This project is a bit off for me, treading into dark waters where I have not gone before. Someone on JoeNet dropped a description of an ultra-linear PP setup that I had not seen before (which is not hard honestly, if you check all my amp projects). His name is Bill, and he knows his stuff. I tried to figure out what he was talking about, wrap my mind around all the points he brought up. Seeing as I am curious about pentodes at the moment, mainly small signal tubes, I thought it would be a nice addition to also look at a different output stage than those I have built so many times.
The UL setup would require me to have an OPT wound specifically, and I really don't need any power for my full-range or compression units, so the PP part kind of seemed overkill. There are advantages like not having DC magnetize the core, needing an airgap, and probably a few other things as well.
As a first soiré into output pentodes and Ultra-linear and cathode feedback (SE OCD kicking in now) I will try a single-ended UL aproach. Initialy I wanted to use either an EL84 pair in PP, or SE with a single EL34. I really only need 1-2W at most, and even a single EL84 should handle that. Then something tingled and I though; what if there was a directly heated pentode out there. I'd have S.E. and directly heated, while also using a pentode in UL. Best of both worlds.
Now I know this other gentleman that has played with just such a tube, and he has shared his love for this tube. The tube in question is the 4P1L, a russian tube that costs peanuts on ebay. I remember reading several of his posts on using it as pretty much any stage in an amplifier, even using the grids as an anode (this guy is nuts!) ;) You might know him too, his name is Ale.
Actually, the final drop in this bucket to get it rolling was when I found a set of 4P1L tubes all manufactured in 1972, the year I was manufactured as well. This is fate!!! This has to happen!
As I have a metric ton (almost litteraly) of other stuff to build first, I will post a concept schematic here and get back to it later when I have more info to share. Bias points for both tubes are not final yet, so don't build this and blame me when your tubes melt ;)