Heater Wiring Do's and Dont's - Will This Fly?

[eschertron]

Hello gentle readers,
I have seen much discussion and read many articles on minimizing hum from heater wiring. I want to toss this out and get some response. If keeping the 6.3VAC pair twisted helps reduce the AC field and resulting hum, tell me why no-one champions the idea of keeping the pair together and *cross* the tube with the pair of wires (see attached photo). In my understanding, this would provide more protection from the AC field than running one wire across the tube and back again. Please tell me where my error lies?



Thanks,
Alan

[soundguruman]

that's not bad at all. the trick being to keep the AC away from the control grids...2 and 7 on a 12AX7
or just use DC for the first stages is better.
As I say, the first stage noise is going to be amplified by the rest of the stages.
So why not use DC for the first stages?
In a normal "Fender" type amp like a twin etc...the gain is not huge. It's a clean amp, and there is lower noise even with AC heaters.
In a high gain amp, like a Bogner or an Egnator...etc...the AC will be amplified much more....and so some folks prefer DC for heaters.
However with DC you are fighting a voltage drop, and larger capacitors / longer warm up time are required...etc...
I mean, if we are talking minimizing hum for real, then we are talking DC heaters.

[R.G.]

It's an OK idea.

However, I notice you're wiring 9A format duotriodes like the 12AX7 and brothers. The 12A?7 family has an internally semi-humbucking setup (or did, back in when most tubes were in production, not reissued) so that if it's powered from 12Vac center tapped with 12Vac on pins 4 and 5 and pin 9 to the center tap, the internal hum fields and leakages tend to cancel. Wiring a 12A?7 for 6V gives up this advantage.

In general, heater hum is less of a problem than some of the other 508,243 causes of hum, though. It might be instructive to run your amp on a 6Vdc battery setup for a few minutes to see what other hum sources are there, and correspondingly how much hum is NOT the heaters.

[Enzo]

I may be wrong, but it seems like you are concerned with the half an inch of wires right at the socket, while the twisted pair of wires is more about not radiating 60Hz into the circuits as it travels across the chassis. And for example, one of those 508,243 hums might be picking up heater AC on a signal wire, so good practice is to wherever possible when signal wires cross AC wires, it should try to do so at a 90 degree angle.

Hum within the preamp tube can come from minor current flow between cathode and heater, whenever the heater becomes more negative than the cathode. That is why in many amps the heater AC is elevated from ground by a positive DC voltage. It makes the heaters more positive than the cathode.

[eschertron]

Thank you all for the replies, I can add more items on my "list of experiments to design" !!

My takeaway from this discussion is that while heater wires at the tube socket is a factor, it's also a given - and so to minimize induced hum *at that point* the designer can go with techniques such as DC heaters or positive CT elevation, etc.

[Enzo]

Think of all of them as potential sources of hum. Not all amps, for example, will have a problem with heater to cathode electron flow.

[R.G.]

... and not all tubes. What is happening inside each tube matters, and that's the one thing we have almost no control over.