13-03-2017, 04:33 PM
The transformers primary is tapped in order to reduce any hum in the transformer from the HT that might otherwise end up at the loudspeaker, the transformers primary has three connections, main HT in (1), the anode connection to the audio output valve (2) and HT out to supply the rest of the receiver (3) It's all designed for hum cancellation.
The HT hum voltage at connection 2 (the tap) branches two ways via winding 2-1 and 2-3, and in effect those two branches consist of winding 2-1 plus the valves internal resistance (ra) and winding 2-3 plus the resistance of R35 (1k), the value of that resistor in conjunction with the turns ratio of those two windings and the value of ra in effect helps to cancel out the hum that would otherwise be present in the secondary winding, cancellation won't be perfect but the arrangement goes a good way to reducing the hum, that's the hum on the HT, and not any hum that might be introduced via the valves grids etc.
Lawrence.
The HT hum voltage at connection 2 (the tap) branches two ways via winding 2-1 and 2-3, and in effect those two branches consist of winding 2-1 plus the valves internal resistance (ra) and winding 2-3 plus the resistance of R35 (1k), the value of that resistor in conjunction with the turns ratio of those two windings and the value of ra in effect helps to cancel out the hum that would otherwise be present in the secondary winding, cancellation won't be perfect but the arrangement goes a good way to reducing the hum, that's the hum on the HT, and not any hum that might be introduced via the valves grids etc.
Lawrence.
