I am probably asking a question where the answer will be way over my head, but here goes...

What is the purpose of the extra tap on the amp side of the T2 output transformer? I can see one goes to power, which makes sense. The other goes to what appears to be the source switches.

The main reason I am asking, is because I am looking for this particular radio, and the only one found so far has no sound output at all. From what little I have learned, that means it probably has a bad output transformer, so I am poking around to see if replacements can be found. Transformer is listed as 5500 ohm with a tap @2% and the secondary is 3-4 ohm. Transformers without the tap seem to be easy to find.

Reason for the tap is hum cancellation - if you can't find one with a tap, larger power supply caps might be needed.

That setup is often called "humbucking". The smaller section of the transformer carries a certain amount of 120-cycle hum that is out of phase with the main winding and provides some cancellation of the overall hum on the audio output.

Thanks, I actually understood that Also nice to know it could be worked around if needed.

Hum bucking windings are more often found on the secondary side of an output transformer, and usually in early 1930s radios with directly heated tubes and very small filter capacitors. With separate cathode tubes and 150 uF worth of electrolytics in the power supply, it's doubtful that there was ever any hum to buck.

The tapped connection on the output transformer adds a small amount of inductance in series with the power supply circuit. This is important in radios which have electrostatic tweeters because they are essentially in parallel with the output transformer and the regular PM speaker. It keeps the audio output stage from "taking off" and oscillating at supersonic or RF frequencies through the electrostatic speakers. Since the purpose is to filter AF and RF, not power supply ripple, only a small amount of inductance is required and it made a lot of sense economically and practically to tap the output transformer rather than install a separate choke.

Hopefully you will find some other problem in the set and replacement of the audio output transformer will not be required. But if it is, the way to do it is to add a small audio choke that has a low DC resistance between the yellow and green wires to take the place the "small" section of the output transformer primary. A standard replacement output transformer having a two-wire primary can then be connected in place of the original.

You don't need the extra winding, I'd just replace it with a 22 ohm 2 watt resistor, bump up the filter capacitors by 50% or so
and add a small capacitor, say .001 1000 volt across the primary of the output transformer.

Cool. I've always wondered how humbucking worked.
Thanks for the simple explanation there Jack. Most appreciative.

~ Mitch ~

If the winding's resistance are close to spec, and not shorted to ground or the secondary, look elsewhere for your no audio problem...

I understand the concept of wanting to keep the audio output stage from oscillating in order to protect the electrostatic tweeters but how does having an inductance between the 1st and 2nd power supply filter caps prevent this? Is it because electrolytics are often ineffective at supersonic and/or RF frequencies? But I still can't see how placing an inductance in series with a 1500 ohm resistor as shown in the schematic could reduce any tendency for the audio stage to oscillate.

I'm almost certain that the tap is used for power supply hum bucking in American-made sets (like Zenith AC/DC models and many others) but since the model in question is European, audio circuits in these tend to be much more complicated.

Dave

I'd suggest that the purpose of the tapped winding is to balance the dc fields of the two winding portions - getting around the problem of dc core saturation in a single ended amplifier output transformer. This would allow the manufacturer to use a lighter, cheaper core.

--Chuck

In the series string sets that used the tapped primary, with 50L6 etc the transformer seemed bigger as I
remember them. It would sure be interesting to dig out the patent on that hookup and see exactly what they were claiming.
Of course these sets were dealing with 60 Hz hum, not 120.

Thinking about it, if the B+ current draw of the output stage was about the same as the current draw of the rest of the set, both hum cancellation and dc balancing of the transformer could be achieved together !!

--Chuck

Also the magnetic field in the core is a function of the ampere-turns of coils in question, so the plate tap
would predominate.