Filaments: Is the 100R/100R center tap inherently better?

[Hardtailed]

Just wondering here... I've read before that an artificial center tap is potentially quieter by way of being able to use perfectly matched 100R transistor to have the ground reference perfectly centered.
Is this still with modern not-so-bad-tolerance power transformers? (In my case a Hammond 290MX)

I'm asking this because I did put two 100R resistors on my board and referenced them to the top of the cathode resistors (cathode biased amp) for some DC elevation.

Now, the PT, despite being a lay-down style with soldering tabs, still has the filament supply on hardwired massive 18 gauge wires. Just feels kind of weird to just cut and heat shrink that big center tap wire and rely on a pair of resistors instead.

I guess I could try the PT's center tap first and see if it hums...
I used it on my Marshall style build and I'm always surprised how quiet it is (Hammond 290GX PT).

On that note: anything inherently inferior about referencing the CT (artificial or not) to the top of the cathode resistor? Easy enough to do and gives a free roughly 10V of DC elevation, but I'm wondering if the occasional bias shift under hard transients might have an effect down the line on preamp tubes.

[Steve Conner]

I once built an amp that had the hum balance pot, and the pot position that gave minimum hum wasn't always the middle! If I changed the first preamp tube, I had to readjust it, and sometimes it would end up at 11 o'clock or 2 o'clock.

So I think in the face of that, the matching of the 100 ohm resistors hardly matters, nor does it matter whether you use the resistors or the winding centre tap.

Elevating the heaters off the power tube cathodes in a cathode-biased amp is a good idea, it has no drawbacks that I know of. The cathode bypass capacitor should stop too much signal getting back down the heater line.

When you elevate the heaters, the hum balance matters even less.

[pdf64]

'I'm wondering if the occasional bias shift under hard transients might have an effect down the line on preamp tubes'
The bias shift on the power tube cathodes will result in a higher elevating voltage, so it shouldn't cause a problem, as I see it.

'hum balance pot, and the pot position that gave minimum hum wasn't always the middle'
I've noticed the same thing with a hum balance pot (on fender 75).

Re elevating heaters, I asked this query in a previous thread, but no-one bit - if the heaters are elevated, what's the benefit of a centre tap / balancing resistors?
ie what difference between the elevated heaters wobbling +/- 3.15V around Vdc cf wobbling +/-6.3V around Vdc (the elevating voltage referenced to a heater leg rather than centre tap)?
The way I see it, as long as the lowest heater potential is always above the highest cathode potential, then referencing the elevating voltage to the heater leg or CT ought to be equivilant. It just means that the elevating voltage needs to be 4.5Vdc higher (leg cf CT) to achieve that effect.
However, given Hardtail's elevating voltage of only 10V, and the peak heater voltage of 9V, then the elevating voltage should be referenced to a CT rather than leg, in order to ensure that the heater potential is always above the cathode potential (say ~1.5Vdc).

[Hardtailed]

I love your replies PDF! Cause you always confuse me a little more, making me go back and read more until I can understand what you're saying.
Yes, this is a compliment!

[pdf64]

It's one of those 'the more you learn, the more you realise you have left to learn' things!

Regards my query in previous post, I guess I was taking Steve's comment

'When you elevate the heaters, the hum balance matters even less'

to it's logical conclusion
ie if heaters are (sufficiently) elevated, why bother to balance them?

[Steve Conner]

Very good question, unfortunately I don't know the answer! Maybe they don't need balanced if they're elevated enough...

[loudthud]

If the cathode is negative with respect to the heater (ie the heater is elevated), then any electrons emitted by the heater inside the tube are repelled! Otherwise there is a small current between the heater and the cathode that can be audible especially if the cathode is not bypassed at the line frequency. Balancing the heater voltage reduces any electrostatic (capacitive) pickup especially in high impedance circuits. Hopefully you get some from each side and they cancel out. Twisting the heater wires helps with the balance of any electrostatic pickup and also balances any magnetic field.

[Enzo]

Fender's pair of resistors was cheaper than making a more complex transformer - a center tapped one. That is what was "better" about them. Either way works.

And that big heavy CT wire? NO current flows through it anyway, so the heaviness is irrelevant.

[jrfrond]

Resistor balancing generally works fine, and I haven't seen any issues with a fixed center-tap either, to tell you the truth. On all new builds, I usually go with a hum balance control though, just for the heck of it. Makes me feel better.

[Steve Conner]

Balancing the heater voltage reduces any electrostatic (capacitive) pickup especially in high impedance circuits. Hopefully you get some from each side and they cancel out. Twisting the heater wires helps with the balance of any electrostatic pickup and also balances any magnetic field.

+1 on this. The capacitive coupling isn't helped by elevating the heaters, so that's one reason to balance them even when elevated.

I should take my amp that has the hum balance pot, elevate the heaters and see how that affects what the pot does. I'll get back to you on that one...

[Hardtailed]

And that big heavy CT wire? NO current flows through it anyway, so the heaviness is irrelevant.

It's true I was wondering why the CT was 18 gauge! No matter what the gauge is, I just always feel weird cutting a hard wired lead from the PT. There's no real reason to that, I'm just that way haha

[jrfrond]

Twisting the heater wires helps with the balance of any electrostatic pickup and also balances any magnetic field.

Oh yeah, this is a MUST on any heater supply. Not by hand either. Twist 'em up by clamping one end in a vise, and the other in a cordless drill, and run them slow, but get a good, tight braid going. I do several feet at a time and just cut it off as I need it, usually green and green/yellow so I can tell the phase. A little trick that some people do not know is that when wire power tube heaters, often flipping the phase on the opposing drive tube (or tube pair, trio or quad) is usually the most hum-free wiring scheme.

[Merlinb]

Not by hand either. Twist 'em up by clamping one end in a vise, and the other in a cordless drill,

I once did that with a non-cordless drill, but the tone wasn't as good.

[Enzo]

Oh yes, the AC field imprints on the wires.

[jrfrond]

Oh yes, the AC field imprints on the wires.

Obviously!