Output Transformer has 3 taps, 2 are bad, 1 is good?

[garytoosweet]

Hey,

Question is, is it ok to use 1 working tap if the other 2 are not working. Anything I should be worried about?

Details:

I have an Output Transformer with 3 taps. 8, 4, and 2.

The 8 and 4 taps are dead. Incredibly low volume, distorted sound. When I received the amp, the 8 ohm tap was driving a 4 ohm load for the past 25 years. The tubes were also biased extremely hot, causing red plating, and I believe prolonged excessive DC load on the transformer.

The 2ohm tap was not in use, and it's currently working. Sounds great. Have it driving a 4ohm load and I've tested it under load for a long period of time.

Appreciate any insight.

[The Dude]

Just curios. What is the DCR of the non-working taps? I presume open?

[garytoosweet]

Just curios. What is the DCR of the non-working taps? I presume open?

I don't recall exactly but all under 1 ohm. They were not open.

[Chuck H]

The way I see it you have a hinky OT that is already compromised and could go hinky elsewhere more readily as a result. Further, you indicated that the transformer has "8, 4 and 2" ohm taps. And then that the "16 and 8 ohm taps are dead". So we don't really know what the criteria or evaluation parameters actually are.?. I say that a partly bad OT is a bad OT. Replace it ASAP and don't count on anything you read on any forum. Especially when specific information can't be concisely provided. On that note, it might be best if a professional handled the repair.

[garytoosweet]

The way I see it you have a hinky OT that is already compromised and could go hinky elsewhere more readily as a result. Further, you indicated that the transformer has "8, 4 and 2" ohm taps. And then that the "16 and 8 ohm taps are dead". So we don't really know what the criteria or evaluation parameters actually are.?. I say that a partly bad OT is a bad OT. Replace it ASAP and don't count on anything you read on any forum. Especially when specific information can't be concisely provided. On that note, it might be best if a professional handled the repair.

16 was a typo, fixed. Thanks for your input.

[Mick Bailey]

An OT that has distorted, low volume output usually means there's a shorted turn, or a burnt spot that's conducting when the output signal is applied. A single shorted turn, or shot between layers, cannot be detected with a DMM. I would use R.G's 'neon trick' as a check, but bear in mind if the OT only breaks down under load it may not detect your particular fault. Usually I find that all the outputs exhibit the same problem and the fault lies almost exclusively with the primary. There may be something else going on given that one output works, but I wouldn't continue to use it.

[Chuck H]

There may be something else going on given that one output works, but I wouldn't continue to use it.

To add... At least for now until the actual mechanism of failure is understood and corrected.

This odd symptom of dead secondary taps, but others working fine may indicate a flaw in the diagnosis and I think it's possible the OT may not be at fault. But, of course, it may.

It could be faulted secondary tap leads? It could be a bad impedance switch? Maybe a switching jack has been incorrectly wired and implemented?

[The Dude]

To add... At least for now until the actual mechanism of failure is understood and corrected.

This odd symptom of dead secondary taps, but others working fine may indicate a flaw in the diagnosis and I think it's possible the OT may not be at fault. But, of course, it may.

It could be faulted secondary tap leads? It could be a bad impedance switch? Maybe a switching jack has been incorrectly wired and implemented?

Yes, I agree. Usually, if there is a shorted turn in any winding, it has an audible effect on all others. If some were open, I could see it working, which is why I asked the question in post #2. Notice I said usually. I suppose it's not impossible, but it would be odd. I too suspect there could be some other problem aside from the transformer itself.

[xtian]

Sounds good as is? Heck! Play it and be happy.

[Mick Bailey]

I've never seen a transformer with a shorted turn where one winding works, but I would't say that in in the history of audio amps this could never have happened. Maybe if I sat down to think of unlikely scenarios I could come up with something. As Enzo says - "Think horses, not zebras". The most likely scenario is possible bad solder joints where the leads are connected.

I had a transformer with the same (or similar) symptoms where I unwrapped the paper layers and found that whilst the winding taps were soldered, the connecting leads were twisted on but had never been soldered at the factory. I think that was a Silverface Twin Reverb, but I certainly had a Carlsbro 'Big Boy' where the same thing had happened.

[Helmholtz]

A hard short between turns of any of the secondaries will reflect to all other windings (including primary) and will render the whole the OT inoperable.

Even a partial short with some resistance will more or less load down the whole transformer and will cause strong local heating leading to total failure after some time.

If the OT actually has low resistance between ground and all secondary taps and delivers power into the 2 Ohm output, it's probably not bad at all. I would remove it from the amp and perform the standard tests: DCRs, neon flash, voltage step-up/step-down.

[Gregg]

Measuring the inductance of primary's halves and leakage inductance will provide some more info about what's going on.

[Mick Bailey]

One of the best things I built to test transformers is a modified Dick Smith ring tester. It's never failed to detect a short or high resistance internal fault - either tending towards short, or open. It's so sensitive I can put a single turn of insulated wire loosely around the outside of a transformer and it will pick it up as a short. I recall it registers a short up to about 100K ohm as well as completely open or dead-short, using a traffic light bar display.

[Enzo]

Come on Mick, if you make a glowing review like that, at least link us to it.

[dmeek]

I built one of those too. Very handy. Here's the manual.

https://music-electronics-forum.com/...1&d=1582935703