6.3v with CT vs no CT

[pjd3]

Hi all,

I have an Edcor power transformer coming for a build. Its secondary has three 6.3volt coils - one coil is a 2A with center-tap and the remaining two coils are 4A with no center tap.

Is there any part of an amp that would benefit more from a center tapped filament coil than a non centertapped?
I've only done 2 builds and they both required a couple of 100 ohm resistors to ground since they were non-centertapped 6.3vac.
All the other specs were favorable on this power transformer so I just went with the extra options of filament coils that were present on the transformer. I figured the selection would cover any bases anyways.

Is there any noise benefit to using a center-tapped heater vs a non-center-tapped heater coil? Better for preamp tubes?

Thanks for your input.
Best, Phil D,

[R.G.]

No matter how carefully you run wires and place components relative to the magnetic fields of high current wires, there is some amount of leakage from heater wires into the signal path. Center tapping at least makes the leakages symmetrical about ground.

If you don't ground the heater voltages somewhere, the leakage is much worse. Ungrounded heater voltages are the worst. Grounding one end makes it better, if not something you will leave that way. Center tapping tends to be lowest, although it may not necessarily make heater hum zero, depending on the layout.

Many older amps and some new ones use a hum-tuning center tap setup on the heaters. This replaces the two 100R resistors with a 200 ohm pot (in concept) with the ends tied to the heater voltages and the wiper grounded. This can tune any possible variation of perfectly CT reference or offset to either side. It can make up for slight offsets of wiring and hum pickup that even perfect centertapping can't fix. It tunes in a cancelling voltage.

With non-CT windings you have the luxury of experimenting. Go for it!!!

[T Wilcox]

Hi all,

I have an Edcor power transformer coming for a build. Its secondary has three 6.3volt coils - one coil is a 2A with center-tap and the remaining two coils are 4A with no center tap.

Is there any part of an amp that would benefit more from a center tapped filament coil than a non centertapped?
I've only done 2 builds and they both required a couple of 100 ohm resistors to ground since they were non-centertapped 6.3vac.
All the other specs were favorable on this power transformer so I just went with the extra options of filament coils that were present on the transformer. I figured the selection would cover any bases anyways.

Is there any noise benefit to using a center-tapped heater vs a non-center-tapped heater coil? Better for preamp tubes?

Thanks for your input.
Best, Phil D,

I'm sure you will get a better explanation of why from another member but adding the 100 ohm resistors to create an artificial center tap made a considerable difference in noise/hum in my last few preamp builds. Before using the Antek torroidal transformers I always used trannies with center taps.

Todd

[pjd3]

Thank you R.G and T Wilcox.

I think I better understand the center tap thing now with filament voltage supply. It sounds to me like the adjustable hum balance is the way to go for the greatest opportunity to cancel heater induced hum. I've taken great strides to position the heater wires away from anything remotely signal related and did use the two 100 ohm resistor ground in my two builds. The amps are very quite noise wise but, why not go for the most effective? It doesn't really cost that much more in terms of money or work. One lowly little pot and a little wiring. I imagine just looking at various parts of the circuit, maybe just the output on a scope would show up the noise level well for adjusting it out.

Thanks guys, good info here. I'll just have to consider the hum balance pots for the non-center tap filament coil at minimum.

Best,
Phil D

[sluckey]

With no signal applied to the amp... Just connect your AC voltmeter to the speaker and adjust the hum balance pot for minimum voltage.

[norburybrook]

also, the 100R resistors add some form of protection too. They will blow/burn out before your transformer. I recently had a ODS#102 amp lose a tube and it took out the 100r resistors, had the amp used a CT the transformer probably would have gone instead. It was a cheap/quick easy fix.


M

[ChopSauce]

So what's the point in making 6.3V windings with center tap...

[T Wilcox]

Thank you R.G and T Wilcox.

I think I better understand the center tap thing now with filament voltage supply. It sounds to me like the adjustable hum balance is the way to go for the greatest opportunity to cancel heater induced hum. I've taken great strides to position the heater wires away from anything remotely signal related and did use the two 100 ohm resistor ground in my two builds. The amps are very quite noise wise but, why not go for the most effective? It doesn't really cost that much more in terms of money or work. One lowly little pot and a little wiring. I imagine just looking at various parts of the circuit, maybe just the output on a scope would show up the noise level well for adjusting it out.

Thanks guys, good info here. I'll just have to consider the hum balance pots for the non-center tap filament coil at minimum.

Best,
Phil D

FWIW after installing the 100r resistors the hum/noise was almost completely removed. I did this in a Jose style high gain preamp. It is just as quiet as my Ceriatone Molecular build with DC heaters so I think you'd have to have dog ears to see hear any difference when adding a hum balance pot. YMMV IMO and all that good stuff

[R.G.]

So what's the point in making 6.3V windings with center tap...

Um, probably because it is HOW IT WAS DONE IN THE GOLDEN AGE. You know, back when only God and Fender could make good sounding amps.

Honestly, I have always had a bit of irritation with the idea that only untouched Golden Age amps could sound good. Mother Nature hasn't issued any change notices to Her Laws in the last century that I know of.

Oh, wait. Humdingers (one name for the pot with grounded wiper) were done by Fender. Maybe Leo knew something secret.

In a less humorous mood, it was traditional for electronic equipment of all kinds in the 50s to have the heaters center tapped. It just rocked on.

Edit: just as I hit "submit" I realized I should comment on transformer winding. Most heater winding centertaps are not really center taps. The way you wind a true center tap (and how all OT CT primaries should be wound, but mostly aren't) is to calculate how long the copper wire is for one half of the center tap winding, then take two such lengths and wind them on side by side, a procedure called winding bifilar (i.e. in two lines). That way, each half of the winding gets exactly the same influences from the rest of the universe, and you're as close to them putting out identical voltages as you can possibly get. As you can imagine, this is a PITA for the transformer maker, so they like to wind on one half, take a tap outside the coil, then wind on the second half. This guarantees that the two halves will not be exactly the same, so there is some asymmetry in output voltage. The transformer maker says to him/herself "it's just a HEATER winding for &deity's sake" and keeps the labor savings.
Real, bifilar CTs are remarkably well balanced, hence their use in the Golden Age. However, modern MBAs like to save money, and the tube-transformer market is not nearly as competitive as it once was, so they don't compete on the quality of their heater winding balance. And anyway, humdingers can balance out other sources of hum, as well.

[xtian]

Can we ignore the CT on a heater winding and use resistors as faux CT? I think yes, but always good to reinforce knowledge.

I know you can't do BOTH (real CT and pseudo CT) without causing trouble.

[R.G.]

Yes, you can ignore the CT. Insulate the lead. The CT in a heater winding does not participate in powering the heaters, only referencing the heater voltage.

In fact, this makes a good test of the improvement of the pot versus the wound center tap. You can switch between them and see whether the pot is an improvement before you install it, or just test out the theory.

[ChopSauce]

Ok, thanks a lot for sharing your knowledge...

FWIW a friend reported he managed to tune a humdinger and the pot ended fully turned. Not sure in which direction, though...

Can we ignore the CT on a heater winding and use resistors as faux CT? I think yes, but always good to reinforce knowledge.

I know you can't do BOTH (real CT and pseudo CT) without causing trouble.

I'm glad you asked. That was my next question...

[R.G.]

FWIW a friend reported he managed to tune a humdinger and the pot ended fully turned. Not sure in which direction, though...

My first guess would be that the pot is going bad and the wiper is high resistance to the resistive track. That might include high resistance in the wire from the wiper terminal to ground, too.
Second guess is that there is something out in the heater wiring that is leaking current to signal ground and the pot is trying to leak a compensating current from the other side.
Third guess is poltergeists.

Debug would be to disconnect the pot, use an ohmmeter to find the resistance from the wiper to the track. If that didn't turn up anything, I'd disconnect the heater wiring from the transformer winding and measure DC resistance from each side to ground.

If you get into poltergeists, call a priest.

[Phil_S]

If you get into poltergeists, call a priest.

Or this: https://www.youtube.com/watch?v=JxVImJjH_BU

[pjd3]

Thats it. You guys got me scared now. especially about the poltergeists.

I"m not going to use any form of CT or hum balance on my filament coils. I'm going to implement a 60 Hz notch filter post power tubes. Shouldn't be a problem.

This is all you fault.

Thanks guys!

Phil D.