Hello!

I've got a Marshall Lead 12 (Stack Version) which has quite a strange "overdrive behavior" - I recorded it via Mic and Line out (mp3 attached - Mic+Line-out/ Mic only/ Line out only) - the "brzzzzzzz" is what i mean....

A shematic and rough information about the amp can be found here:
http://www.ssguitar.com/index.php?topic=2826.msg20791
http://www.drtube.com/schematics/marshall/3005.gif

Giving a singal to the power-stage only gives a clear and *loud* sound - This lead me to look a the preamp section:
Measured to Chassis/0V

C18: 17.8V
C17: -17.8V
IC1/Pin8: 13.4V
IC1/Pin4: -13.4V

From Tranny: AC 13.7V (x2)

Done so far (Problem & Voltages were monitored and stayed the same): New Bridge Rectifier, ElCaps, InputJacks, Gain Pot.

Do you have any advise for me?

Thanks in advance
Chris