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Man that's a tricky analog circuits problem I'm struggling to answer appropriately. My first instinct is to say, no cathode heaters are meant to be isolated in all cases. Then I was thinking they're arguing for a (B+) coupled to the heater circuit instead of a ground. So heaters would be -(B+) relative to cathodes (in all other tubes) instead of 0 (assuming standard anode coupled load).

However since the cathode of the rectifier is going to be at a higher potential relative to all other cathodes in circuit, I could see how that might be an issue? but I still can't see how coupling heater circuit to B+ makes a better option.

> Not only that, but the DC voltage is divided by inherent resistance of the heaters, so really only half of B+ sits on the heaters.

That doesn't really add up to me since there should be no current flow, therefor no voltage drop.

micahcmiracle 2w ago

So, the heater (pins 3 and 4) sit very close (perhaps directly connected) to the cathode. AC is applied (in series) to the heaters of each tube in the circuit.

The cathode of the rectifier spits out a DC voltage for the rest of the circuit (tube plates, screens, etc.).

The DC appears on the heaters of the 35W4, and subsequently each series connected tube in the heater string.

Elevating tube heaters to an appropriate DC potential is a common hum reduction trick in amplifiers.

Usually, it's accomplished with a voltage divider from B+ injected into the heater string.

The DC voltage (B+) from the cathode of the 35W4 appparantly does this automatically (happy accident maybe).

And the inherent resistance of the heater pins reduces the DC voltage roughly in half, making the DC potential on the string appropriate for hum reduction.

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micahcmiracle 2w ago

The B+ is still referenced to chassis ground.