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Old 28th Jan 2005, 11:16 pm   #1
corvair
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Default Push-pull transformer resistance anomaly

I have two output transformers, taken from an EMI model 557 amplifier. This was a stereo amplifier, and each channel used two EL84.

The primaries of the transformers have five tags - so, one might think, the configuration was ultralinear.

On measuring the DC resistance of the primaries, the results are most odd:

Centre - tap to either tag alongside it shows about 72 ohms.
Centre - tap to one anode tag shows 257 ohms.
Centre - tap to the other anode tag shows 162 ohms.

Each transformer is virtually identical with these DC resistances. The resistance from anode to anode is 420 ohms.

I do not understand why the DC resistance is so very different from the centre tap to each anode tag. Is there some odd circuit configuration which calls for assymetry either side of the screen-grid taps?

Or a configuration using two valves with a transformer with what appears to be an ultralinear type which isn't, in fact, ultralinear at all?

If anyone can throw some light on this, it will save me a fortune in aspirin. If someone has a circuit diagram for the EMI 557 stereo amplifier, I will be eternally grateful!

David

Last edited by corvair; 28th Jan 2005 at 11:18 pm. Reason: To correct an error ... re. assymetry
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Old 28th Jan 2005, 11:34 pm   #2
Station X
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Default Re: Request for assistance; EL84 push-pull transformer anomaly

It is not unusual for transformer windings with the same number of turns to have different DC resistances. There was a similar discussion to this on the forum a while back concerning Quad output transformers. It's to do with the way the transformer is wound. Windings close to the core use less wire per turn, so they end up with a lower resistance when compared to a winding further from out the core which uses more wire per turn.

It's the turns ratio which counts, not the resistance. You could check the turns ratio by connecting say 6.3V AC from a heater line to one winding and measuring the AC voltage on another winding.

Graham.
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Old 29th Jan 2005, 10:09 pm   #3
corvair
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Default Re: Request for assistance; EL84 push-pull transformer anomaly

Graham, you were spot-on.

1.8 volts applied to the secondary produced 48 volts from the primary's centre-tap to one end of the winding ..... and 48 volts between the centre-tap and the other end of the winding. The screen-grid taps were similarly balanced.

I cannot believe now how blinkered I was, suckered into a mental hole by the resistance readings!

Thanks, Graham.

David
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