Vox Super Foundation Bass problem

[rf7]

I've been banging my head against the wall on this one.

Preamp Schematic

Everything works, except when you are playing in clean mode, with the distortion off, the output distorts on louder low notes if the distortion pot is cranked.

The distortion control is on the collector of the second transistor from the left in the distortion channel. When the distortion switch lines are open, that transistor has 0 volts dc on its legs. When the switch is closed, you have 0.7Vdc on the base. I'm seeing the output of the first stage clipping early when the distortion pot is turned up with the 2nd transistor turned off.

I have re-capped the whole amp.

Any ideas why that pot and transistor is affecting the gain of the first stage? I've lifted the pot wire from the 2nd transistor's collector and the problem is eliminated (of course).

The amp sounds great except for this one issue.

Thanks in advance.

[loudthud]

Have you tried another transistor in the 2nd position? You might just be seeing one of the shortcomings of that circuit.

[rf7]

Yes, I have tried a replacement transistor in there and it did not make a difference. And yes, I was afraid that it just might be the way the circuit works. Oh well. Thanks for weighing in on this.

[mozwell]

Try using input 2 instead of input 1. From the diagram input 2 is 100mV is suitable for higher signal levels on both the distortion channel & the tone X channel. If your bass has a high output it might just be overloading everything too much....