Oscillating DIY Blues Breaker...any suggestions?

[Rockhorst]

Ok, this is a bit of a long shot, but I'm hoping someone can help me out. I've been working on a modified Blues Breaker circuit, running on 18V, and designed a PCB for it that I had fabbed by OSH Park. I tried the circuit before hand on a bread board but it was oscillating, which I thought was caused by long wires to the IC. But I just fired up my prototype and it's oscillating as well.

I tried audioprobing the circuit and the oscillation is all through the circuit, the pitch responds to turning any of the controls. Interestingly, I got the loudest sine tone when touching the input jack, so I suspect that something is going wrong right in the beginning of the circuit. This is supported by the IC voltages:

Pin 1: 9V
Pin 2: 9V
Pin 3: 8V
Pin 4: 0V
Pin 5: 9V
Pin 6: 9V
Pin 7: 9V
Pin 8: 18V

Notice Pin 3 with a 1V drop compared to the others. I'd expect 9V there?

I really don't know what to look for. Everything seems to be connected properly as far as I can tell with my DMM. I checked the schematic that I made to the one at GGG and the Morning Glory project, and I don't see any faults (but may of course be wrong). Any and all suggestions welcome.

Schematic


Board

[Rockhorst]

As a quick test, I clipped off the second half of the opamp and the oscillations went away. So my guess is that it's the second part that's oscillating, supported by the fact that the frequency is influenced by both the gain and the tone pot.

[Rockhorst]

Could it be that C2 at the input of the second opamp stage is the cause? Should I swap C2 and R6?

*Edit* Just checked the schematic of the Madbean King of Klones (also a BB type thing) and that suggests that the order of C2 and R6 shouldn't be an issue.

[Rockhorst]

More tests, more info: it seems pins 5 and 7 of the opamp are working as transmitters? I came close to them with an audio probe (not touching) and it picked up the sine wave through the air. I'm guessing this should set off alarm bells for someone with more electronics know how than me...

The first part of the circuit, up to C2 in my schematic has been confirmed to work properly.

[jkokura]

I'm interested to hear the results on this. I've never been able to suss out oscillation.

Have you tried switching the IC?

Jacob

[oldhousescott]

The low reading at pin 3 is fine. Your DMM is loading the circuit at the point making the reading low.

I don't see a cap across R13 to make Vb a virtual ground. Instead Vb looks like 23.5k to (audio) ground. You can tack an ecap of 10-22uF across R13 for starters to see if that helps.

[Rockhorst]

You seem to be spot on oldhousescott. Adfding a cap there stops the whining. Now all that's left (hopefully) is to check it with guitar later this afternoon. Over at DIYSB someone suggested making it a split rail design. I have a few issues with the tone control as well. All in all, enough to do a little redesigning

[Rockhorst]

And it's working
Thanks all, on to the next version, not done with this thing.

[garfo]

How does it Sound running on 18 volts?i have a rc4559p and a opa2134 ready to experimental on my bb that would probably Sound good runing on more than 9 volts.