Filament grounding- center tap or virtual- which is better?

[gsr]

Hello,
I am starting another build, and my pt has a heater center tap. I would like to get opinions as to whether to use it, or to go virtual. (My first build had no center tap, and the virtual center tap worked well.) I'm just curious to hear opinions of those more experienced than I. Thanks

[mooreamps]

Hello,
I am starting another build, and my pt has a heater center tap. I would like to get opinions as to whether to use it, or to go virtual. (My first build had no center tap, and the virtual center tap worked well.) I'm just curious to hear opinions of those more experienced than I. Thanks

If your power transformer has a center tap ; then use them both.

-g

[Topper]

I too have a center tapped heater. Can I connect the heater center tap to the power tube cathode to elevate the heaters in the same way one would connect the virtual center tap to the cathode? Also, what is the advantage to using both a real center tap and a virtual center tap?

[Bruce / Mission Amps]

Hmmm .... both?
Here's a very simple and elegant solution:
Take the common point of the virtual center tap, or a real center tap, ... to a small voltage divider off the last high voltage filter cap where the B+ is the least.
Use something like two 1/2 watt resistors in series to ground with a 22uF@100v cap to ground across the grounded resistor.
Something like a 270K resistor and 27K resistor in series to ground with the cap across the 27K resistor.
Then attach your filament winding's "center tap" to the junction of the two resistors.

[mooreamps]

Hmmm .... both?

Well, for power tube filaments, in which I use A/C power, I put a pair of matched 100 ohm resistors across each of the secondary windings, but not to ground, but elevated to abouts 80 volts or so. For preamp filaments, it's still a matched pair of resistors, but then into a full wave bridge.

-g

[Diablo]

On one of my amps, I used the bias supply winding on the PT to elevate the heater windings. I used a diode to rectify the bias voltage and a voltage divider to drop the voltage to 25V, then an electrolytic cap to filter the voltge. The heater windings were connected to the offset DC voltage with a pair of matched 100R resistors. It works. Any reason this might be a bad idea vs using the B+ as a source for the DC offset voltage?

[mort]

Correct me if I'm wrong but the only goal here is to reference the heater voltage to ground. If there's a center tap just use it. If not then do the virtual. No gain in making it any more complicated than that, right??

[The Dude]

On one of my amps, I used the bias supply winding on the PT to elevate the heater windings. I used a diode to rectify the bias voltage and a voltage divider to drop the voltage to 25V, then an electrolytic cap to filter the voltge. The heater windings were connected to the offset DC voltage with a pair of matched 100R resistors. It works. Any reason this might be a bad idea vs using the B+ as a source for the DC offset voltage?

Honestly, not trying to be a "Richard Noggin'", but what is the point of this? I'm simply curious as to why you'd go through so much trouble to elevate the heater supply. Or maybe I'm not understanding what you did?

[Diablo]

Honestly, not trying to be a "Richard Noggin'", but what is the point of this? I'm simply curious as to why you'd go through so much trouble to elevate the heater supply. Or maybe I'm not understanding what you did?

I was taking the shotgun approach to quieting the hum. Balanced the heater windings with the matched resistors, and then gave them 25V of DC offset. Some smart folks here said that elevating the heaters helps to reduce hum. Not so much trouble, and it's a hobby, not a business for me. Or are you saying that it would have been simpler to use a voltage divider off the B+? I think the only difference is that with the bias tap, I added an extra diode and e-cap. The only reason I picked the bias winding is that it's filtered DC voltage of 50V comes out closer to my target of 25V for the offset, so it just felt right.

[The Dude]

I was taking the shotgun approach to quieting the hum. Balanced the heater windings with the matched resistors, and then gave them 25V of DC offset. Some smart folks here said that elevating the heaters helps to reduce hum. Not so much trouble, and it's a hobby, not a business for me.

Reiterating: I wasn't being critical- just asking. So,......did it work? Did it do anything for your hum?

[Justin Thomas]

My (former) Ampeg had the Heater CT, which was tied to the cathodes to elevate them... seems you did the same thing, just got the elevation voltage from a different place, and made your own CT, right? Is this a fixed-bias amp? And is that why you used the bias tap instead of the cathodes?

Justin

[Diablo]

Reiterating: I wasn't being critical- just asking. So,......did it work? Did it do anything for your hum?

Well the amp is quieter than it was, but I didn't take the scientific approach and try one thing at a time. So I can't say if the heater work improved anything, but it didn't make it worse....

[Diablo]

My (former) Ampeg had the Heater CT, which was tied to the cathodes to elevate them... seems you did the same thing, just got the elevation voltage from a different place, and made your own CT, right? Is this a fixed-bias amp? And is that why you used the bias tap instead of the cathodes?

Justin

Yes, you guessed right, it's a fixed bias amp, so no cathode resistor to supply the voltage offset.The amp also has a heater center tap, but I disconnected that to try the virtual center tap to see if that would improve things.

[g1]

The amp also has a heater center tap, but I disconnected that to try the virtual center tap to see if that would improve things.

Well, you could have just run that center tap to your elevator supply, instead of the resistors. That's maybe what Mort and Dude meant about why complicate things. Now you have an opportunity to try it that way and see if the virtual or (elevated) real center tap is better.

[Diablo]

Well, you could have just run that center tap to your elevator supply, instead of the resistors. That's maybe what Mort and Dude meant about why complicate things. Now you have an opportunity to try it that way and see if the virtual or (elevated) real center tap is better.

I read somewhere that the filament windings might not be balanced via the center tap, and you could achieve better balance by using matched 100R resistors with a virtual center tap. One other thing I found was they wired the heaters out of phase on the push pull output tubes, so I rewired that part of the heater circuit.