When to use heater center tap

[dcoyle]

Is it better to use the center tap of a heater winding, or leave it disconnected and use 100 ohm resistors to ground from each leg of the heater winding? Thanks.

Dan

[MarkusBass]

I think that these are two alternative ways to achieve the same results. The resistors are used when there is no center tap. So when you have the center tap, I suggest to use it. With resistors you loose some power due to the current flowing through the resistors. This is another reason to use center tap. Also with center tap you don't need to buy 2 resistors so you save some money .

Mark

[frus]

Those 100 ohm resistors make "nice" fuses when the plate/screen shorts to heaters, which happens a lot evidently (with today's tubes?), people come to me all the time with those resistors burnt. A real fuse would be better of course, but it's very rare

On the other hand, I myself had a few tubes go that way (with center tap and no fuse), and neither PT nor heater wiring went up in flames (although I immediately realized something was wrong and shut the amp down)

[pdf64]

The CT is a much more robust option.
A common power tubes failure mode is a short that allows B+ current into the heater circuit.
A CT will sustain that current, thereby forcing a fuse to blow (B+ fusing is highly recommended).
A resistive ground ref will tend to blow open circuit, not blowing any fuses, allowing the short to pull the heater circuit up to a new reference of the VB+.
That is way above the tubes h-k insulation rating, and all the tubes in the amp may be damaged by the time a fuse eventually blows or the amp is switched off.
If there's no CT, I prefer beefy resistor types (eg >3W >350V) that have a chance of maintaining that fault current for a few seconds at least.

On the other hand, 1% resistors may likely provide a more accurate halfway point, as there tend to be few turns on a heater winding.

[es345]

Another possibility:

take a 5W 250 Ohm Poti with outer legs to heater and center tap to ground. It allows to trim to minimize hum.

Hans- Georg

[Enzo]

1%??? it really isn't that critical. I much prefer the CT given the choice. In any case, I don't see one method over the other providing "better" results.

The 100 ohm resistors make a 200 ohm load across the 6v winding, which amounts to 30ma or so draw, the 6v winding provides several AMPS of current and 30 mills won't be missed at all.

[dcoyle]

"Better", for me, was lower hum.

Thank you all for your time and thoughts, gents. I'm going to go with the center tap, grounded. I've never got much effect from the humdingers I've encountered, possibly because the imbalances in the tubes were more than it could compensate or the hum's source was not the filaments, more likely.

Thanks

Dan

[Chuck H]

Humdinger circuits come into their use with unmatched power tubes. Since probably 98+% of the power tubes installed in amps are either purchased "matched" by the amp owner or of the same batch and within a few mA idle current by a manufacturer it doesn't come up that much. Add to that the popularity of higher gain circuits now and yeah, hum is usually something other than what a humdinger can solve for. That doesn't mean it's not a good idea. It is. It won't hurt a thing and it's there if you need it.

I don't know if a false CT (the pair of 100R resistors or humdinger pot) actually protects against plate to heater or screen to heater shorts such that there is less likelihood of damage than if a winding CT is used. My logical mind says it does, but I've never analyzed it critically. Nor do I have all the chops to know everything I should be looking at in this regard. I use the false CT most of the time.

Another out loud thought... If you elevate the filaments on a cathode bias circuit and a power tube shorts plate to heater you now have a plate to heater to cathode short. With a false CT there may be some hope of less peripheral damage.?. I've never taken steps to determine what the wattage rating for a false CT should be to act effectively as a fuse.

[dcoyle]

Speaking of elevating the heater's CT on a cathode bias push-pull output stage, would there be noise cancelling benefit from the summation of the heater CT current and the output tubes' currents at the junction of their cathodes?

Thanks,

Dan

[Enzo]

I could be wrong, but I never had the idea that the heater hum balance pot or the simple pair of resistors or the CT grounded were there for the power tubes, seemed to me the preamp tubes were the ones sensitive to the heater hum. Same with elevating, that is why we often see only the first tube or two elevated or on DC.

Power tube mismatch hums because the uneven draw through the OT will not be balanced to cancel B+ ripple in the primary. I never attributed it to heaters.

[Enzo]

Speaking of elevating the heater's CT on a cathode bias push-pull output stage, would there be noise cancelling benefit from the summation of the heater CT current and the output tubes' currents at the junction of their cathodes?

???

What CT current? Current flow is through the heater, there is no current through the CT, not unless one side of the 6v has resistance to ground, or the tube is shorting to some other supply.

The noise benefit is that the cathode is just an easy place to steal some positive DC voltage, instead of making a power supply for it.

[Gaz]

I could be wrong, but I never had the idea that the heater hum balance pot or the simple pair of resistors or the CT grounded were there for the power tubes, seemed to me the preamp tubes were the ones sensitive to the heater hum. Same with elevating, that is why we often see only the first tube or two elevated or on DC.

Power tube mismatch hums because the uneven draw through the OT will not be balanced to cancel B+ ripple in the primary. I never attributed it to heaters.

This is true. The hum balance pot is to null the hum in preamp tubes. In my case it always manifests as more of a buzz, but the hum balance gets rid of it every time. The null point also changes with different preamp tubes. They will fuse like the resistors, of course, and also take out the heater elevation filter cap if there's a short (assuming the filter cap is not rated for full B+).

[Merlinb]

Is it better to use the center tap of a heater winding, or leave it disconnected and use 100 ohm resistors to ground from each leg of the heater winding? Thanks.

There is a third option which is to use the CT but connect it to ground through a small (e.g. 47R) fusible resistor. Thus you get the same fusing benefit of the artificial centre tap, but with a tiny bit less wiring.

[es345]

This is true. The hum balance pot is to null the hum in preamp tubes. In my case it always manifests as more of a buzz, but the hum balance gets rid of it every time. The null point also changes with different preamp tubes. They will fuse like the resistors, of course, and also take out the heater elevation filter cap if there's a short (assuming the filter cap is not rated for full B+).

In the OP there was there was no hint that there are 2 heater windings. My comment is based on one heater winding only. I should have mentioned that.

Hans- Georg

[Gaz]

In the OP there was there was no hint that there are 2 heater windings. My comment is based on one heater winding only. I should have mentioned that.

Hans- Georg

Hans, I'm not sure what you are talking about, but I was responding to Chuck and Enzo. I don't see anyone mentioning two heater windings regardless...