PI cathode resistor value

[iknowjohnny]

I generally see PI cathode values usually in the 1k range going down to 470R and up to maybe 1.2k. But i never see much higher values. However, in my high gain pre equipped 50 watt marshall style build i tried a few larger values up to 3.9k, and it seems like the higher i go the smoother and richer the amp sounds. I'm now going to raise it again and keep going till i hear a decrease in tone quality then back off to the last good sounding value.

My question is, why am i hearing improvements by going to values larger than i have ever seen?

[Jana]

Is this at low volumes or at higher volumes where you are pushing the amp?

Have you check the outputs of the PI on a scope to see what is happening?

Checked power output of the amp?

My guess is that the PI is distorting sooner. Raising this resistor would change the bias point of the grids. Right?

[iknowjohnny]

This is at any volume up to the point i play it at, which is about as loud as a small bar gig. I would be surprised if it's because of PI distortion tho because i hate PI distortion. besides, doesn't a higher cathode R make for less gain ? it does on gain stages so i figure on the PI too, no?
But my question is also why am i hearing an improvement when i never see other amps with such large values there? It's not ME. In other words, it's not like i have some odd definition of good tone. My idea of good tone in a amp of this type follows the general consensus, so it's not that.

[Firestorm]

The cathode resistor in a long-tailed pair mostly sets the bias for the PI. The place you may want to experiment is the "tail" resistor. The higher this goes, the better balance you achieve between the two sides of the inverter (which can reduce PI distortion due to imbalance). But if you get too high, you'll kill your headroom. Just another thing to play with.

[iknowjohnny]

Thanks for that FS. I haven't messed with the tail forever....at least not since 3 or 4 different incarnations of this amp ago. i just swapped the 10k with a 33k and preliminary listening seems to really sound/feel (more feel even than sound) good. But i need to listen more and swap the 10k back in there a few times to A/B it real good. It seems much more than subtle, but i don't trust my ears after a day of tweaking. I'll hear better tomorrow first thing. But thanks....this may have been a great change for the better.