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johi 13th January 2012 09:16 AM

SE OT impedance question

As newbie in diy world I purchased a pair of SE OT from eBay Germany not knowing much more that they were type S2 (whatever this means) and that theyare suited for SE projects, 4 Ohm out, 4 - 8 W. Seller proposed EL95, ECL86 tubes as candidates.
So I wanted to check impedance of the OT to see which more tubes could be suitable. I fed 16,8 VAC, 50 Hz in and got 0,2 VAC out, which gives turn ratio 84:1. Further calculation gives 28k primary for the 4 Ohm. I do not see how this fits with ECL86 or any other tube.
So please help me to understand 1) where this OT can be used, or 2) what I did wrong when checking turn ratio.
Thanks in advance

DF96 13th January 2012 10:29 AM

A small transfomer, probably intended for a radio set or guitar practice amp, might not work too well at 50Hz. Measure the primary and secondary inductance (some DMM can do this). The ratio of inductance will be fairly close to the impedance ratio, as both depend on turns squared.

Another issue is that some transformers need some DC through them to boost inductance.

Tom Bavis 13th January 2012 02:39 PM

If your meter resolves only 0.1V steps (and has a typical accuracy of 0.7% plus or minus a digit) your measurement accuracy is +/- 50%, possibly worse. Use a higher voltage (even 240VAC is NOT out of the question) or a meter with better resolution. Obviously take precautions for safety - connect meter with clips before connecting input. I would expect 6-10K primary.

cerrem 13th January 2012 07:01 PM

Place mains voltage across the primary..... Just be cautious...
Then measure the secondary unloaded to obtain the voltage ratio of the windings....I belive you know the rest of the steps...


johi 15th January 2012 09:33 AM

Thanks for advice. True that transformer does not handle low frequencies well so I used pc sound card as tone generator, attached it to the secondary and measured voltage on primary. Quite as you guys wrote impedance Z=~7,5k for 8 Ohm. Of course neither pc or DMM are wonders of high precision but these two OT are on their way to a small SET.
Thanks again

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