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Newbie questions on power transformer
Newbie questions on power transformer
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Old 10th June 2018, 02:27 PM   #1
aslee is offline aslee  Hong Kong
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Smile Newbie questions on power transformer

Hello,

I have 3 questions on tube amp building:

1. Can I connect 2 power transformers with primary 110V to use them for 220V mains?

2. For secondary tap with 250-0-250, can I use it as 0-500V with bridge rectifier with same current rating?

3. In case of a power transformer with primary 110V & secondary 6V, can I use it as 220V primary with secondary 12V with half current rating?

Thanks very much in advance!

Lee
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Old 10th June 2018, 03:21 PM   #2
Alllensoncanon is offline Alllensoncanon  United States
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Newbie questions on power transformer
1. It is not safe. Chances are that the 110V transformer lack the inductance to operate at 220V. I wouldn't want to mass with high current and high voltage at the same time.
2. Yes. Just expect half the current output. Nothing is free.
3. Same as one.
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Old 10th June 2018, 03:24 PM   #3
rayma is offline rayma  United States
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Join Date: Apr 2011
Quote:
Originally Posted by aslee View Post
1. Can I connect 2 power transformers with primary 110V to use them for 220V mains?
2. For secondary tap with 250-0-250, can I use it as 0-500V with bridge rectifier with same current rating?
3. In case of a power transformer with primary 110V & secondary 6V, can I use it as
220V primary with secondary 12V with half current rating?
Just to reinforce, 1 and 3 are VERY dangerous, do not attempt!
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Old 10th June 2018, 03:44 PM   #4
aslee is offline aslee  Hong Kong
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Thank you for the replies.

For question one, I mean to put two 110V primary in series. So I guess each is having 110V.
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Old 10th June 2018, 03:59 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by aslee View Post
I mean to put two 110V primary in series. So I guess each is having 110V.
Only the twin, identical primaries of a single transformer can be connected in series.
Not for two physically separate transformers.

Last edited by rayma; 10th June 2018 at 04:02 PM.
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Old 10th June 2018, 04:29 PM   #6
H713 is offline H713  United States
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Quote:
Originally Posted by aslee View Post
Hello,

I have 3 questions on tube amp building:

1. Can I connect 2 power transformers with primary 110V to use them for 220V mains?

2. For secondary tap with 250-0-250, can I use it as 0-500V with bridge rectifier with same current rating?

3. In case of a power transformer with primary 110V & secondary 6V, can I use it as 220V primary with secondary 12V with half current rating?

Thanks very much in advance!

Lee
If you need 220V operation, either buy a transformer with a pair of primaries (can be wired for 110 OR 220) or buy a transformer wound directly for 220. Most (or all) of the Antek Toroidal transformers I believe have a dual primary. They're nice PTs for the price- I like the one I've got, but they can pose a bit of an aesthetic challenge with some builds.

Second, you can absolutely use the 250-0-250 as a 500V winding, but 500v x 1.414 is around 700V, which even with the drop from a tube rectifier is still going to be pretty high, certainly in the 650v range. That's pretty high for most tubes (ignoring of course the broadcast tubes).

If you're new, make sure to read up on high voltage safety... If you come across these voltages, they have the potential to kill you. The power supplies in most tube amps are not all that far off of a defibrillator circuit...
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Old 10th June 2018, 04:46 PM   #7
Alllensoncanon is offline Alllensoncanon  United States
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Newbie questions on power transformer
Quote:
Originally Posted by aslee View Post
Thank you for the replies.

For question one, I mean to put two 110V primary in series. So I guess each is having 110V.
I see, that is a different story. But you have to ask yourself. Why? If you are doing it at the bench, I would look at two things.

#1 How to balance the load between between the two transformers at both start up and while in operation. Using two identical transformer is a good suggestion.

#2 The other is putting two serial floating power supply together. With two same transformers, voltage double and current remain the same.

Last edited by Alllensoncanon; 10th June 2018 at 04:49 PM.
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Old 10th June 2018, 10:43 PM   #8
aslee is offline aslee  Hong Kong
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Thank you very much, indeed! Appreciate all your advice!
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Old 23rd June 2018, 12:00 PM   #9
FauxFrench is offline FauxFrench  France
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It has come up more times if a dual rail supply can be made with two transformers or if you need more current, can transformers be connected in parallel?
For those interested:
Appended are three figures on how to use two transformers for a single amplifier:

The straight forward parallel connection of primary and secondary windings is not advised. You risk both cross-currents and poor load sharing. In particular if the transformers are not identical, the cross-currents can be a big problem.

The parallel connection of the primary windings and the secondary windings through separate rectifier bridges is acceptable for identical transformers. Cross-currents are avoided by the two rectifier bridges but the load sharing may not be perfect.

Parallel connection of the primary windings and series connection of the secondary windings is “clean” in that each transformer and corresponding secondary winding is concerned with feeding a different supply rail momentarily so they never act in parallel and cannot source one another. This two transformer arrangement mainly has mechanical layout advantages. Electrically, it performs as a single transformer with two secondary windings and double the power (VA) compared to each of the two (identical) transformers.
Attached Images
File Type: jpg TransformersParallel.JPG (6.1 KB, 68 views)
File Type: jpg TransformersParallelRectifiers.jpg (50.4 KB, 68 views)
File Type: jpg TwoTransformersSeries.jpg (3.7 KB, 72 views)
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Old 26th June 2018, 02:52 PM   #10
aslee is offline aslee  Hong Kong
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Your diagrams are very helpful! Thanks!
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