21-09-2019, 03:52 PM
(This post was last modified: 21-09-2019, 03:55 PM by Mike Watterson.)
The audio current is irrelevant as that is AC.
The DC and AC voltages are also irrelevant.
All that matters is the DC currents (especially as it's Class A). So feeding rest of set from one winding and output anode from the other does work. Obviously it works best if the ratio of the windings equals the ratio of the currents.
I'm sceptical that the idea was patentable, though lots of things that should not have patents did and do. I've seen this on a number of brands older radios. It's arguable if it saves much money or reduces hum much compared with suitable R & C after main cap. I'd think it fell out of favour when Electrolytic capacitors got bigger & cheaper, resistors replaced chokes and suitably gapped SE transformers were cheaper. Earlier there were Field coil Electrodynamic speakers with hum bucking windings, so only saturation of O/P TX core rather than hum would have been the issue.
I'd think it wasn't used widely because there was little advantage to it then compared to now.
It's a very useful technique today for SE class A as it allows use of a mains transformer. The DC current to rest of set vs the Class A quiescent current can be balanced with a shunt HT to 0V resistor or additional taps/windings on the O/P primary or rest of radio primary. The CT goes to HT as the DC current of the rest of set is in anti-phase to O/P valve, as in case of the twin quiescent currents of Push Pull Class AB or Class B. Class A on Push Pull is wasteful.
The DC and AC voltages are also irrelevant.
All that matters is the DC currents (especially as it's Class A). So feeding rest of set from one winding and output anode from the other does work. Obviously it works best if the ratio of the windings equals the ratio of the currents.
I'm sceptical that the idea was patentable, though lots of things that should not have patents did and do. I've seen this on a number of brands older radios. It's arguable if it saves much money or reduces hum much compared with suitable R & C after main cap. I'd think it fell out of favour when Electrolytic capacitors got bigger & cheaper, resistors replaced chokes and suitably gapped SE transformers were cheaper. Earlier there were Field coil Electrodynamic speakers with hum bucking windings, so only saturation of O/P TX core rather than hum would have been the issue.
I'd think it wasn't used widely because there was little advantage to it then compared to now.
It's a very useful technique today for SE class A as it allows use of a mains transformer. The DC current to rest of set vs the Class A quiescent current can be balanced with a shunt HT to 0V resistor or additional taps/windings on the O/P primary or rest of radio primary. The CT goes to HT as the DC current of the rest of set is in anti-phase to O/P valve, as in case of the twin quiescent currents of Push Pull Class AB or Class B. Class A on Push Pull is wasteful.
