too much gain at PI?

[iknowjohnny]

heres a issue that i have ignored for the most part because the amp is mainly a high gain rock amp. But i would like to be able to turn the gain way down and turn the master (pre-PI master) up and have a clean sound. But the problem is, when i do that the preamp is clean as i can hear, but only until somewhere between 12:00 and 2:00 on the master. then the clean goes away and the amp distorts like crazy all of a sudden. So i guess the PI is getting slammed too hard. But this is with the preamp gain turned as low as it can go. The PI is configured like a typical marshall with NFB and 470R cathode with 1 meg leaks and 10k tail. I have tried many other values but the same thing happens. I have no scope so i can't figure it out that way, but does anyone have any suggestions as to what i might look at?

The amp is typical JCM 800 in most of the key aspects such as PI values and cathode follower with typical tone stack values. The PA is now cathode biased but that doesn't matter because it's done this always even when it was fixed bias.

EDIT: i guess the preamp is just too gainy. I tried lowering the gain in the pre and the issue subsides to a great degree, tho not totally. So i can lower it more and the issue WILL go away. What i am assuming from this is that because i am generating a ton of gain without using as many stages as most amps of this amount of drive, the LEVEL in the pre is higher than it would need to be in a amp with more stages. so it's high enough to get as much gain, but the level itself has to be too high to allow the PI and later to remain clean with the master up. Does that make sense?

Anyways, with less gain in the pre i still have enough to keep me happy but the issue is much better tho not perfect.

[Jana]

need accurate, up-to-date schematic of the entire amp.

[iknowjohnny]

I can't....at this point it's too far from the last one i made and would take more time than i have right now. But see the edit in my initial post, as you posted at the same time i posted that edit.

[chief mushroom cloud]

raise the B+ on the PI......Marshalls (and voxes, etc) are notorious for PI clipping before power tubes

but you have to compensate next power supply dropping resistor...
w/o a sch of what you have.....how can anyone help you?

[stoo]

How about a lower gain tube ahead of the PI. 12at7 for example.
Stew

[iknowjohnny]

Actually i realized i had forgotten that i changed some PI values. Changed them back and the problem is much lessened. It's still a bit harsh but nothing like it was. I may still try your 12ax7 idea tho stoo. good idea. I will likely also jack the B+ up at the PI and just compensate at the next node like cmc said. as for a schematic, like i said it will take time and i don't have a lot of that right now.