Harsh oscillation w/ Jerkulator and Tillman preamp built into guitar, thoughts?

[Barn_Explosion]

Title says it all. Bear in mind that my rendition of the harmonic jerkulator omits the input pot (and accompanying 1k resistor to ground), has a couple 1N4148's and resistors as an asymmetric clipper section, and uses a 47uF emitter cap, rather than the 22uF Escobedo calls for. The transistors are a high-ish gain 2N3906 (hfe ~380) and two 2N3904s arranged piggyback style with a 10k resistor (hfe ~80)

The neighboring Tillman preamp it feeds into uses an NTE457 (though I did get a shipment of 5 J201s, so I'll probably swap it out and put one of those in) and an incoming 100k volume trimmer (á la the jerkulator's output pot) to ensure that max output from the jerkulator doesn't overdrive the fet. Still bumps up the signal output a smidge above unity gain.

I'm fairly certain it's not the circuit or the input pot (the guitar's volume), considering this never happened on the breadboard (when I just had it wired from the board to output with no volume). And I'm confident the volume pot's not faulty either; it works fine when the gain is maxed.

Other folks on reddit suggested better power supply decoupling between the two (100uF and 100nf in parallel to ground), or a small cap from the emitters to ground, in parallel with the 47uF. What do you all think?

Important details about when exactly I get oscillation (or lots of extra noise that wasn't there on the breadboard):

* The vast majority of the oscillation only occurs (and does so consistently) when the neighboring Tillman preamp is engaged

Preamp off:

* Oscillation occurs only when input is around 1-1/2 and output is 9 or greater

* Extraneous noise is in the signal when input is 1-1/2 or greater and output is past 7.5 (no noise at max input, though)

Preamp on:

* Oscillation occurs when input is 1-1/2 or greater and output is 7.5 or greater

* Extraneous noise is heard when input is 1-1/2 or greater and output is past 6 (lots of noise past 7.5, even when input is maxed)

Thank you for your help, it's very much appreciated!

[LightSoundGeometry]

I have a similar preamp inside my guitar ..I am guessing its a wiring issue ..they need to be spread apart a bit because of magnetism of two wires.

not sure, but you have to be careful inside guitar cavities, there is a lot room under the pickguard to work with and is left alone for a reason I am thinking..probably has to do with the pickups and its magnetic field causes interference of some sorts; or guitarist would have all kinds of on board effects ..I have a swimming pool route and coudl fit 3 pedals inside if I wanted lol..I tried a octave and got super oscillation like you are stating.

modified:

my preamp is bypassed like a pedal inside with a dpdt - otherwise coming right off teh volume it was very harsh as you words explain as well

one of the resident engineers will explain more eloquently  here soon

[Barn_Explosion]

Yeah, I see where you're coming from with that, but there were definitely guitars that did this sort of thing (check out the Vox Special series, in particular). Looking over the wiring of those things, I didn't even see any real power filtering! But from clips of those and other built in distortions/ preamps, I haven't seen this sort of oscillation, so I feel like it's not just inherent to the design. FWIW, I'm having these wired with true bypass switches, too (3pdt, after tone and volume pots).

[GibsonGM]

Well, the 'magnestism' mentioned is really called capacitive coupling (or inductive...).  Happens because the wires act like small capacitors with each other...and it really does have a major effect sometimes.     I'd totally go for decoupling the power supply! 

I don't think the cap changes you made would do much...if it worked ok on the breadboard but just not playing nice when close to each other, that is what I think you should look at!   Even wire routing, as LSG said.   

That said - too much gain all at once can turn an amplifying device into a really nice oscillator!  I'd maybe consider looking at finding the 'balance' of signal levels to make the 2 play nice together...try decoupling first.

[Barn_Explosion]

Turned out to just be a grounding issue (forgot the wire connecting the electronics to the shielding + bridge wire)! I still added the coupling caps for good measure, though (100uF and 100nF in parallel from +9V DC to ground). Thanks everyone!

[GibsonGM]

Wow, you got out of that easy, Barn, LOL!  Glad it worked out!  Can't go wrong with decoupling, clean power is a wonderful thing.