"out of tune" feedback

[dave g]

Hey guys, I just fired up an express for the first time this afternoon and it's sounding pretty darn good! Everything is working as a 'wreck should, except for one thing: sometimes, not all the time, the feedback will "go sour" for lack of a better term. You can be playing and then hold a note and let it go into feedback, but sometimes a really bad sounding note starts layering on top of it, and at other times it sounds like there's some intermodulation distortion going on. However, it all goes away when you mute the strings. Swapping tubes hasn't seemed to help, and the lead dress is a mirror image of Fran. Right now I've got an Amperex in v1, Eis in v2/3 and winged C EL34s. Anybody else been here and have any advice?

[lewilson]

Check to see if the phase invereter plate resistors are on the correct side. 100k to the side with the presence control and the 82k to the other side.

[strato17]

Check to see if the phase invereter plate resistors are on the correct side. 100k to the side with the presence control and the 82k to the other side.

Ditto this, in one of the preamp board layout diagrams, the 82k and 100k are not in the right position and need to be switched. I also made this mistake and had to switch them. However, I dont know if this will fix your problem.

[dave g]

You were both correct - I had the PI plates reversed.

Thanks for catching that!

[GainMaster]

Hey guys, I just fired up an express for the first time this afternoon and it's sounding pretty darn good! Everything is working as a 'wreck should, except for one thing: sometimes, not all the time, the feedback will "go sour" for lack of a better term. You can be playing and then hold a note and let it go into feedback, but sometimes a really bad sounding note starts layering on top of it, and at other times it sounds like there's some intermodulation distortion going on. However, it all goes away when you mute the strings. Swapping tubes hasn't seemed to help, and the lead dress is a mirror image of Fran. Right now I've got an Amperex in v1, Eis in v2/3 and winged C EL34s. Anybody else been here and have any advice?

Feedback is the result of sympathetic ringing of other guitar strings in tune with the original note. That is why it goes away when you mute other strings. It has nothing to do with the guitar amp except that the amp needs enough gain and compression to properly amplify the sympathetic ringing of the guitar strings. You may have to change your playing style some and deaden some of the strings when an offending note appears.

Brian

[bigtone96]

What you are describing is just one type of feedback. Other types of feedback can be generated internal of the amp thru the chassis or other components. Oscillation when the treble and presence are turned up is one type of undesirable feedback. Other types of desirable feedback greatly add to sustain and harmonics.

***Sorry, I just read where the original poster mentioned it went away when he muted the strings. My mistake.***