Heater CT

[trem]

The Heater CT is for a "Ground Reference".
So I assume there is no current in the Filament Center Tap.....would it matter then, if you grounded the Heater CT with say.....the HT CT.?
Would that create a ground loop somehow...or be a source of noise.?
I guess what I am really asking is.....Does the Heater CT need to be isolated from any of the other PT Grounds/Center taps.?
Thank You

[Chuck H]

I commonly ground all the CT's to a star with the first filter. I never considered the possibility of a ground loop. Any hum on the HT CT should (ideally) be canceled in a push/pull amp. And I don't think any unfiltered ripple from the HT CT could ever compete with the filament winding. I say go for it. My amps don't seem to hum more than any others. Maybe less.

[Leo_Gnardo]

would it matter then, if you grounded the Heater CT with say.....the HT CT.?
Would that create a ground loop somehow...or be a source of noise.?
I guess what I am really asking is.....Does the Heater CT need to be isolated from any of the other PT Grounds/Center taps.?
Thank You

Fender did it on gazillions of their classic amps, so I guess it's OK.

[trem]

Well...I guess if it was good enough for Leo Fender (or Gnardo) it is good enough for me.
I typically ground the (if the PT has one) Heater CT with the HT CT...but maybe I have been lucky, or maybe my amps could have been better.
Anyway...thanks for the info.

While I have the forum's attention.....Are there any links, or opinions that address the theory of a Heater CT.?
That is (assuming the PT has a ct for the heat) is there any evidence that using a "virtual ct" is better than using the heat ct of the PT.?
You guys know what I am asking...is one method better than another...or maybe it varies from one build to the next.?
Can both methods be used at the same time...would THAT be problematic.?
Thank You

[g1]

You don't want to be using both methods at the same time. The real CT negates the virtual CT. You end up with a real CT but a couple extra resistors creating an extra current load on the heater winding.
Just going from various experiences I've read about, which type to use depends on the build. Sometimes real CT is quieter, in other amps virtual CT may be quieter.

[mac dillard]

Can only use one method. There is some thought that a humdinger is the best way to go. Can balance the two legs that way.

[Chuck H]

Considering the very low impedance of an actual CT for a winding tap I'd say that adding two 100R resistors is a waste of components and shouldn't make any notable difference. The virtual CT has the advantage of not requiring a CT on the winding (cheaper) and will likely blow up resistors before taking out the PT in the event of a gross tube failure (provided low wattage resistors are used). An actual CT probably has the advantage wiring simplicity and perhaps lower hum simply because of the low impedance, but I'm just guessing on this.

[trem]

Considering the very low impedance of an actual CT for a winding tap I'd say that adding two 100R resistors is a waste of components and shouldn't make any notable difference. The virtual CT has the advantage of not requiring a CT on the winding (cheaper) and will likely blow up resistors before taking out the PT in the event of a gross tube failure (provided low wattage resistors are used). An actual CT probably has the advantage wiring simplicity and perhaps lower hum simply because of the low impedance, but I'm just guessing on this.

Hey Chuck.....Thanks for the reply. THAT is the kind of "stuff" I was wondering about. Not saying I WANT to use both for some reason...I was just curious...and of course wondering about the theory behind which of the two might be "better".
Thanks Again

Good Grief...I see you quoted ME in your signature.
I cannot remember the exact topic(s).....but it had something to do with tubes, maintenance, tube technology Vs. modern technology and the way so many things are now just Throw-Away items...nothing is worth fixing anymore.
I was just opining about "the good old days" ...that I am kind of old fashioned...a bit older than my years, and have always liked older things.....but being born in 1960, I kind of missed out on a lot of good stuff, and was wishing I had "Been Born Sooner".
I did not realize you had immortalized me.

[Chuck H]

Re: the sig. It's funny as hell. You actually get to watch the thought process unfold in a well considered manner. And easy to identify with for most of us here. I loved it (and still do!).

[Alan0354]

I did not use the CT. When I grounded the CT, the voltage on each side was slightly different( Weber). Also, with my Bassman prototype, the Fender PT does not have a CT for filament. And I had to use rectified DC for the filament for the preamp tubes because I have a high gain channel with 4 stages. I ended up using like Fender with two 100 ohm in series across the DC output and ground the middle. The amp is quiet like a mouse. So I don't want to rock the boat and did the same way on the KMD.

[Enzo]

The real and virtual center taps are equivalent in my mind. The resistors are cheaper to add than a center tap is to include. Ther is no current through the center tap, but do ground it to the other grounds. We have discussed elsewher the idea of elevating the heaters by connecting the CT to some positive voltage. That prevents heater to cathode current from forming in a preamp tube. Those are tiny currents, but I see no need to add them to anything else, so that is why I'd ground my heater CT with the other CTs. Or go ahead and elevate it.

[loudthud]

Two points on the subject.

The heater winding is probably the last thing the transformer engineer adds to the design. Maybe RG will weigh in on this but I think for most transformers of the scale used in guitar amps, you end up with something on the order of 1 to 2 turns per Volt. So for 6.3V you need 6 to 12 turns. If it comes out to an odd number, are you going to split the winding on the top side of the bobin? My point is that the "Center Tap" might not be in the center of the winding. Is the engineer willing to fudge the numbers and increase either the copper or the iron just to make the center tap more in the center?

When you have an actual center tap, any short on one of the green wires will probably burn the wire unless by luck it blows the line fuse or a fuse on the heater winding. I've seen this happen at the pilot light on a Fender. The other side of that is the short at a power tube from pins 2 to 3. In that case it's nice to have a real center tap. Otherwise the virtual center tap resistors smoke and send high voltage to all the preamp tubes. Will the B+ fuse blow before the resistors vaporize? No good answer here. You can't protect against every possible fault. The humdinger has all the peril of the virtual center tap, but probably gives the lowest hum.

[Alan0354]

Two points on the subject.

The heater winding is probably the last thing the transformer engineer adds to the design. Maybe RG will weigh in on this but I think for most transformers of the scale used in guitar amps, you end up with something on the order of 1 to 2 turns per Volt. So for 6.3V you need 6 to 12 turns. If it comes out to an odd number, are you going to split the winding on the top side of the bobin? My point is that the "Center Tap" might not be in the center of the winding. Is the engineer willing to fudge the numbers and increase either the copper or the iron just to make the center tap more in the center?

I did measure slight difference in voltage between the two legs respect to the CT.

[trobbins]

The HT CT connects directly to the first filter capacitor. Any reference to the HT CT connecting to other 0V points is really the node at the capacitor negative terminal.

Stray AC heater voltage capacitive coupling from the ends of the heater to the grid of the input valve can be unbalanced due to valve construction and external wiring. A tuned humdinger can certainly minimise that hum ingress level, which may be the dominant hum contributor at idle.

I guess paralleling a high value resistor with one of the humdinger sides can sidestep the problem of using the humdinger as a poor-mans fuse for certain fault mechanisms. Similar to paralleling a high value resistor to a cathode fuse. Another example of Loudthuds Fender pilot light issue, is if an amp uses say a 6X4 rectifier, with the heater powering the other valves, and the 6X4 heater-cathode insulation fails.