Hi All,
Im a newbie, great forum.

Im curious if anyone can tell me the function of each of the preamp tubes in the Marshall TSL602 [JCM200 2x12 combo]
The tube layout is as follows when looking at the rear of the amp...[hope this translates]
[]-transformer 1-pretube 2-pretube 3-pretube/sleeve
A-pwrtube B-pwrtube 4-pretube []-transformer

Im most interested in decreasing the gain of the clean channel and increasing the gain of the dirty channels, just not sure which tube is responsible for which.

Any thoughts?
Thanks,
Topo

the closest 12AX7 to the input is the preamp 1 the next tube is the tone stack driver the 3rd is the phase inverter then your power (EL34) section. The preamp tube 1 has the most effect on the amps tone because it's the first stage that the signal hits, the tone stack driver has an effect also and the phase inverter splits the signal to send to each respective power tube also has an effect but less than the first 2. The power tubes take the signal and amplifys it to drive the speakers through the output transformer. I'm no electronics major but this is the most basic amp set up. amps that have more than 3 12AX7's use tubes to drive and recover reverb signal (usually the tubes in front of the phase inverter and after the tone stack driver in 5 preamp tube amps) and some utilize tubes for tremelo. If there is a big tube over by the mains transformer that would be the rectifier that converts the AC to DC for the amp to function. some an=mps have a solid state rectifier which is probably the case in your Marshall. Hope this helps.

Hi George,
Thanks for the 411...

RE:
I took the previous information and ended up with a hodge podge of pretubes and achieved some workable tones.
Now I have a problem where I cannot turn up the amp with out it squealing like a banshee on the both the crunch channels. [honestly Ive never cranked this amp so it may have done this all along]
At any rate, this happens even with the guitar vol off... Power tubes are new and freshly biased Svetlanas. Conversely the pretubes are whatever I had laying around. However, a tap test doesnt reveal any unusual crackling, ringing, etc....
Any ideas what it might be?
Thanks,
Doug