Here is my problem. Both kaiser and Helga have rather long necks which are cylindrical. So if I adjust my (leerburg bought Herm Sprenger) prong collar to fit comfortably behind their ears then gradually it slides down their neck.

yesterday I removed one link from each of their collars and fit it behind their ears. If they pull on the leash slightly (to go sniff the side walk) they start choking and coughing.

I see the Mals and the GSDs they seem to have necks that widen towards the base. But with my dobes the neck is almost the same size.

Am I doing something wrong here?

My problem is a collar adjusted not to cause discomfort slides way down their neck and if I remove just one link then they choke.

Have you tried a collar with smaller size links? The smaller links would provide smaller incremental adjustments. You might need to
purchase 2 collars to have a big enough collar.

Same to my mal puppy (7month). Looks like his neck is long and cylindrical too.
When I put it behind his ears he start to shake it down. When I remove a link he feels confined and acts like a bucking horse.
Since a bucking horse doesn't learn I added the link again.
It still works though.
Maybe not so effective but I'm ok with the results.

where can I get the smaller link one? I got the small herm Sprenger with out the quick release.

It would appear that Leerburg sells the small-link prong collars, too. You may need to buy a few extra links for it to be large enough to fit a Doberman.

I own Dobermans, too, and I can tell you that I have constant problems with the collar slipping down. As you said, their necks are almost the same diameter from top to bottom, plus, they have very short, slippery hair. It is my experience that you just can't take out enough links so that the collar doesn't slip. (Or if you do, the dog will be in constant pain, as you have discovered.)

One of my instructors insisted my collar wasn't tight enough and that I take out more links. I complied because she was the instructor, after all, but my dog ended up with welt marks all over his neck, so I never made it that tight again. (And the collar still slipped a little.) Of course, that instructor owns Belgian Tervurens, so I'm sure her prong collars never slip.

So it is my habit to fit the collar snugly, high on the neck, and just pay close attention to it and pull it up when it starts to slip down.

I have the same problem with my mutt. He has the short slippery hair as well. Glad to hear I'm not the only one.