"Virtual center tap" w/ center tapped transformer?

[tubenoob]

What if you have a pair of 100 Ohm resistors making a virtual center tap on your 6.3v winding when the power transformer has a center tapped 6.3v secondary?

I replaced a transformer and the new one has a CT and I'm wondering if I need to snip those out or if there is some benefit to leaving them?
Seems like I heard something about raising the ground plane but I don't really know what that means.

[martin manning]

Use one or the other, but not both. I would use the physical center tap, which can be grounded with the high voltage winding's center tap, or with the reservoir capacitor's ground, and remove the two resistors creating the artificial CT.

[matt h]

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[tubenoob]

Cool, thanks.

[pdf64]

The CT on the winding will have a way lower impedance than the 100 ohm resistors, so will take precedence.
I can't see a problem if the resistors were left in place? They'll just draw ~30mA, the same as if the CT wasn't there.
To leave the resistors in is a pointless waste of ~0.2 watts but then so is the use of a pilot lamp (as opposed to a neon or LED indicator), ie it's trivial.
If the winding has an even number of turns and the voltages aren't as precisely balanced as with 1% resistors, then so what?

matt, if there's a Vdc offset reference then I can't see the need for any method of balancing the reference point? More a case that the Vdc offset makes balancing a contradiction in terms.
Pete

[martin manning]

My thought is that if two grounds are used and there is any potential difference between them, then there will be some current flowing through the chassis. It would be best to avoid that.

[matt h]

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[pdf64]

Martin, yes, I can can see that there's the potential for current to flow between the two reference points.
It would be tiny though, and given the common implementations, well separated from the signal path, so might not cause hum.
But I agree that it tips the balance, for me it would be in favour of removing the resistors. The winding CT provides a much more robust path to ground, in the event of fault currents, eg a tube short resulting in B+ flowing to ground via the heaters. Low power reference resistors blow quickly, thereby allowing the the fault to pull the heaters up to VB+, with the potential to damage the heater-cathode insulation of all the tubes.

Matt, yes, I didn't consider the benefit from balancing the radiated ac field, as I was focusing only on the Vdc elevation.

Thanks for the help! Pete

[martin manning]

Good point re the potential damage to h-k insulation Pete

[pdf64]

I've come to the conclusion that it's good insurance to replace regular balancing resistors (if fitted) with 3 watt wirewounds.
The idea being that they will be tough enough to withstand such fault currents for long enough for the B+ or primary fuse to blow, thereby maintaining a ground reference and so protecting the tubes.
Pete