I've had a US made Fender Pro Junior from '94 for a few years but not played it much until recently. It's loud and has great tube sound, especially for a small amp but has always had a strange sounding harmonic as the note or chord deteriorates. I've removed the middle chassis screw from the back panel because I've heard that can be an issue. Today I removed the springs and bracket that keep the power tubes in the socket. That I think, actually helped. Has anybody heard of that? Are there any alternatives to keeping the power tube from falling out of the sockets? BTW it's got fresh J J tubes in both pre-amp & power amp.