Nostr Web Client

So... I've been arguing with AI for an hour about this.

The image is from a specification sheet for the 35W4 half-wave rectifier tube, common in old AA5 radios.

My AI (Brave assistant) insists that the DC voltage that comes out of the cathode (B+) sits on the heaters as well (which run on AC, obviously).

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.

Further, the DC voltage runs to all other heaters for the other tubes placed in series with the 35W4.

This (according to AI) elevates all heaters to an appropriate DC potential, reducing hum by minimizing stray electrons that tend to flow between the heater and cathode in the preamp section.

Thats awesome if true.

Reply to this note

Please Login to reply.

Discussion

micahcmiracle 1w ago

nostr:nprofile1qqs0wgf8etnayvkcs4c636fdvepy73jc9q2xlcln64srjdm5e98p8ncpzdmhxue69uhhqatjwpkx2urpvuhx2ue0qythwumn8ghj7un9d3shjtnswf5k6ctv9ehx2ap0qy2hwumn8ghj7un9d3shjtnyv9kh2uewd9hj72sttqc what do you think?

Ryan 1w ago

I'm so confused.

ChipTuner 1w ago

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 1w 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.

micahcmiracle 1w ago

The B+ is still referenced to chassis ground.

micahcmiracle 1w ago

I just wanted to circle back to say that of course you are correct. The AI version of events makes no sense. Even with the cathode physically connected to one side of the heaters, B+ does not flow through the series filament string as DC is unidirectional and grounded at the power supply.

AC does appear on the cathode, however, but is filtered by the 1st cap of the power supply.

ChipTuner 1w ago

Yeah thanks for following up. I definitely had a hunch something was off.

micahcmiracle 1w ago

Let me know your thoughts on this:

In many old, transformerless, series filament (AA5) radio/amps, one side of the AC is indeed tied to the chassis, along with the ground for B+ ...

In such a case, is it possible then that the B+ on the heaters would be carried through the heaters?

This assumes that the cathode really is tied to one side of the heaters, which I havent fully verified, but seems likely.

micahcmiracle 6d ago

Oof. I'm in circles. ⭕️ ... 🤣

The tube in question (35W4) is not directly heated. The cathode and heater is not connected.

In fact it can hold its isolation up to 360v: https://www.amplifiedparts.com/sites/default/files/associated_files/35w4.pdf

So there's that. Such misinformation spread through AI. 🤷‍♂️

This is all moot now. Thank you for your attention to this matter.

ChipTuner 5d ago

XD Yeah thanks again for clearing that up. I use AI for software, every time I've tried for electronics I get spun in circles and always end up back in my college textbooks and notebooks lol.