21-09-2019, 11:13 AM
Hi Amie,
Theory predicts what you suggest, won't work.
You can indeed get currents flowing in opposite directions in the two primaries, which will at least partly cancel out the standing magnetisation in the core. However, the output valve is trying to put audio voltages across one primary, but the HT capacitors across the other primary are trying to keep the voltage here constant. So the poor little transformer would be beset by two fighting forces.
There is a technique as Lawrence above has posted, where the windings are unequal, and the 'extra' winding has a resistor in series (most important as it allows some audio voltage to appear across this winding). By getting the value right, it is possible to completely cancel any hum on the HT rail from appearing in your speaker.
Jeffrey asks why it is not used widely? I'm thinking that it was a Philips patent, and they only allowed a few other companies to use their idea.
Amie, if you start experimenting, good luck, trying something in practice and proving or disproving in ten minutes is sometimes quicker than sitting with pencil and paper with a wet towel round your head for an hour. My own technique is to use whichever method is likely to get me the answer fastest!
Theory predicts what you suggest, won't work.
You can indeed get currents flowing in opposite directions in the two primaries, which will at least partly cancel out the standing magnetisation in the core. However, the output valve is trying to put audio voltages across one primary, but the HT capacitors across the other primary are trying to keep the voltage here constant. So the poor little transformer would be beset by two fighting forces.
There is a technique as Lawrence above has posted, where the windings are unequal, and the 'extra' winding has a resistor in series (most important as it allows some audio voltage to appear across this winding). By getting the value right, it is possible to completely cancel any hum on the HT rail from appearing in your speaker.
Jeffrey asks why it is not used widely? I'm thinking that it was a Philips patent, and they only allowed a few other companies to use their idea.
Amie, if you start experimenting, good luck, trying something in practice and proving or disproving in ten minutes is sometimes quicker than sitting with pencil and paper with a wet towel round your head for an hour. My own technique is to use whichever method is likely to get me the answer fastest!
