Hey all you circuit junkies. I have been slowly working on putting together an amp for a friend who is a harmonica player. He had played his green bullet through a champ circuit I built and loved it. I thought I might built him something a little more flexible and harp focused. I chose a 5F2a for the added tone control and bought one of those 15 watt upgraded OT from Classic Tones before they closed. I build these by depopulating old radio amp chassis and wiring the schematic point to point inside. It's a bit of a brain teaser but it's yielded some very sweet sounding and stable amps. So far this Harp-amp ain't one of them. It's quite prone to feedback, without having much of that singing, crunchy midrange distortion that sounds good with an amplified harmonica. When you plug a guitar in it sounds nice at lower volumes and has pretty decent headroom, but once you turn it past half way the distortion you get is not good sounding. It's choked, there is no sustain and it cuts out noisily like a gated fuzz. I've tried a number of different tubes, I've played with the voltage on a variac, I put the NFB resistor on a potentiometer so I can set it anywhere between 10k and 60k. I've tried .1, .022 and .01 coupling caps. Nothing makes this sound better. So... What would you do next? Does that stepped up OT compromise the overdrive?