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Old 18th April 2012, 07:51 AM   #1
Jaap is offline Jaap  Netherlands
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Default simple question

Perhaps a stupid question, but just to be sure:

I have a transformer with a centertapped secondary. I need two seperate supplies (for a tube amp) with different voltages and currents. Can I connect the centertap to ground and use the two windings for seperate half wave rectifiers ?
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Old 18th April 2012, 08:15 AM   #2
Elvee is offline Elvee  Belgium
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Originally Posted by Jaap View Post
Can I connect the centertap to ground and use the two windings for seperate half wave rectifiers ?
Yes, you can, but I am not sure it makes sense: since it is a center-tap, the raw voltages at both the rectifiers outputs will be similar.
With that in mind, why not use a classic full-wave rectification, and from there split your supply the way you want: two different CLC's or regulators or whatever.

That method has a number of advantages: there is no risk of power unbalance between the windings, and there is no risk of magnetization of the core if one supply draws more current than the other.
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Old 18th April 2012, 08:50 AM   #3
Jaap is offline Jaap  Netherlands
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Originally Posted by Elvee View Post
Yes, you can, but I am not sure it makes sense: since it is a center-tap, the raw voltages at both the rectifiers outputs will be similar.
With that in mind, why not use a classic full-wave rectification, and from there split your supply the way you want: two different CLC's or regulators or whatever.

That method has a number of advantages: there is no risk of power unbalance between the windings, and there is no risk of magnetization of the core if one supply draws more current than the other.
Yes, I think that is what I do. Use a full bridge and then split. At one channel I will put a tube rectifier in series to get the voltagedrop I need to feed the driver tube in the amp.

In the meantime I found another solution on the net. If you use a full bridge on the outer taps of the transformer you can have about halve of the raw dc at the centertap. Filter that and you also have your second dc voltage.

I wonder if there is a preferred solution
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Old 18th April 2012, 10:55 AM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Jaap View Post
Yes, I think that is what I do. Use a full bridge and then split.
If you use a full bridge, you'll end up with twice the voltage of your initial solution.
With a center tap, only two diodes are required for a full-wave rectification


Quote:
In the meantime I found another solution on the net. If you use a full bridge on the outer taps of the transformer you can have about halve of the raw dc at the centertap. Filter that and you also have your second dc voltage.
Yes, using a full bridge is the same a 2x full wave rectification with two diodes, one positive and one negative.
Quote:
I wonder if there is a preferred solution
It all depends on the voltage you need vs. the windings voltage.
It would be silly to end up with 600V when you only need 270V.
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Old 20th April 2012, 07:24 AM   #5
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by Jaap View Post
Yes, I think that is what I do. Use a full bridge and then split. At one channel I will put a tube rectifier in series to get the voltagedrop I need to feed the driver tube in the amp.

In the meantime I found another solution on the net. If you use a full bridge on the outer taps of the transformer you can have about halve of the raw dc at the centertap. Filter that and you also have your second dc voltage.

I wonder if there is a preferred solution
Usually you see one high voltage supply, with lower voltages tapped off using a drop-down resistor and an isolating cap.
The reason for this is that the 'regulation' gets progressively better as the voltage (and hum) is stepped down, and hum is shunted off in a cumulative fashion.
Usually the lower voltage supplies (in an audio amp say), need less and less current,
but require better and better isolation and are more noise-sensitive.

The disadvantage of trying to tap off a lower (half-voltage) near the source (c.t., and/or diode bridge) is that you usually need a divider-resistor network, and

(1) this both wastes power and
(2) the lower voltage will sag with any sag on the H.V. due to extra current draw, and
(3) the lower voltage will vary with current draw from the lower voltage load, and
(4) the lower supply will have a higher impedance anyway, because of the dropping resistor, or else the resistor-divider will draw excessive current if the values are too low.
(5) there is poor isolation from noise when tapping close to the main source for the supply.
(6) you rarely need exactly half the main HV for your lower voltage supply.

TYPICAL MULTI-VOLTAGE POWER SUPPLY DESIGN:

Click the image to open in full size.



NOT SO EFFICIENT DESIGN:

Click the image to open in full size.
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Old 20th April 2012, 07:53 AM   #6
Jaap is offline Jaap  Netherlands
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This is the variant with clever use of the centertap
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Old 20th April 2012, 10:52 AM   #7
DF96 is offline DF96  England
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I have had a bright idea: why don't you tell us what you are trying to achieve, and what your transformer is? Then we can all stop guessing.
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Old 20th April 2012, 11:26 AM   #8
Jaap is offline Jaap  Netherlands
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Sorry, I thought this was clear. I decided to use a full bridge on the outer taps of a 285-0-285 V secondary and feed 6V6 output tubes of a pp amp with that.
From the centertap i feed the drives tubes.

Of course everything with filters, regulators, gyrators and ccs's :-)
Problem solved
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Old 20th April 2012, 06:12 PM   #9
nazaroo is offline nazaroo  Canada
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Originally Posted by Jaap View Post
This is the variant with clever use of the centertap
I really like the 6L6 idea for a 400v supply:

What is the current capability and what is the control tube, 12AU7?
Did you design this, or is this an example from somewhere?
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Old 20th April 2012, 08:30 PM   #10
Jaap is offline Jaap  Netherlands
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I am going to use this design from RCA. I will use the 6sn7 at the front and perhaps will experiment with outputtubes (807 ?) after it works. I will use solid state regulator and combination of gyrator/CCS on the first tube. Possibility is to make the outputstage differential with two CCS (LM317) in the tail. But perhaps best to make as a first step the original design.
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