what does this mean?

[iknowjohnny]

I have tried using a cross line master in the past several times on this amp and just tried it again with the same results as always. that would be that the amp gets very thin and ratty sounding with a lot more distortion than when using the pre PI MV. I added the cross line then turned the pre PI master up so that it's out of circuit except for acting as a 1 meg ground ref at the PI grid. using the cross line as master, the sound becomes as described.

I think this says something about the circuit that may not be right BEFORE the PI, but thats what i'm asking you. any ideas what may be happening? the amp is a high gain marshall style build with a high gain pre similar to a JCM 800 but without the cold biased 2nd stage. It's not the same even then, but the amp is similar with a 800 style EL34 fixed bias output. Anyways, it makes sense that the PI is getting slammed and that must be whats causing it, yet i have seen other amps with high gain preamps that have post PI masters. Any ideas you have may be helpfull. thanks.

[Alexo]

the amp gets very thin and ratty sounding with a lot more distortion than when using the pre PI MV.

So you are basically isolating the pi distortion, in a sense, or at least you are able to compare the tone of pure preamp distortion to the tone of preamp + pi distortion.

When you introduce the pi distortion, the amp sounds ratty to you.

Seems to me that you're just not into the sound of pi distortion (you are not alone)!

yet i have seen other amps with high gain preamps that have post PI masters

Without specific examples, I don't think anyone can speak to this, there may be other factors involved.

I can tell you that pi dist. does have its place, in the overall cranked tone of a non-mv amp, you have preamp distortion, pi dist., power amp dist., power supply reactions, speaker distortion, etc., all contributing to the overall tone and feel, but when you isolate one or two of these, things naturally sound different from the gestalt effect.

I suggest playing with the value of the cathode resistor on your pi, that will change its distortion characteristics a bit, anything between 470 and 2K2 is fair game, imo.

[iknowjohnny]

Thanks, i'll give that a try.

[Jana]

I tried the post PI master, once. I remember thinking, "and people LIKE this sound?" I sure didn't, I took it out right away and have never used one since.

[iknowjohnny]

I agree. But i think theres something in my design (and the one you tried) that makes it not work right. Because theres not a chance anyone would like what i heard ! I've been playing for decades and what guitar players like and don't like is second nature to me. And i can say w/o a doubt that the tone i'm talking about is one of those things that most all players would consider horrible. Like blocking distortion. How many players would like that? Mayeb one in 1000, but surely not the majority, and it's the majority that seems to prefer the master after the PI. therefore i have no doubt theres a reason for this tone.

anyways, i tried a 2.2k at the PI cathode and while it didn't cure that sound with the crossline, it DID improve the amp's tone. And by the way, the amp sounds great with a pre PI master, and i had no intention of using a PPI one. the only reason i tried it is to see if that ratty sound is still there to give me an idea if all the tweaks i have done since i last tried it may have improved whatever it is in the amp that causes it. i feel that whatever does cause it may also improve the overall amp's tone if i were to find and cure it even tho i'll not be using a crossline.

[David Root]

I've used a dual 250K pot across the line post PI MV and it sounds fine. It is a design that has been on this forum some time ago, a modification of one of Ken Fischer's designs. The dual pot is expensive (I use PEC) but worth it.