Heater Wires - Elevate or Twist?

[Thegrateoz]

I'm working on a 1986 Plexi Bass build and I'm to the point of installing the heater wires.

I like the clean look of elevated wires, neatly placed parallel to one another and have seen this on several builds, but Marshall seems to have twisted them and placed them into the corner of the chassis. I know the goal is to keep hum to a minimum, but is one method better than the other? Am I overthinking this and it's just potayto/potahto?

Thank you.

[R.G.]

Twisting works by the electromagnetic fields confining themselves to between the wires, and by both wires being almost exactly the same-but-opposite by external fields. This is a differential-voltage/current thing.

Running the wires near the chassis works on the common mode stuff. That is, if there is a voltage different from the average voltage of the wires, moving them close to the chassis increases the capacitance to the chassis "ground" and makes the composite chassis/plane and wires have a lower transmission line impedance at rf. Which shouldn't matter, except in very, very rare cases. It also moves those wires away from other stuff and decreases the capacitance to other sensitive wires.

A neat trick is to use shielded twisted pair for heater wires. You can then put them where you like as long as you solidly ground the shield.

In general, twist them except where they have to break step at a tube's pins. Running near the chassis will help knock off some more hum from common mode stuff, as well as making the loop area of the heater wires and chassis be smaller for lower magnetic field pickup - which is a common mode effect on both wires. Shielding is another step toward quiet.

[Stevem]

Do it the way Fender mostly did it, levate and twist.

Even when powering the heaters of 6 preamp tubes that's only 1.8 amp of heater current so once your passed the output tubes and feeding the preamp tubes you can very safely step down to 18 ga wire and then get the benift of making more twist / turns per inch.

Smaller and tighter twist will also allow the twisted section to go right up to the tube pins.

[TUBEDUDE]

Elevating fights hum by eliminating hum induced in the cathode by the heater.
Twisting reduces hum by reducing radiated hum, by
, as RJ said, antiphase nulling, capacitance to ground, etc.
So both strategies combined will give less hum.

[sluckey]

Do it the way Fender mostly did it, levate and twist.

I suspect this means elevate the heater wires in the air above the sockets. Not talking about elevating the CT to some VDC level. Right?

[Stevem]

Yes.

[CrocMule]

Even when powering the heaters of 6 preamp tubes that's only 1.8 amp of heater current so once your passed the output tubes and feeding the preamp tubes you can very safely step down to 18 ga wire

Hey Stevem, do you run larger wire than 18g to your power tubes? I can't even imagine stuffing 16g or larger through the holes in the tube socket solder tabs. That's not necessary. I know this has been discussed many times before, but look at the chart below. Even with temperature and multi conductor cable correction factors, 18g wire with a 105C insulation rating (very common) can safely carry 8 amps. (Two 18g conductors with 105C insulation twisted pair at an ambient temp of 90C). Now, voltage drop is usually also a consideration in ampacity calculations, but the distances involved are less than 5 feet total (round trip), so it would add up to a minuscule amount. I think you're wrangling thick wire for no reason.

I don't know how hot it really gets inside a tube amp. I'm going to guess it's not over 90C (194F). Here are max amps heater wire calcs (twisted pair, 90C ambient, 105C wire insulation):

18g, 8.0A
20g, 4.8A
22g, 3.7A

So even in a hot amp, 18g for preamp tubes is way overkill. 22g is fine, and it's much easier to work with. The wire gauge to the power tubes depends on quantity and current requirements of the tubes, but I can't imagine needing 16g -- just trying to save you some hassle!

Best, Russ

[Yoda]

What about Soldano? How the heaters are wired on an SLO flies in the face of everything posted here. So I think heater wires are not as critical as some make it out to be.

[Phil_S]

Use zip cord (lamp cord.) No need to twist.

[maxkracht]

I think heater wires are not as critical as some make it out to be.

Yep. There is a big difference between best practice and what you can get away with. Took me a while to figure that out with heaters, still figuring it out with everything else.

[Reeltarded]

I also find them not to be critical as the quicksand so pervasive on TV from 1962-1984.

I do them in the corner on Marshall things, sometimes sheathed and parallel to each other. Fly them to the headshell shield and twisted on a tubes down amp.

Mark Fowler built an amp with them routed outside.

[xtian]

I spent some time years ago TRYING to induce hum into my preamp by poorly routing heater wires. I failed.

[ViperDoc]

I'm happy to hear it doesn't matter. This is how I routed the heaters on my 5F8A build a while ago. Tight and tidy. And yes, that's a turret board in a vintage Tweed chassis. SOMEWHAT of a tight fit, but it all worked out.

[TUBEDUDE]

Well it doesn't matter a great deal on low gain builds. As more stages are used, the hum/noise floor increases, requiring adoption of these strategies up to a full D.C. heater supply for preamp tubes.

[bepone]

Correct, hi gain doesnt have chance with AC heaters... I like to DC supply V1 tube on any amp, it is easy to do, small current, rectification is easy, 12VDC regulator, 12VDC heaters supply to pins 4-5.

Also why people using 6,3VAC for heaters, only complications with wiring and heavy wires for no reason?