Thor oscillation

[m-theory]

I recently built Thor from this layout
http://www.aronnelson.com/gallery/main.php/v/Phils-FXs/ThorFinal.gif.html

It works beautifullly, and sounds fantastic...I really like it a lot, BUT, it's got this weird oscillation thing that I need to unravel.  It only happens with the high boost switch "off," and the gain at about 3:00.  Any other position for the gain knob, and any position at all with the high boost switch "on" doesn't create this situation.  Just this one particular gain setting, with the high boost "off." 

The noise is a high pitched "squeal" that's not terribly overpowering, but is definitely noticeable and annoying.  It seems like the kind of noise you'd hear with a tube amp build with wire layout issues, so that's where I dove, in order to find this problem and resolve it. 

I found that when I loosened the screws from the board mounts and allowed the board to free float, the noise continued, but the moment I touched pretty much anything on the board while touching the box at the same time, it would quit.  This would seem to fit into my belief that it's an oscillation-type feedback, since the additional grounding that I provided by touching the board and box at the same time made it stop. 

I then pulled the board up and away a bit, and reached in to move wires around.  I found that I could get the noise to stop when I had JUST the right placement for the boost switch wires.  Of course, the moment I re-attaced the board to the mounts, the noise came back.

I'm wondering if anybody else has had this happen, and if so, what was done to alleviate it.  Or, if not, what I might consider doing to resolve it.  It's really not a huge thing for me, since it's only that one very specific, narrow gain level setting, and only when the high boost switch is "off," but it's annoying to me that the pedal isn't 100%, and I'd like to fix it. 

[Uma Floresta]

I had this issue temporarily on my Umble. It went away after I tried some different JFETs. Could be a bad JFET maybe?

[MohiZ]

Have you used shielded wire? I always end up replacing most of the wires with shielded wire in my high gain distortion boxes, if I haven't done that from the start, to stop the oscillations. You could try using shielded for the switch wires.

[m-theory]

Could be a bad JFET maybe?

Fortunately, I socketed them, so it's easy enough to swap and see. 

Have you used shielded wire?

There's really no room in this build for shielded wire (both Thor and a 3 knob fat boost are stuffed into a BB box, so it's fairly tight), and frankly, this isn't an overly gainy circuit, so I don't think this is related to excessive gain.  It only happens when the high boost switch is off and the gain knob is turned down to about 3:00.  With the gain knob pegged and the high switch off, no squeal.  With the gain knob at 3:00 and the high switch on, no squeal.  It's just this one particular setting that's got the noise. 

[caspercody]

I just built the Thor, and also I am having a high pitch sound. When I turn the trim pot to the voltage specified for Q2, it makes a loud high pitch noise especially when I turn the gain down. So, I set the gain about half way and adjusted the Q2 pot till the high pitch went away. Now I am reading around 8.3v, which is almost twice the voltage specified. I would like to get it down to the specified voltage to get the true distortion from this.

Also, I did rewire this with shielded wire, no big difference. I did try switching transistors, no difference. I am using Fairchild J201, wonder if different manufacture would make a difference. Any ideas on that?

[Darkness, Darkness]

I think you should have a look and try the tip that head_spaz advised me for my Thor build. This might help to fix your oscillation problem
http://www.diystompboxes.com/smfforum/index.php?topic=76635.msg627796#msg627796

Good luck 

[caspercody]

Thanks! I'll give it a try.

[m-theory]

FWIW, I resolved mine by shortening the leads to the switches.  I had a bit of bare wire on one of the wires, that got clipped away when I shortened the leads, so I'm sure that's what was causing the problem.