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Old 7th December 2003, 07:31 AM   #1
Dags is offline Dags  United States
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Question Specific Transformer Question

Hi everyone, I'm planning on building a GainClone.

The first thing i'm working on is transformer selection. I happen to have come across a transformer at work, but no one knows anything about it. I'm new to transformers so I don't know if it's center tapped or what. Here's what it looks like:

3.75'' tall, 4.5'' wide, 3.5'' deep. It's not a Toroid, it's a standard looking transformer. The primary side has two wires (duh) brown and blue. The secondary side has two red, one black, one gray, and two yellow.

My question is, what exactly would I have to measure to see if this is usable for a GainClone? I presume either the black or the gray is 0v? But what would the other be then? I have a multimeter and everything, I just need to know which wires to touch the probes to, so i can figure everything out! I seem to remember getting 52v out of both reds when I was playing with it at work, so I know it is capable of delivering enough voltage. I just don't know if the configuration is right.

Sorry for such a beginner question, but can anyone help me?
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Old 8th December 2003, 03:38 AM   #2
AJT is offline AJT  Philippines
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looks like the two red wires are of higher voltage than the two yellow ones, use a dmm to confirm which of the black or the grey goes with the reds or the yellows, finally, you can check the secondary voltages using a voltmeter to measure secondary outputs with a 110 volts ac primary input...take care!!!
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Old 8th December 2003, 11:52 AM   #3
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Default testing the transformer carefully.

Usually the high voltage secondary will have a higher dc resistance than the low voltage coil. For 100VA transformers the high voltage coils could be 100 ohms or thereabout. The low voltage coils could be 10 ohms or less. It all depends on the VA ratings.
Touching the high voltage secondary wires is like touching the mains. It could potentially be lethal . Do be careful. You could always tape the meter leads to the wires and switch on the power to check. Keeps your hands well away from danger.
Cheers.
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Old 8th December 2003, 12:22 PM   #4
sreten is offline sreten  United Kingdom
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First use a multimeter to check what secondary wires are
connected to each other - you could have two isolated
secondaries.

Then measure the resistances to get a good idea of the connections.

For example if red-black-red is an isolated secondary centre-tapped :
The resistance to the other wires will be very high.
Resistance red to red will be twice red to black, both
red to black resistances will be the same.

I suspect the transformer has two centre-tapped secondaries,
red-black-red and yellow-grey-yellow.

Having done this for all wires you should have a good
idea of how to connect it up and measure secondary
voltage.

Once you know the secondary voltages you need to
establish / estimate the VA rating of each secondary.

You can use the resistances measured to help you
estimate this - probably best to post your measurements.

You need to avoid trying to use a low VA secondary of
a large transformer even though it has the right voltage.

/sreten.
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Old 8th December 2003, 08:25 PM   #5
Dags is offline Dags  United States
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Okay guys, first off, thanks for the replies! I tested resistance & voltage, and here's what I found:

Yellow to Yellow Resistance = 1
Gray to Black Resistance = 0.7
Red to Red Resistance = 0.5

Any other combination gave me infinite resistance. I assume the transformer is not center tapped then. Am I correct?

Here are the voltages @ 120vAC input:

Yellow to Yellow = 54.5v
Gray to Black = 27.5v
Red to Red = 11.35v

Okay, so this looks like it has 3 isolated secondaries, correct? The "black" wire is only half black. It is gray coming out of the transformer, and somebody spliced a black wire to it. I didn't notice that before, sorry!

So what does this tell me about my transformer? Is it pretty much useless for a Gainclone? What would the VA rating be? If it is useless to me for Gainclone, are there any other easy to build amps that I could use it for?

Thanks in advance for all your help!
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Old 8th December 2003, 08:36 PM   #6
Dags is offline Dags  United States
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I'm sorry, that didn't come out right. the "" is supposed to be an Omega.

So it should read:
Yellow to Yellow Resistance = 1 Ohm
Gray to Black Resistance = 0.7 Ohms
Red to Red Resistance = 0.5 Ohms
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Old 27th April 2011, 02:12 PM   #7
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This is a really old thread but i was keen to know if Dags had a transformer with three secondaries ? Or was he measuring two center tapped incorrectly ?
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Old 30th April 2011, 01:46 PM   #8
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Traveller View Post
This is a really old thread but i was keen to know if Dags had a transformer with three secondaries ? Or was he measuring two center tapped incorrectly ?
Hi,

it looks like he did have a three secondaries transformer.

For a power amplifier you want two isolated or centre tapped identical secondaries.

For a integrated amp you want an additional two isolated or centre tapped identical lower voltage secondaries.

The above when isolated is also useful for Mosfet amplifiers, where you can use the extra taps for higher drive voltage.

rgds, Sreten.
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