Bias adjustment for Marshall JCM 800

[lynx]

Is anyone familiar with the bias adjustment for a Marshall JCM 800? (combo head). I'm told that on this amp there is an adjustable pot that can be rotated to the desired position instead of changing out resistors. I only see one such item on the board- behind and to the right of the bass pot. My tech says you can tune this "by ear" since ultimately it's going to affect the way you like to hear it.(I know that you are supposed to use a scope or tester that goes between tube and socket and set to a specified value). Can anyone comment on this? I've got it open right now doing a minor repair and would like to tweak it with my old G.E's and new Svetlana's (6550's-both matched sets) for comparison.

 

Thanks

 

lynx

[myles_rose]

Originally posted by lynx:

Is anyone familiar with the bias adjustment for a Marshall JCM 800? (combo head). I'm told that on this amp there is an adjustable pot that can be rotated to the desired position instead of changing out resistors. I only see one such item on the board- behind and to the right of the bass pot. My tech says you can tune this "by ear" since ultimately it's going to affect the way you like to hear it.(I know that you are supposed to use a scope or tester that goes between tube and socket and set to a specified value). Can anyone comment on this? I've got it open right now doing a minor repair and would like to tweak it with my old G.E's and new Svetlana's (6550's-both matched sets) for comparison.

 

Thanks

 

lynx

 

Lynx -

 

There is a bias pot on your amp.

 

As far as adjusting by ear, until you know the max and min range of your amp, the current draw of your particular tubes, how close the two sets you have are to each other, your B+ voltage, etc., doing this only by ear is asking for trouble.

 

Remember, a race car runs best just the moments before it blows up ... and an amp is the same!

 

Amps like to run hot ... but it is too easy to under-bias a Marshall and have the tubes run way too hot. Bias too high, and it runs too cold, sounds grainy and no harmonic content.

 

Since you are in the area, you may want to drop by one of my free amp clinics. I can show you on the scope what your amp is doing, and I will look at your current draw and see if your amp's bias is at least in the ballpark.

 

I will repost my amp clinic info here in another post.

 

Regards