In article ,
Andy GD1MIP wrote:

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)
+++++
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)
+++++


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.

By using it the way it is your only design worry is that you will be
dumping more power across the 2N3055 Regulator Transistors, and will
need more of them, for the same current output, due to Dissipation
Limits of the heatsinks and transistor cases, because of the higher
Voltage Drop across the regulators. Also recognise that you will have a
much higher DC Voltage going into the regulators, after some Capacitor
Filtering. Likely up near 25 Vdc, which means you will be dumping
roughly 50% of the provided transformer power in the Regulation System.