WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
If the OT has two secondaries, wired in parallel for 3 Ohms and wired in series for 15 Ohms, there will be no 3-15-tap.
Then you will have to take the 3 Ohms arrangement. You will loose some power, though ... but in general it would work this way!
What VEK says is true. But I think the OTs on this particular amplifier have three wires on the secondary side; Black - common, Yellow - 3 ohms, and Brown - 15 ohms. Try the speaker between the brown and yellow wires.
You need to remember that with transformers the impedance ratio is the square of the turns ratio.
Talking in terms of 4 and 16 Ohms
2 off 4 Ohm Taps in parallel doesn't change the numberr of turns so it still 4 Ohms.
2 off 4 Ohm taps in series doubles the turns so X4 of the impedance that is it becomes a 16 Ohm output.
the 3 and 15 Ohms mentioned for your transforemer are nominal impedances. The 3 is probably about 3.5 Ohms. It is extremely likely that the number of turns between the 3 and 15 Ohm taps is the same as between the Common and 3 Ohm tap. That means connecting a load between teh 3 Ohm tap and the 15 Ohm tap will just be another 3 Ohm nominal output.
If you wish to connect an 8 Ohm load I'd just hook it to the 3 Ohm output.
I had one of these amps - with (I think) the same output trannies. Do yours have ultralinear taps. If so then they are the same as I had. The Raa was about 6K and the Ultralinear Taps were at 20%.
I used the trannies in the ECL86 (6GW8) prototype which led directly to the "Baby Huey" design. I ran 8 Ohm speakers of the 3.5 Ohm tap
Thanks for all your input guys, I havent picked up the amp yet, but I do know it needs quite some work, Loads of those nasty old 'Plessy' elect-o-lytics to go, plus who knows what else...!
Hows this for a mad idea....
Replace the current 6bm8/ECL82 with summit like a EL86, or even a PL508 that 'likes' O/P impedance of 2.5K or 5Ka-a, Add summit like a ECC83 to replace the triode sections of the ECL82, along the same lines as the original scheme for phase-split and gain stage, and run a 8 ohm speaker from the 15 ohm taps....
Thanks for the input, Gingertube. I did find the error in my numbers but it comes from trying to do the math and calculate square roots in my head. If these are the same transformers you had then I am sure you know more about them than I do. I am not that certain of any of the impedances ratings or turns ratios. What you said makes sense and there may very well be two 3 ohm or 4 ohm windings in series for 12 ohms or 16 ohms. I can't say. One could measure the transformer, if so inclined.