Long Tail PI: Later clipping

JJman

Member
Messages
994
I'm using a 470ohm > 20k > 5k-presence > ground for the PI's cathode of my build. LTPI type. It gets a strong signal on it's input. I want stronger clean outputs from it before the PI’s outputs start to clip. What new values should I try? Or maybe I'll need to increase the plate voltages by reducing the dropping resistor?

 

keiths

Member
Messages
172
What are you building? You could add negative feedback if you don't have it or try raising the 470ohm to 820ohm. Look at BF or SF Fender PI. What you described sounds like tweed values.
 

JJman

Member
Messages
994
I have some feedback but not much. The 6v6s are in Tweed Deluxe arrangement. The rest is based on the Trainwreck Express but with somewhat lower voltages.

Based on some scoping the PI starts to clip on the top and bottom of it's output wave(s) at the same time. I don't know if this means I must move to a higher PS voltage on it's node to get stronger output before clipping. I'm not familiar enough yet with PI's and all those resistors on their cathodes and grids.:messedup

If I saw this on a triode I would think higher PS voltage would be in order. Then a cathode resistor adjustment to "re-center" the bias.

Or should I mess with the plate resistors?
 

Jackie Treehorn

Silver Supporting Member
Messages
2,416
Assuming you get it up to Express voltages, 920 - 1k ohm on the cathode, 39k tail, and I like a 68k feedback resistor from the 8 ohm tap with the 5k presensce pot. You'll get a nice window of power tube crunch with the PI clean, but I wouldn't go more -31 bias volts.
 

JJman

Member
Messages
994
Thanx Jackie. This is what I’m looking for. I’m using adjustable cathode bias on the 6v6s but even running them hot (~12watts) wasn’t enough to clearly see them clip before the PI clips. I’ll have to play with the NFB too since I think I’m using a 150k or so. Very little effect from the presence because of that. I get PI distortion pretty soon on the dial as also confirmed with the ears when running the PPIMV on quiet settings. The 3rd stage clips after the PI on the volume dial so this change should open that 1st window that I think I’m missing now.

Thanx again.
 




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