion!" in a big heavy duty military-looking box.  I probably trashed the whole thing after my experiment failed.  Well, I think I saved some parts.  Pretty sure the speaker binding posts on my stereo came from that thing.  I might even still have the PT in a box somewhere.  Hmm...maybe I should look!

[GibsonGM]

Sometimes the 2nd set is better, y'know?? LOL, you may find it was just a bad cap or something!!   Or being young maybe didn't have your wire runs clean enough...coupling....the things we do when we don't know better, ha ha!

I threw something like 30V into a transistor base (oh, that was some biasing!) with no base resistor when I was starting out.  Glad I wear glasses    

[thermionix]

Well that PT is long gone, but digging around I found a cool OT that I forgot I had.  Allen/Heyboer TO35MT.  Like a Pro Reverb OT.  When did I buy that?  Hey I've got a PT and a choke too...

Also found a roll of 60/40 solder.  It's a good day!

[Chrisfromiowa]

So, after going through .000001% of the information on valve casters on this site alone, it seems like I can achieve Big Muffish distortion with as little as 12v, so I'm going to give it a try... And I'll be sure to name my first born after y'all..... Thanks again, regards CfI

[Ice-9]

Nothing better to build than the 'Bajaman Real Tube'. or keep it for a valve replacement spare for your real valve amp.