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RE: Alex...

"I'm still not sure I understand the need to split the secondary."

The original secondary is probably 9 turns, for an output of 12V.
After installing the cap across the rectifiers bridge, the output is going to rise probably to 16.68V (this depends on various factors, but can be assumed as a rule of thumb to simplify the calculation).

Now 16.68V are too much for 10Y, aren't they? Instead of burning voltage across resistors (more unnecessary heat), and adding a second unit for the other tube, we can do a simple calculation: 7.5V + 7.5V = 15V.

This means that we already have the voltage and the space for two windings, the only thing we need to do is split the original winding into two (now separate) windings. While doing that, we are going to loose 1 turn, which turns out to be beneficial, since 16.68V/9 turns = 1.853V per turn.

This leaves us with 2 separate 4 turns windings, each of them will yield 4x 1.853V = 7.413V (which is an almost perfect approximation of 7.5V).

Not only have we avoided to burn voltage across resistors, but we are using the same original wire, which was good for 50W (i.e. 4.16A) so it must be more than good for our purpose (1.25A).

You could probably use the same 50W transformer for one 211, which requires 10V 3.75A -- in this case you would not split (i.e. cut in half) the 9 turns winding, but rather unwind the superfluous turns. Assuming the same unit and the same voltages, with 6 turns you are most probably going to get 11.12V - which is slightly too much for 10V, and thus would leave you with two alternatives:

1) Burn the excess 1.12V with resistors (0.3 ohm would suffice) which is rather easy to do, or
2) Count the number of turns on the primary and calculate how many do you have to remove in order to get as close to 10V as possible (usually 2 or 3 for Europe, probably 1 or 2 for US) by setting an adequate primary to secondary ratio.

Removing a couple of turns from the primary will not endanger the transformer. Take into account that we plan on using a 50W unit for 37.5W, so there is enough room to play and retain all the reliability.

NOTE:
In practice, most electronic transformers delivery slightly less than 12V, usually between 11.4 and 11.8 depending on the load - the higher the load, the less the output voltage. Since we are planning to use those units at less than rated load, their original output will most probably be 11.6-11.8V and from there should a precise calculation start. But assuming 12V instead leaves us in the worst case with slightly lower voltage, which is not going to endanger the tube: it is not so low to be considered as starvation, and when it comes to thoriated thungsten filaments, most are aware that 9.6 or 9.7V are a better option than 10.2V ...
******

http://rh-amps.blogspot.com/


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