Multiple filament windings

[debk]

I have a Power tranfo with two 6.3V filament windings neither are center tapped. Each 6.3V winding is rated at 2.5A

The project I am going to use this in will have two tubes. Each tubes filament draws 750mA, so 1.5A total both tubes.

So the question is should I just use one winding for both tubes, or since I have the extra winding, use a separate winding for each filament. There is adequate current for either method. I plan on running the heaters with DC.

Are there advantages or disadvantages to either of the scenarios.

Would I be better off connecting the two filament windings in series and then using the the 12.6V in a regulated circuit for the heaters?

Thanks

Debra

[HollowState]

Well Debra, you haven't said what the tubes are, or what they will be for, so a good answer will have to be a guess. However, since you want to DC heat them, the 12.6 volt option seems best at the moment.

[debk]

The tubes are russian 6N6P
I am going to use them in a preamp

Debra

[ryuji]

Filaments are often mismatched in even same brand tubes for resistance. A lot of tube datasheets will advise against series operation for this reason. if your transformer has four wires and not a internally connected center tap its better to paralell the windings for even loading if you run as 6.3

[boywonder]

Rectifying 6.3VAC to make 6.3VDC can be a challenge........That being said, if you are considering AC filaments, one 2.5A rated winding is plenty for 1.5A. The voltage will probably be a little high since the winding will be under-loaded. You can derive a center tap with a 100 ohm resistor from each leg to ground.

Rectifying 12.6VAC to make 6.3VDC will probably end up dropping quite a bit of voltage as heat...either in resistors or regulators.

[Eli Duttman]

Debra,

You said you want to DC heat 2X "6" V. tubes each of which draws 750 mA. Phase up the pair of filament windings, connect them in parallel, and Greinacher voltage double the 6.3 VAC into a stack of 2X 15000 μF./15 WVDC 'lytics. Use that DC to drive a 7812 3 terminal regulator IC. Heat sink the IC VERY well, as it's working hard. Use the regulated 12 VDC to energize the heater pair wired in series.

I usually suggest Schottky diodes in heater supplies, but not this time. The larger forward drop in PN junction diodes will reduce the power the regulator IC has to dissipate. To ensure PN diode switching noise is not a problem, connect a 1 μF. Panasonic ECQ-V (stacked film) cap. across the IC's I/Ps. Mount that cap. as close to the IC as you can. BTW, the cap. improves regulator action too.

[ryuji]

Rectifying 6.3 and adding two extra diodes for extra voltage drop would bring it down to 6.1 to 6.4v?

[debk]

Originally Posted by Eli Duttman

I usually suggest Schottky diodes in heater supplies, but not this time. The larger forward drop in PN junction diodes will reduce the power the regulator IC has to dissipate. To ensure PN diode switching noise is not a problem, connect a 1 μF. Panasonic ECQ-V (stacked film) cap. across the IC's I/Ps. Mount that cap. as close to the IC as you can. BTW, the cap. improves regulator action too.

Eli
Not sure where you want me to put the capacitor.
In the typical 7812 circuit I can find there is a capacitor from pin 1 to ground, another capacitor fromm pin 3 to ground, and a diode from pin 1 to pin 3. Does the cap you are referring to go between pins 1 and 3, bypassing the diode?

Thanks
Debra

[debk]

One other question. Instead of using the 7812 could you use a LM 317?

With it rated at 1.5A would it have less thermal issues than the 7812?

Debra

[ryuji]

Originally Posted by debk

One other question. Instead of using the 7812 could you use a LM 317?

With it rated at 1.5A would it have less thermal issues than the 7812?

Debra

You can use it, indeed. but you cant break ohms law. He comes to beat you up.
12.6v rectified becomes 17.81v, minus ~1.2v from diode forward voltage drop, you get 16.61v. to cut 16.61v down to 12.6 volt you need to drop 4v, at 1.5 amps you need to dissipate 6 watts, which is way more then a TO-220 package can passively handle. even at 750 mA its best to have a heat sink on there. TO-220 is about 50C/watt without a heat sink.

neat thing about the LM317 is unless you short one out or way overvoltage it, you cant really burn one up/out. it has overcurrent and overtemperature protection. however, it will underperform without a heat sink as it only does as much as it can handle