Determining the rating of a transformer

DeadSpeaker · 8th March 2008, 03:26 AM

On my quest to build the perfect guitar amp, I've dug this old power amp out of my closet. I was thinking about selling it to help fund a new transformer that will work in Rod Elliot's 100watt amp, but then I decided to see if it's transformer would work (it has to be a minimum of 25-0-25 150v 3amps).

On Rod Elliot's site, he has a great wealth of information on transformers. http://sound.westhost.com/articles/xfmr3.htm

I don't have the right equipment for all the tests that would be needed. Is it possible to measure the transformer at the rails with a volt meter? Or is there a simplified formula? Thanks

-Josh

TheMG · 8th March 2008, 03:54 AM

Well, measuring the voltage is only half of the pie.

You need to know how much current the transformer can handle before either sagging too much or heating up like miniature toaster oven.

If you're lucky, you may be able to lookup the part no. of the transformer. If not, you may be able to find some service manuals for the particular amp which would have more detail about the voltages and whatnot.

DeadSpeaker · 8th March 2008, 03:58 AM

That's why I want to measure the thing(or calculate it some how.) I can't find any info on the amp, and there is a sticky pad conveniently placed on top of the code. I think removing it would remove the code as well.

TheMG · 8th March 2008, 04:07 AM

For that, you're going to need a variable test load.

DeadSpeaker · 8th March 2008, 04:16 PM

Hmmm well It looks like it is going on ebay then. Thanks

yaron · 8th March 2008, 09:32 PM

Hi
You can also order a suitable transformer from a workshop of transformers. You need to tell them the current it shoul take, voltage, frequency -3db, and they can create one for you.
Yaron

TheMG · 8th March 2008, 09:53 PM

This company offers very good bang-for-buck transformers:

http://www.toroid-transformer.com/

Haven't extensively tested mine yet (still waiting on UCD modules) but many people have used them with great success. One thing's for sure though: the price is right.

DeadSpeaker · 9th March 2008, 09:35 PM

Wow those are good prices. They have dual secondaries though. Do you know if they have the center tapped variety?

DeadSpeaker · 10th March 2008, 02:06 AM

I should have done a little research before I asked about center tapped toroids. This is a quote for Rod Elliott's site

"Many transformers are supplied with two (or more) secondaries. In many cases, the data sheet will indicate that the windings may be connected in parallel or series. For example, a toroidal transformer may be rated at 2 x 25V at 5A (250VA). With the windings in parallel, the available current is 10A, but only for a single voltage of 25V AC. Connect the windings in series, and you get 50V at 5A, or by referencing the centre tap to earth, the familiar 25-0-25 designation."

http://sound.westhost.com/xfmr2.htm#8.series-parallel

EWorkshop1708 · 10th March 2008, 04:05 AM

The 25V 1000VA transformer has 4 secondaries!

8th March 2008, 03:26 AM	#1
DeadSpeaker diyAudio Member Join Date: Jul 2007	Determining the rating of a transformer On my quest to build the perfect guitar amp, I've dug this old power amp out of my closet. I was thinking about selling it to help fund a new transformer that will work in Rod Elliot's 100watt amp, but then I decided to see if it's transformer would work (it has to be a minimum of 25-0-25 150v 3amps). On Rod Elliot's site, he has a great wealth of information on transformers. http://sound.westhost.com/articles/xfmr3.htm I don't have the right equipment for all the tests that would be needed. Is it possible to measure the transformer at the rails with a volt meter? Or is there a simplified formula? Thanks -Josh

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8th March 2008, 03:54 AM	#2
TheMG diyAudio Member Join Date: Jan 2008	Well, measuring the voltage is only half of the pie. You need to know how much current the transformer can handle before either sagging too much or heating up like miniature toaster oven. If you're lucky, you may be able to lookup the part no. of the transformer. If not, you may be able to find some service manuals for the particular amp which would have more detail about the voltages and whatnot.

8th March 2008, 03:58 AM	#3
DeadSpeaker diyAudio Member Join Date: Jul 2007	That's why I want to measure the thing(or calculate it some how.) I can't find any info on the amp, and there is a sticky pad conveniently placed on top of the code. I think removing it would remove the code as well.

8th March 2008, 04:07 AM	#4
TheMG diyAudio Member Join Date: Jan 2008	For that, you're going to need a variable test load.

8th March 2008, 04:16 PM	#5
DeadSpeaker diyAudio Member Join Date: Jul 2007	Hmmm well It looks like it is going on ebay then. Thanks

8th March 2008, 09:32 PM	#6
yaron diyAudio Member Join Date: Mar 2008	Hi You can also order a suitable transformer from a workshop of transformers. You need to tell them the current it shoul take, voltage, frequency -3db, and they can create one for you. Yaron

8th March 2008, 09:53 PM	#7
TheMG diyAudio Member Join Date: Jan 2008	This company offers very good bang-for-buck transformers: http://www.toroid-transformer.com/ Haven't extensively tested mine yet (still waiting on UCD modules) but many people have used them with great success. One thing's for sure though: the price is right.

9th March 2008, 09:35 PM	#8
DeadSpeaker diyAudio Member Join Date: Jul 2007	Wow those are good prices. They have dual secondaries though. Do you know if they have the center tapped variety?

10th March 2008, 02:06 AM	#9
DeadSpeaker diyAudio Member Join Date: Jul 2007	I should have done a little research before I asked about center tapped toroids. This is a quote for Rod Elliott's site "Many transformers are supplied with two (or more) secondaries. In many cases, the data sheet will indicate that the windings may be connected in parallel or series. For example, a toroidal transformer may be rated at 2 x 25V at 5A (250VA). With the windings in parallel, the available current is 10A, but only for a single voltage of 25V AC. Connect the windings in series, and you get 50V at 5A, or by referencing the centre tap to earth, the familiar 25-0-25 designation." http://sound.westhost.com/xfmr2.htm#8.series-parallel

10th March 2008, 04:05 AM	#10
EWorkshop1708 diyAudio Member Join Date: Nov 2004 Location: Florida	The 25V 1000VA transformer has 4 secondaries!

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