DIY guitar amp ringing

[sa230e]

Hey everyone. I'm still pretty new to electronics but I think I'm starting to get it. I designed and
built myself a little 9v battery powered practice amp as a first project and I'm having some trouble
with oscillation, I guess. I'm hoping someone here can help me out. It sounds pretty good a low to medium gain but as I turn it up more it develops this fairly high pitched (still easily in the audible range though) noise which sounds kind of like a ring modulator effect which just kills the tone.

I don't have a scope so I can't tell what the frequency is or exactly where it orginates. It doesn't happen in LTSpice but real life is often different from a simulator as per Murphy's law of :p . I think it comes from the second gain stage as turning the tone pot to the bass side reduces it significantly. Actuall just touching the tone or gain pots changes the frequency of the ring. If I touch the input jack or even the surface (not the traces) of the PCB the ring stops but the gain is also much reduced which I've never seen happen before.

I built it on a fairly generic piece of prototype PCB and I'm just bench testing it right now. I did try putting a metal container over it to shield it but it didn't change the sound. I've attached a schematic. Hopefully someone can provide some insight.

Any help is much appreciated!

[J M Fahey]

You must ground your metal container or it does nothing.
Also this is a very high gain circuit, easier to have problems.
I would first plug the guitar straight in the LM386, play at will, and only after some time add one extra stage , going slowly step by step towards the input.

[sa230e]

Thanks for the reply! I didn't ground the container so I'll have to try that next.

I did plug the guitar straight into the LM386 and had no problems. This project actually started off as a Ruby (http://www.runoffgroove.com/ruby.html) and sort of evolved from there. The JFETs are not there for lack of gain but for the tone. I didn't care for the sound of the LM386 by itself when driven into distortion. The idea is for the distortion to come solely from the preamp. I designed it specifically so the LM386 would not clip even at full gain.

[sa230e]

Tested it out with the container grounded this time and it made a HUGE improvement! Thank you so much!

[sa230e]

D'oh!

So it turns out I had the input jack wired up backwards. Oops! I simply reversed the wires and now it's nice and quiet - even without shielding. With a problem this frustrating I knew the cause had to be absurdly simple!

Thanks again for you help, though. I'm looking forward to participating more in the future with this great community!