Which PS give better perf. with a Tube rectifier - PT wCT or noCT + 2 diodes ??

[kagliostro]

I'm waiting to put an order because the material is out of stoch at the moment

my circuit requires an Hammond 369GX  (225v CT @ 75mA - 6.3v @ 2.5A)

the supplier give me two choices (other brand of course)

1 = 230v CT @ 100mA - 6.3v @ 3A                 (dimensions 88mm x 70mm)

2 = 230v @ 120mA - 6.3v @ 4A - 20v 120mA  (dimensions 78mm x 65mm)

Initially I was thinking to keep the first option as it has the CT, then I was wondering that we usually use CT transformer more for tradition

than other (... or so seems to me)

So my question is ... mantaining the use of a tube rectifier, is more performant (ripple fequency and other) to use a PT that has a CT or

to use a transformer without the CT and use two diodes + the tube rectifier arranging an hybrid rectifier bridge ?





Many Thanks

K

[sluckey]

Regarding the circuits in the pics... I think performance will be very similar, maybe even identical.

the supplier give me two choices (other brand of course)

1 = 230v CT @ 100mA - 6.3v @ 3A                 (dimensions 88mm x 70mm)

2 = 230v @ 120mA - 6.3v @ 4A - 20v 120mA  (dimensions 78mm x 65mm)

But these two transformers will give different B+ outputs. #2 will have twice the B+ of #1. #1 would need to be 460v CT (like the PT in your pic) to put out the same voltage as #2.

[kagliostro]

Ciao Steve

But these two transformers will give different B+ outputs. #2 will have twice the B+ of #1. #1 would need to be 460v CT (like the PT in your pic) to put out the same voltage as #2.

that seems odd to me

a 230v-0-230v AC used with 2 diodes full wave rectify I think gives ~322v DC

a 0-230v AC used with a full bridge I think gives the same ~322v DC

 

May be the trouble (my trouble) is I refer as a 230v AC CT instead to say 230v-0-230v AC ?

The vendor specified the PT as 2 x 230v 0.1A und 6.3v 3A

---

Yes, those transformers are a bit different in current handling, but my question is independent of that

what I would like to say, and may be I incorrectly esposed, is if the transformers had the same voltage and same current handling

will be the noCT version, used with a tube rectifier and a pair of diodes as to have an hybrid bridge, more performant in terms of

ripple (and other aspects that I at the moment didn't suppose) ?

Franco

[sluckey]

The PT in your first diagram is 230-0-230. Or you could say 460VAC with CT. But a 230-0-230 is not the same as 230v CT. A 230v CT would be the same as 115-0-115.

It's easy to miscommunicate this stuff. Be sure that you and your supplier know exactly what each other is talking about.

[kagliostro]

You are perfectly right Steve, it was my fault

the vendor say 2 x 230v @ 0.1A - 6.3v 3A

Franco

[MadMax]

I'm reading this post and trying to understand these transformer ratings. But the water is still muddy.

IF: "A 230v CT would be the same as 115-0-115." THEN: A 230-0-230 would be the same as a 460v CT, right?
Therefore, if the latter is what Franco has pictured in his first diagram with the CT grounded, why isn't it the same as the 230v situation in the second diagram? He shows 230v on each half of the transformer in diagram #1, so regardless of his dialog, are the DIAGRAMS equivalent?

-MadMax-

[HotBluePlates]

IF: "A 230v CT would be the same as 115-0-115." THEN: A 230-0-230 would be the same as a 460v CT, right?

Yes. If you add the letters "CT" to a transformer secondary voltage, it implies the voltage is that of the entire secondary winding with a center-tap halfway. So 230-0-230v has a secondary of 230+230 = 460v CT.

Therefore, if the latter is what Franco has pictured in his first diagram with the CT grounded, why isn't it the same as the 230v situation in the second diagram? He shows 230v on each half of the transformer in diagram #1, so regardless of his dialog, are the DIAGRAMS equivalent?

Yes, the diagrams are equivalent.

The center-tapped transformer at the top uses a full-wave rectifier, wherein during any instant one rectifier diode is conducting and the other is shut off. The rectified voltage is that from the transformer's center-tap to the conducting diode, or half of the full secondary's voltage.

The transformer at the bottom uses a full-wave bridge rectifier (although that might be harder to see with the tube/solid-state hybrid setup). At any instant, 2 of the 4 diodes present are conducting, one attached to each end of the secondary. Therefore, the full secondary's voltage gets rectified. A bridge rectifier is then more-efficient use of transformer materials as all of a secondary winding is in use at all times.

The all-secondary vs half-secondary is also why some think a bridge rectifier doubles voltage, because when they attach a bridge rectifier to their secondary which previously used a CT, they get double the voltage output they did before with a full-wave rectifier. The bridge is not a "doubler" of itself, the user just unknowingly went from using half-secondary to full-secondary voltage. Simple errors like this is why Sluckey keyed on the discrepancy between what Kagliostro drew (where outputs are the same) vs what he said (where outputs would not be the same).

my circuit requires an Hammond 369GX  (225v CT @ 75mA - 6.3v @ 2.5A)

You will have another 6.3vac winding for the tubes other than the rectifier, right? 230*1.414 = ~300vdc after rectifier losses. None of your other tubes will be happy having 200-300v from heater to cathode.

The 6X4's heater-to-cathode voltage rating is 100vdc, requiring the 6.3v for that tube be referenced to the B+.

[sluckey]

You will have another 6.3vac winding for the tubes other than the rectifier, right? 230*1.414 = ~300vdc after rectifier losses. None of your other tubes will be happy having 200-300v from heater to cathode.

The 6X4's heater-to-cathode voltage rating is 100vdc, requiring the 6.3v for that tube be referenced to the B+.

Lot's of small amps use the 6X4 with it's filament on the same string as the other tubes. Here's one...

     http://el34world.com/charts/Schematics/files/danelectro/danelectro_centurian_275.pdf

That 100vdc rating applies as a maximum if the heater is positive with respect to the cathode. But the heater will be negative in respect to the cathode so the maximum should be 450vdc. Or did I read that wrong?

[HotBluePlates]

Oops! You're right!!

I looked at the first rating I saw instead of seeing the spec which applied to this circuit. Thanks for the correction!

[kagliostro]

Is correct to assume that a 2 diodes full wave rectify give a ripple like this



and a 4 diodes bridge rectifier give a ripple like this



so there is a change in ripple frequency and also the ripple voltage amplitude is smaller ?

or the first example is to be referred to a single wave rectifier (1 diode) ?

Franco

[sluckey]

They are both FULL WAVE rectifiers so they will both look like the second drawing. A single diode (half wave) rectifier will look like your first drawing.

[kagliostro]

Thanks Steve & HBP

Franco

[MadMax]

Thanks HBP for the edification. So if you took the 230-0-230 transformer from example #1, lifted the center tap and used a full-wave rectifier, you would then have the full 460v (or the rectified equivalent). If you say that is correct, my understanding of this concept is adequate.

-MadMax-

[HotBluePlates]

Yes, a full 460vac input to the rectifier.

You were right all along, I just said it a different way. Hopefully, that just reinforced the concept for you.

[sluckey]

there is a difference between the transformers outputs

That's right.

and the second case 60 cycle hum

That's wrong.

I do not believe this has been discussed in this thread.

It has...

They are both FULL WAVE rectifiers so they will both look like the second drawing. A single diode (half wave) rectifier will look like your first drawing.

[PRR]

The 460V CT (230-0-230) with two diodes, and the 230V (no CT) with four diodes, give the *SAME* output.

Choice depends on the cost of diodes versus windings.

Before solid rectifiers, vacuum rectifiers were expensive. We want a minimum number of rectifiers. In particular cathodes (and their heater windings) are expensive, so we want a minimum number of cathodes.

The CT with a common-cathode rectifier is the cheapest plan for vacuum rectification.

Now with small cheap floating Silicon rectifiers, things are different. The high-voltage CT winding has double the turns of thinner (more fragile) wire and a tap. A single winding is less turns of tougher wire and significantly cheaper. You "pay" for that with two more diodes, but they are 10 for a buck now.