Log of tracking down a whistle

[markr14850]

[First post here.]

I recently picked up a used Express clone. I had a an issue, that I've tracked down and resolved. While the solution was one I read here, I figure that it's good to write it up and post - just to make it easier for the next guy to find it via search.

The amp was making a high pitched whine/whistle/squeal/oscillation. This would happen without anything plugged into the input. The volume level didn't need to be up very high. Rolling down the presence and/or treble controls would delay its onset.

At first I was thinking microphonics. Picking it up off of the 2x12 combo-cabinet it was sitting on caused the whistle to stop, at least up to some greater volume level. However, various forms of foam and whatnot between the cabinet and the chassis didn't seem to do anything.

The speaker cables from the cabinet were pretty short. I made up some longer ones, moved the chassis a few feet away, and the whistle was completely gone, at all volumes. So, yeah, I figured it had to be microphonics.

But... At one point, I had the chassis upside-down and happened to move the speaker cable near the input section of the preamp. Weeeeeee.... The exact same sound.

It turns out, it wasn't microphonics at all. (There are some, but that's not this issue.) It was just an induction from the speaker wire in the 2x12 cabinet back into the preamp. (Someone here mentioned that they had a whistle into a 2x12, but not into a 4x12. This may have been the same thing, just the speaker wiring taking a path further from the input section.)

The solution, as has been suggested many times here: Make sure that the bottom of the chassis is somehow closed/shielded.

As a side note, I found it very interesting to make a sharply pointed loop with the speaker cable and "aim" it at various parts of the amp. The area around the bright switch was particularly susceptable. Looking at the way the switch is wired, it seems to me that at least one of those caps & wires are always an antenna.

Anyhow, a large sincere thank you to all that came before me and were willing to share!!!

--Mark