05-02-2022, 03:45 PM
The filament winding supplies nominally 6.3 VAC across its terminals. The hum balance pot is connected across the winding terminals while the pot slider is connected to chassis ground.
The pot serves as a virtual center tap of the winding so the slider can be set at half the filament winding voltage. Since the slider connects to chassis, when measured to the chassis 3.15VAC appears from chassis to each winding terminal. It acts as if there is a center tap on the filament winding, but since the pot slider can move, the location of the center tap can be adjusted to compensate for any imbalance. When I say imbalance I am speaking of the precise setting so exactly half of the filament voltage appears from each end of the winding to ground. It does not refer to any difference between the output voltage of each of the two separate filament windings.
Because the 6B4G tubes have a directly heated filament powered by AC, the polarity of the filament voltage reverses 60 times a second. This causes a 60 Cycle AC signal to appear in the output. But if the ground connection for the filament is connected to exactly the electrical center of the filament winding, the AC signal in the output cancels and no hum appears at the output. That's the purpose of the hum balance pot. It allows you to set the filament to the exact center of the filament winding so the filament voltages is balanced with respect to ground which causes the AC hum in the output to cancel.
This is all a bit difficult to explain, but in the end the hum balance pot has nothing to do with why you measure different heater voltages on the heater pins of different tubes.
Getting back to your issue, are you saying that only the 6J5G AVC tube reads a higher filament voltage. All the other tubes read lower?
Do both 6B4G tubes have exactly the same filament voltage, or does one read higher than the other?
The pot serves as a virtual center tap of the winding so the slider can be set at half the filament winding voltage. Since the slider connects to chassis, when measured to the chassis 3.15VAC appears from chassis to each winding terminal. It acts as if there is a center tap on the filament winding, but since the pot slider can move, the location of the center tap can be adjusted to compensate for any imbalance. When I say imbalance I am speaking of the precise setting so exactly half of the filament voltage appears from each end of the winding to ground. It does not refer to any difference between the output voltage of each of the two separate filament windings.
Because the 6B4G tubes have a directly heated filament powered by AC, the polarity of the filament voltage reverses 60 times a second. This causes a 60 Cycle AC signal to appear in the output. But if the ground connection for the filament is connected to exactly the electrical center of the filament winding, the AC signal in the output cancels and no hum appears at the output. That's the purpose of the hum balance pot. It allows you to set the filament to the exact center of the filament winding so the filament voltages is balanced with respect to ground which causes the AC hum in the output to cancel.
This is all a bit difficult to explain, but in the end the hum balance pot has nothing to do with why you measure different heater voltages on the heater pins of different tubes.
Getting back to your issue, are you saying that only the 6J5G AVC tube reads a higher filament voltage. All the other tubes read lower?
Do both 6B4G tubes have exactly the same filament voltage, or does one read higher than the other?
![[-]](https://philcoradio.com/phorum/images/bootbb/collapse.png "[-]")
