Hi I am building a (240volt in) 13.8 volt (out) PSU along the lines of this plan..
http://www.warc.org.uk/proj_psu.php it is very similar to the old Marchwood practical wireless unit from the early 1980's using 2n3055 transistors.

My question. The spec says the transformer must have a secondary of 16.5 volt. Now when I last built a PSU using a similar plan the transformer output could be much higher, in the region of 15 to 20 volts.

I have a Talema branded toroid I want to use, its spec is as follows (from maker)
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Toroidal transformer type 5028-P3S03:
Prim: 0-110V-230V-240V orange-yellow-violet-blue
Sec1: 20V/6A green-red
Sec2: 20V/6A black-white
Sec3: 55V/2,5A braun-braun
If you use only the 2x20V you can get out 9A of each winding (no load on the 55V)
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as you can see the secondary output is 20 volt, now folks do you think this transformer will suffice at 20 volts, or should I removed a little winding to bring it down?

73 Andy

PS the variac is just there for scale.