12 V zener causes 1/2 volt too much?
Yay! I was pretty close predicting 11.65 V for the zener.
If you need to lose 1/2 volt, R1 only calculates to use up 1/2 volt at .05 amps.
Reducing R1 to zero would be necessary to trigger 1/2 volt sooner so that method won't work.
You're going to have to work with the zener to fine tune this.
If you can drop in an 11.5 volt zener, you're done.
If you can't get an 11.5 V zener, go lower and increase the output voltage by increasing the resistance of R1.

In other words, I'm with debe on this.

 

Wouldn't it be possible to use a TRIAC? This is an AC generator?

 

...You're going to have to work with the zener to fine tune this....

Thanks. I should have been ordered different zeners. I must check a local store if they have. Thanks to the screw terminals, it's easy to change and test by "trial&error" as debe said.

 

Wouldn't it be possible to use a TRIAC? This is an AC generator?

Why would you if an SCR does the job?

 

Why would you if an SCR does the job?

Yes correct, but I mean to use a TRIAC similar to a dimmer (lossless).

I have various Thyristors here and TRIACS as well, have built a small TRIAC circuit, which works, but have not yet found much time to build thyristor circuits.

As far as I understand here SCR is used as some kind of shunt, and my suggestion was to build a lossless regulation.

I don't know if the AC frequency is variable/fixed.

If this would be my own generator setup, certainly worth consideration/experimentation.

 

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I have various Thyristors here and TRIACS as well, have built a small TRIAC circuit, which works, but have not yet found much time to build thyristor circuits.

As far as I understand here SCR is used as some kind of shunt, and my suggestion was to build a lossless regulation.

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A Triac is just the equivalent of two parallel, inverse connected SCRs. A Triac in this circuit would no more or no less efficient than an SCR, the difference is that it can conduct current in both directions, rather than only one.