I built an amp using parts from an old knight amplifier. It is working but one side sounds distorted. I can't see any problem with my circuit so I think it might be the transformer. The transformers are push pull and when I measure the resistance on the secondary side, I get .7ohms on both. When I measure the primary side, I get 350 ohms on the good transformer and about 290 ohms on the one I think is bad. Should these measurements be exactly the same or is there some variation between transformers?
It looks like a partially shorted primary. Several turns are shorted on one side of the primary. It will only get worse.
I was afraid you were going to say that. I guess I will have to buy some new transformers as I am sure it is not worth repairing.
Would repairing it be as simple as unwinding, recoating with enamel and rewinding?
Would repairing it be as simple as unwinding, recoating with enamel and rewinding?
Measuring the DC resistance of the windings is not the definitive test to determine the condition of a transformer, only a rough indication. There are sometimes wide variations between identical transformers and the test is only conclusive for open,shorted or significantly lower measured DC resistance. A few shorted turns will not be revealed by this test and usually remains unnoticed. A more valid test for tube amp P.P output transformers can be done by connecting an audio generator to the secondary winding (speaker output) and measuring the reflected voltages between half primaries (plate connections) and common (B+ connection). You can use an AC voltmeter or better a (dual trace) oscilloscope which will shown any waveform distorsion. Using the center B+ connection as a common measuring point both halves (plate connections) must give nearly the same indication regarding AC voltage and waveform. It is advisable to remove the output tubes during this test as the stepped up generator voltage may reach a few hundred volts on the plates. I've never seen a defective transformer passing this test succesfully,except for intermittent H.V insulation breakdown/arcing issues. (which need a different test procedure anyway)
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Nah ! ! !
With nothing else connected to the transformers measure the current flowing in the secondary when feed with a low voltage (no more than 6V ac).
They should be the same on both transformers.
Yves.
With nothing else connected to the transformers measure the current flowing in the secondary when feed with a low voltage (no more than 6V ac).
They should be the same on both transformers.
Yves.
Connect the primary winding of the suspicious transformer, in series with an incandescent light bulb, to your mains voltage. Be extremely (!) cautious, better use an insulation transformer. Does the bulb remain dark? Measure the voltage from the center tapping to each end. Are both readings the same? If the answer to one of these questions is no, your tranny probably is faulty.
Best regards!
Best regards!
I put an ac signal on the outputs and measured the primaries, the transformer is shot. I just ordered some edcores but I had to buy the 10watt g series because that was all that would fit on the chasis.
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