I had my 78 JMP 50 watt converted from 6550s and now I'm running the GT 6CA7s. Since the conversion, the amp has lost its tight low end, and has a piercing upper end that can't be completely dialed out. I've tried many different preamp tubes to tone down the harshness, but to no avail. Does the power tube conversion adversly affect the entire amp tone? I'm using the MV and playing at pretty low volume, so I wouldn't think that the power tubes are having that much affect (they're not working that hard). I've removed the cap on the MV and that helped some, but there is still some harshness. I'm thinking of having it converted back to 6550s as this amp was incredible before the conversion. One last thing, could new caps cause this? Any advice would be appreciated. Thanks.