Humbucking Filaments?

[jeff]

 I read someone on this site say that when you hook up the filaments in parallel it's humbucking but series it's not. Is a 6.3 parallel quieter that a 12.6V series heater? I don't understand how that would work off one 6.3V supply, but it did get me thinking. If you had a 12.6V CT heater supply, and grounded the CT then put one 6.3V leg on pin 4 and the other on 5 and grounded pin 9 now each filiment would have 6.3V AC out of phase.

Can that be done?
Would that be humbucking?

[sluckey]

I read someone on this site say that when you hook up the filaments in parallel it's humbucking but series it's not. I don't understand how that would work off one 6.3V supply, but it did get me thinking. If you had a 12.6V CT heater supply, and grounded the CT then put one 6.3V leg on pin 4 and the other on 5 and grounded pin 9 now each filiment would have 6.3V AC out of phase. I'm going to be using a 12.6V heater supply anyway so it would be simple enough to jump a wire from pin 9 to ground. My concern would be the transformer. Let's say it's a 12.6V@150mA running one tube, is it ok to use as two 6.3V@150mA for each filiment? I think that should be OK but just checking.

Can that be done?
Would that be humbucking?

Yes, using a 12.6v CT transformer, it can and has been done in a lot of military equipment. I guess you could call it humbucking. The 12A?7 tube family is special in that they can be heated with 12 or 6 volts. Be careful if you use a 12 volt transformer in a system that also has 6 volt ONLY heaters.

[jeff]

Does grounding pin 9 actually do anything vs. floating it?

[sluckey]

Yes. Grounding pin 9 (and CT) gives the 'humbucking' out of phase heater current you mentioned. If you don't ground it, current simply flows thru the series filaments (pin 4 to pin 5) in the same direction (phase).

[RicharD]

Center tap grounding consitutes humbucking.  6.3v is better (quieter) because there is less source potential difference for amplifiers to pick up.  Twelve volt is better when doing DC because of the lower current draw.

[jeff]

I'm not trying to be argumentitive I'm just trying to understand better. With a 12.6V with the CT grounded, isn't pin 9 always at 0V?
When pin 4 is at +6.3V pin 5 is at -6.3V and pin 9 is at 0V relative to ground. So how does grounding pin 9 make a difference?

In a humbucking pickup, each coil picks up hum equally and opposite and is combined at the output to cancel out. But is this what's going on it a tube?
There's two filiments in a tube tied together at pin 9. I assume pin 4 to 9 heats one cathode pin 5 to 9 heats the other. If so is the hum of one filiment picked up by one triode and the hum of the other picked up by the other section?

Each filiment has equal but opposite hum but is that hum being combined and canceled, or are you just putting equal but opposite hum on each triode section?

If the hum of each filiment is independant, this may wind up being more noisy than using a 6.3V CT. With a 6.3V CT you're gyrating around ground. Using a 12.6V CT is like using two 6.3V grounded on one end but each filiment is out of phase.

Thanks for talking this out with me,
Someone had a quote- If you ask a stupid question and you may look stupid, but if you don't ask, you'll stay stupid.
         Jeff