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Post subject: Finding impedance of unknown OPT's
Posted: Sat Jul 28, 2012 8:31 am
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I posted this in another board. Maybe useful for those checking out garage sales this weekend. To find secondary and primary impedances of an unknown, but good looking OPT. :D

1.) Remove output transformer (OPT) from amp.

2.) You need a signal generator (SG)

3.) Measure the VAC across the signal generated at, say, 1000Hz.

4.) Should be around 0.25 to 1 VAC.

5.) Place the SG leads onto each the secondary winding.

6.) Connect your DVM (reading VAC) onto primary ends. Do not connect center taps (CT), if present. Just leave the CT connected together, but not to the DVM leads.

7.) Get primary VAC reading.

8.) Divide the primary VAC by the secondary VAC = turn ratio.

9.) Square the ratio value and divide into the primary impedance load = secondary impedance.

10.) Say, for a dual push-pull 6V6GT amp, primary = around 6600 ohms. And you get a ratio of 28.7 to one volt (primary to secondary).

Square it to get 823.69. Divide 6600 ohms by 823.69 = 8.01 ohms (approximate secondary impedance).


You can also use this method to find the primary impedance of an unknown OPT.

1.) Place measured VAC SG (at 1000Hz) onto secondary ends.

2.) Now, measure VAC across the primary leads (CT not connected).

3.) Ratio of Primary VAC to Secondary VAC = turn ratio. You can
also apply VAC to Primary and read the Secondary VAC.

4.) Square the ratio of Primary/Secondary VAC TIMES secondary impedance = Impedance of the Primary.

For instance, you find an unknown OPT. You measure a 1 VAC SG at 1000Hz across two secondary leads at 29.2VAC on the primary. Ratio = 29.2.
Square it to get 852.64.

If you multiple this ratio by the desired secondary impedance of 8-ohm you get:
8 X 852.62 = 6821.12 ohms

This is around the impedance of two 6V6GT's (in push-pull, AB1 mode --- ala BF/SF Deluxe Reverb).


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Post subject: Re: Finding impedance of unknown OPT's
Posted: Sat Jul 28, 2012 6:26 pm
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Joined: Fri Jun 26, 2009 6:53 pm
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Very cool, Beemer!!! Now where is that caculator? Art

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