Blues Deville overdrive mod oddity

[Teleman55]

I recently upgraded a friend Blues Deville with some cap and resistor changes including making the prescence into a clean boost.I mentioned a mod to add V2B into the overdrive.Everything sounded great then I noticed that when the Bass control is turned up to 8 there is a fast low intensity vibrato,and as the control is turned up to 12 the vibrato slows while the depth increases.Nice effect and the owner loves it but I don't know what causes it.I told him that I threw it in as a freebie.

[Enzo]

Sounds like the amp is motorboating, and my guess is if you added some gain, you don't have it decoupled properly. The amp is feeding back on itself through the power supply. The fact that the control affects the level and frequency of the oscillation adds support to the hard wired feedback theory.

[Teleman55]

Thanks Enzo;I will have to study the diagrams a bit more,there was an option of adding a filter cap for this stage with the suggestion that it may not be necessary.Is this condition harmful to the amp?

[Enzo]

I doubt it will hurt anything but your pride. Scope the B+ on the top end of the plate load and see if it dances with the signal.

[drewl]

"dance signal dance"

I think we should all chip in and get Enzo a gift certificate to a nice restaurant or sumpin'