Hey guys..

I have a 79 Marshall JMP 50W MKII Lead head. It has only the 2 inputs high and low. Anyways it has 6550's Svetlana winged.I replaced the 3 sovtek 12ax7s with GTs (just before the Mullard series came out )I havent taken care of this amp very well over the years and its the only amp I have. I am ssure the bias is off. I am not too worried because those 6550s are huge, however I am concerened it might be effecting the sound. Can non biased tubes cause a harshness? I have always liked my base clean sound to be fat. I like the amp a little below break-up and prefer to use my volume knob on guitar and pedals to bring out the drive sounds. Anyways I usually adjust the bass kinda high and keep the treble and mids rolled back some. Well lately I have noticed that the treble and mid knobs on my amp seem to cause a biting irritating sound if I put them above like 2. Should I replace my 12ax7s. I had a bud tell me he puts a 5771 tube or something weird like that with his Fenders. I have no idea what to try. He also suggested trying a 12aw something or other instead. I am confused here and looking for any suggestions. I really am pretty lame when it comes to knowing jack about the inside of my amp. I am not looking to get a clean sound the amp is not capable of or anything. I understand its a Marshall not a bassman. But I know it didn't used to sound this harsh. Any suggestions about tubes, settings, etc would be greatly appreciated. I use a Les Paul mostly on the bridge PU. Its a Duncan jazz. I run it out of a 4x12 cab, Many thanks!

Typically the amp will sound thin and brittle if the output tubes are biased cold and fat and warm if they are biased hotter. I would take it to a qualified tech and have it biased if I were You.