Trainwreck Express

[Oddvar R]

Just wondering why the presence pot gets very noisy when turned to full? Almost like oscillating?

[Colossal]

Just wondering why the presence pot gets very noisy when turned to full? Almost like oscillating?

When you say noisy, do you mean the amp hums or whistles? A whistle is oscillation. A hum might indicate a bad ground. The routing of the negative feedback wire from the speaker jack to the Presence pot is sensitive and a point of trouble for many that build these amps. If you are getting a scratchy, rustling sound when you turn the knob, that sounds like someone blowing across a microphone, that is OK as there is about 6VDC across the pot. The Express Presence 5k/0.1uF is wired like the old Marshalls and that's what they do.

[Oddvar R]

Ok, thanks.

[Roe]

oscillation might indicate positive feedback