Quote:
Originally Posted by Adam
But I don't understand where the DC on the heater string comes from, or how the amount of AC vs DC voltage on the heater line changes with the tube position.
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Think of the heater string as a resistor voltage divider. The midpoint of the heater string (in respect to voltage, not the number of tubes) will look a bit like the centertap on a transformer secondary, one feeding a bridge. You've seen centertap secondaries feeding bridges in power supplies that produce a positive voltage, and a negative voltage of the same absolute value. If you measure from the centertap. If you call the negative the ground, then the centertap will look like 1/2 B+, and the positive will be full B+.
You wouldn't be able to draw much current off the heater string "centertap", but in this application, you don't need to.