P channel for J-tube - details? photos? drawings?

Hey Folks!  Looking for more details on a P-channel for a J-tube RMH - I'm familiar with the basic concept, i.e, an extra piece of metal in the feed tube to improve air flow and prevent the fuel from blocking air. but I can't seem to find any details on design, fabrication, installing.  Any info and/ or links would be much appreciated!!

Thanks!!

Few nice ones in that thread https://permies.com/t/32099/doubling-power-RMH-cooling-feed
Few drawings here.

https://donkey32.proboards.com/post/2987/thread

But that's posh ones.

Thomas's version https://permies.com/t/60402/channel-explain#513664

Bother version  https://permies.com/t/25147/rocket-mass-proven-design-normal#200003

And another.

thanks for the detailed links Satamax, these are great!!  Just what I was looking for!

The post by Konstantin Kirsch mentions experiencing smoke in the room from the feed tube, is this pretty common w/ J-tubes?  can it be mitigated with a sliding door/ bricks, or even a bot of water like he uses?  My wife, who is all in on the RMH, is sensitive to smoke, so it's gonna be an important issue.

Hi Mark,

Each system is different as to how much they can smoke back. There are a lot of factors that seem to contribute to it which can be mitigated but from my view, not entirely eliminated 100%.
The biggest factor I have experienced with smoke back is lack of proper draft. Whether that be because the system was built to be too efficient (trying to extract too much heat from the chimney) has too much drag (from too many elbows or restrictions), weather conditions - wind, Delta T etc.
The second factor is whether you allow wood to stick out of the feed tube. Fine if your there to watch it, but sometimes it can bite you in the butt and the fire can climb up when your not looking!
A great example I remember was in Luke Perkins video comparing J tubes to Batch boxes. You were there.  
The third factor is how much you want to babysit your dragon. No matter what system you use to help control draft - bricks, sliding plates, water pot etc, your going to need to keep a regular eye on it. This is one of the main reasons I switched over to a batch box. I just love the J tubes simplicity and fiddling with the fire, but when I have work to do in and out of the shop all day, its not possible to watch it all the time. Even if you keep the wood short, one time or another, a piece of wood will get lodged beside another and just hang there in the feed and increase the risk of bringing the fire up close to the surface and increase the chances of smoke getting into the room.

Well, my opinion, having played with J tube a fair bit.

Batch boxes are better. Nowhere near as much tending, nor smokeback. Far more powerful. Plenty of design tricks to cook and heat. Even hot water and central heating possibilities for the one willing to invest in doing it right.

I would not mess with J tubes, anymore, besides outside cooking.

HTH.