Two positive rails from a center-tapped transformer?

[KT]

Hi,

I have a center-tapped transformer that I'd like to use to power two independent heaters in a dual-triode tube.

I'm curious if I can configure the center-tapped transformer to produce two positive rails, one for each of the heaters.

If this is possible, what is the best way to do it?

Thanks!
KT

[ianpengelly]

You could, but it would only be half-wave rectified, which is far from ideal. It is possible to create a single positive rail, full-wave rectified and use that for both. I shouldn't think cross-talk will be an issue since it is for heaters, not signal.

Sorry, should have said to do the latter use the centre tap as your zero reference and put a single diode on each of the taps in the same orientation then join together post diode.

[tomchr]

Transformers tend not to like half-wave rectification as it leaves a DC current running in the transformer. I've had transformers growl at me (increased hum) when using half-wave rectification.

The heater doesn't care if the voltage is positive or negative. As long as you aren't violating the heater-to-cathode voltage (Vhk) spec, you should be fine. If you're worried about hum induction, then you can elevate both heaters to ensure a 0 or small positive Vhk on both tubes - as long as you don't get in trouble with the Vhk spec... You could use attached circuit where the "0" node is raised to the appropriate level to keep the Vhk's happy.

What's the point of this if you don't mind me asking? Why do you need two positive supplies? Just curious... Maybe posting a schematic would allow visual people like myself to help you out in more ways than you'd imagined...

~Tom

[sq225917]

Maybe someone should tell Naim, they've been making dual positive power supplies from centre taps for decades.

[tomchr]

Originally Posted by sq225917

Maybe someone should tell Naim, they've been making dual positive power supplies from centre taps for decades.

Schematic?

[2wo]

I built a 2A3 amp a wile ago. I needed 2, 2.5V filament transformers. They were backordered. I had a Hammond 5v ct on hand, so I did a little surgery and found that I was able to separate the ct into 2 windings of 2.5V each…John

[KT]

Originally Posted by tomchr

The heater doesn't care if the voltage is positive or negative. As long as you aren't violating the heater-to-cathode voltage (Vhk) spec, you should be fine...

What's the point of this if you don't mind me asking? Why do you need two positive supplies?

Hi everyone,

Thanks for the input and advice. To answer your question, I'm inquiring about this center-tapped heater circuit because I already have a transformer with these windings.

The preamp in question is the WOT output transformer preamp from Diego Nardi of Audio Note Italia. It was featured in Sound Practices #10 some years ago (interestingly, the Euridice WOT preamplifier, which was featured in the same Sound Practices issue seems to have a much stronger following around the world. Information regarding the Diego Nardi WOT design is hard to locate on the web. Discussion about the Euridice are much more common).

At that time, I purchased a compliment of transformers from Diego to build this preamp. Of course, it's something like 15 years later and I'm only thinking now of actually doing it.

Your first point about the heater is useful, because in the original Nardi WOT design, the two heaters of the solitary 5687 are powered by the two opposite voltage rails from the center-tapped winding: the positive rail and and the negative rail of the center-tapped heater windings powering both of the individual heaters. Both sides are regulated with discrete regulators.

As you can tell, I'm a builder, but not a designer or engineer: I had no idea that powering heaters from both the positive and negative rails was a common practice. If this is acceptable, I may go ahead and proceed with this layout.

The reason I was unsure about doing this is that instead of using the 5687, as specified in the original design, I will be using a 6N6pi. I understand these two tubes are similar in their voltage and current requirement, just the pinouts and, perhaps, optimal operating points may be different. That's why I was unsure about using the same pos/neg rail scheme for powering the heaters; I thought the 6N6pi might require two positive rails.

If this sounds like the way to go, please let me know. And again, thanks a lot for your advice.

I don't have a scan of the WOT schematic currently, but will try to post it soon.

Best,
KT