OK John, I see where you are coming from now,
Re the connection of the pot, yes I accept what you are saying, although I have several similar power supplies used for my Hifi to power large valve filaments.
In twenty years I have yet to see one let the magic out (smoke) The theory is sound though.
We have no need for a regulated supply here, after all we are only powering a motor, nothing critical. So the zenner is really overkill in this application whilst at the same time it becomes a limiting component. I'm more concerned with back EMF when switching the motor off. Normally one would use a reverse diode, but difficult here because we have a reverse polarity switch. I need to think some more on this. However, it may not really be a problem. More "correct" theory.
Simple is always best

If I were to really want a proper regulated supply I would probably use a 317 as a variable voltage device to reference a 2n3055. Or as I have done in the past a pair of 2n's if the current demand warrants it. Sometimes it does. Valves with heaters at 10V/10A can be pretty demanding to build a supply for, esp when you have four of them to supply.
Remember, we don't want to feed them AC and this is where the problem starts.
However,
This thread was not intended to be an in depth electronic topic. It was about making a simple variable power supply for those with no real electronic experience.
I believe I have shown what can amount to not only the simplest method to achieve this, but the cheapest as well. These are important factors as this is not an electronics forum. Guys here mostly chop metal, not solder.
This thread was deliberately never intended to become technical, that really scares peeps from trying altogether. Quite honestly we have discussed things that didn't belong in this topic.
If anyone other than John and I are still reading this, what I have proposed is really quite simple and cheap. Ignore all the other stuff, none of it applies to what we need here.
I have now powered the mill with this power supply and motor. I was going to show you but my phone battery went flat. I'll take a video tomorrow.
BTW, nothing too exciting as I'm holding the motor by hand, but I have made a nice linkage in delrin....

You should be able to make this supply for about a fiver, tenner at the most if you are unlucky.
If anyone decides to go down this path, there is always help available. Even if I have to do it for you !!