JCM600 Sound Problem

[jworle01]

Ok guys, I am not very good at describing things tonally related especially when it comes to amps. However, I will do my best. I have a jcm600 that has had the diodes bypassed through a 390k resistor as described on the metro amp forum. I thought it sounded really good but I have recently noticed this faint farty distortion sounds that is kind of like an after note. If you crank up the gain and strike a power cord it sounds good but then it has the raspy ripped speaker sound behind it. However, the speakers are in great shape (g12m heritage) and I do not get this on the clean channel. Could this be related to the diode bypass or does it just need a preamp tube replacement? I am 99% sure this is not tube rattle as it doesn't sound like tube rattle or microphonic problems I have experienced in the past. HELP!!!

[jworle01]

Maybe it is my imagination? I replaced all the tubes still seems like something is there. Plus this thing seems to be vibrating internally like crazy? I actually seem to only notice this little strange distortion with the gain turned down. It kind of has a buzzy farty kind of noise when you try to play clean but when you turn up the gain and dig in it is all Marshall and sounds very ac/dc (seems the resistor mod is working well in that regard?)

[jhdc]

I had a 600 - they had major problems with heat, and a lot of people added fans to help disapate some of it. does this problem occur immediately upon firing it up, or after playing for a little while? if the problem occurs after it heats up, chances are it has something to do with that.

honestly, i hated the distorted sound on these, but loved the sound of the clean channel dimed - classic marshall crunch, especially when you dig in.

[jworle01]

Originally Posted by jhdc

I had a 600 - they had major problems with heat, and a lot of people added fans to help disapate some of it. does this problem occur immediately upon firing it up, or after playing for a little while? if the problem occurs after it heats up, chances are it has something to do with that.

honestly, i hated the distorted sound on these, but loved the sound of the clean channel dimed - classic marshall crunch, especially when you dig in.

No it happens all the time. I need to do some more playing around but I don't want to take it apart again. I did the diode bypass on it so it does classic rock very well but has lost that 90's marshall buzz distortion. I just cleaned the input jack again and the problem may have disappeared but I need to plug in another guitar or two and see what is going on. I am actually supposed to be trading the amp possibly this weekend and I want to make sure it is in top shape beforehand.

[jworle01]

Bump