Output Transformer Resistance

[Tavda3172]

Hey everyone,
I had a quick question about the primary resistance of the output transformer in the build I am working on.

I was getting really low, distorted output in this amp, which is an original build, so I measured from the center tap to one of the plate leads and got a reading of about 90 ohms. From the center to the other plate lead was 30 ohms. This is a 4k 35 watt transformer with UL taps. I happened to have a Fender Pro replacement transformer lying around and that measured about 90 ohm on both sides.

I assumed there was some thing wrong with the transformer in the amp so I pulled it and replaced i t with the Fender while I waited for new transformer to arrive and this solved the problem. Now, the new transformer is here and I am getting the same resistance readings as before: 90ish on the one side and 30ish on the other. Is this normal? What would cause this? Have I gotten two duds from the factory or am I overlooking something? Any help would be greatly appreciated. Thanks,
Kevin

[martin manning]

That doesn't sound right. Normally you'd expect ~10% difference. Are you sure that you're not measuring the ultralinear taps due perhaps to an error in the wire colors? Check all of the leads to center and also that the total resistance a-a is the sum of the lead-to-lead resistances.

[Tavda3172]

Thanks for the prompt reply Martin!

Normally you'd expect ~10% difference.

Agreed! I am used to seeing the ~10% difference which is why I though the first transformer was toast. Usually when I measure from center to each plate lead everything is as it should be so I must say I feel a little embarrassed when you suggested to measure end to end to make sure the the resistances add up . Of course they do not and I feel a bit stupid!

I am so used to the plate leads being a different color (e.g., blue and brown) that I assumed the solid blue wire was one plate lead and the solid green wire was the other. I assumed the "blue/white" and "green/white" wires were the corresponding UL taps. Measuring end to end it turns out that the blue and "blue/white" wires are the plate leads and the green and "green/white" wires were the UL taps. Shame on me.

Everything adds up now and I greatly appreciate the simple wisdom as it saved me a lot of frustration. I won't make that mistake again. Tremendous thanks,
Kevin

[martin manning]

Glad to help. That's what I would have guessed for the color codes too. Even with a diagram I measure to check for errors, and check the phasing too.