Newbie questions on power transformer

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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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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!
 
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...
 
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.
 
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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.
 

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This arrangement will work properly if, and only if the loads on the + and - supplies are identical: if it is not the case, the transformers will see a DC component, which can lead to a number of problems.

Note that even in the case where the loads look balanced, as in an audio amplifier, this will no more be the case if the amplifier has to handle frequencies < 2*Fmains.


Regarding the series-connection of two 110V primaries, it is never advisable, but there are (strict) conditions under which it is tolerable:
-The transformers need to be absolutely identical
-The secondary loads must be identical too. There are two ways to fulfill that condition. The most reliable is to parallel the two secondaries. The other one is to connect them in series, with nothing else connected to the junction point, but under no-load condition, one of the transformer might take most of the voltage because the magnetic circuit is never perfect, and one of them will have a higher reluctance, due to mechanical tolerances, steel quality, etc
 
Thanks for your information. You "strict" conditions may be applicable to me.
If I buy two identical Japanese power transformers with 105V primary, connect the primaries in series with the secondaries in parallel. The mains in my locality is 220V. So the secondary voltages will be a little bit higher. Will this work? If so, I don't have to use a step down transformer then.
 
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