What are "proper" ohm readings for an OT?

[disorder]

Peavey Classic 50 (older one from '93) with a suspect output transformer.

Primary windings measure ok. End to end is ~140 ohms... one end to center is ~65ohms and the other end to center is ~74 ohms.

Secondary seems to be a total short. The 8ohm winding to 16 ohm winding is totally shorted... 8ohm winding to common is ~0.2 ohms and 16phm winding to common is ~0.3 ohms.

Should the secondary of an OT measure as such or is it beat?

[pdf64]

Those readings are normal. No reason to suspect it on that basis. What's the issue?

[disorder]

Powers up, no sound at all. Tried multiple speakers/cabs, and different tubes.

[Enzo]

Is the high voltage fuse blown? is B+ getting to BOTH plates and screens of the output tubes? Is 400 ohm (or sometimes 390 ohm) R58 open? That would mean B+ on powert tube plates, but none on the screens or any other tubes.

Without B+ on ALL tubes, you get no sound.

And are ALL the tube heaters glowing? The power tubes and the phase inverter all run on 6VAC wired in parallel, but the preamp tubes V1, V2 have their 12v heaters wired in series across 24v of DC. There are separate fuses for those two heater systems.

And i agree with pdf64, those resistance readings sound normal to me.

[R.G.]

And another one.

Why is it that people leap so easily to "My output transformer must be dead." ?

[J M Fahey]

Because it´s a big ugly heavy part?

[Chuck H]

And another one.

Why is it that people leap so easily to "My output transformer must be dead." ?

Maybe because (not a sarcastic 'MAYBEEEE because', but an actual maybe because) most players and gear tinkers know just a little about it and are aware of all those times when they tried to use their amp without the speaker plugged in. Or maybe it's because it's one part of the amp that has "output" in it's name. "No output? Must be the output part." Or maybe it's because a lot of after market part vendors talk a high game about what a critical part it is, so that's all anyone thinks about. Probably less than half the electric guitar playing community knows anything about what a filter capacitor is or what it does. But many do know (or think they know) something about what an output transformer is and has heard tales of blown output transformers.

It IS an annoying trend though. Maybe there's a joke to be made of the subject for the sake of comic levity... A priest walks into a bar with a dead amp in one hand and a dead fish in the other...

[Danelectron]

A priest walks into a bar with a dead amp in one hand and a dead fish in the other...

And???

[Prairie Dawg]

Of all the things in an amp that go wrong output transformers are among the easiest to test. Maybe it's because there's wires in, wires out and some mystery thing goes on inside it/ I've only changed two for cause in my entire career.

[Chuck H]

A priest walks into a bar with a dead amp in one hand and a dead fish in the other...
The bartender says "what's up with that amp?"
The priest says "Not sure. Probably the output transformer."
bartender: "And the fish?"
priest: "It's dead."
bartender: "Well what did you bring them in here for?"
priest: "Why should I know? I don't even know why priest would walk into a bar."
Bartender: "Your right... It's probably the output transformer."

[Chuck H]

I modded amps for years before I was building my own. And the ones I build are designed to be run into OD most of the time. And I've NEVER seen an OT failure in two decades.

[J M Fahey]

Well, I have, but let´s say that although I have the pleasure of working with Pro clients (which almost never "break" anything, their problems usually come from moving and bumping around their stuff), but must recognize that the steady breadwinner is the regular kid who knows little and cares less .... and they *do* kill their properly working stuff out of carelessness (or plain old stupidity).
I *have* repaired a JCM900 where the original OT was replaced 3 (three) times in less than 6 months, still under warranty, until the very pissed off Importer sent it to me to see what could be done.
They also sent the pullouts for analysis.
Yes, it was the kid's fault, who tested that the speaker was connected by putting everything on 10 and playing a few power chords (he complained that "music came out of the head, not the speaker, and that at very low volume), but I also found that the transformers were wound with what we call "self soldering wire" (regular English name?), the kind which has a *very thin* layer of enamel, designed to evaporate at, say, 130ēC and leave perfect bright copper, no residue behind, as to avoid scratching and such.
Excellent for wall warts and such, where 10 extra seconds peeling a wire for soldering mean bankruptcy, or in pickups, but it has *no* business at all inside a big, heavy, expensive amplifier.
What do they save, 60 seconds per transformer?
Clearly it stands "regular" use or there would be zillions of dead Marshalls, but not abuse (which must be figured in the equation if we speak of the Rock World).
I rewound them with Class F 180ēC guaranteed motor or generator duty wire, dried them in an oven for 24 hours before vacuum varnishing .... they are still working, same owner after many years.
I also found the same kind of weak wire in (obviously burnt) Mesa Boogie transformers. Shame on them.
These self soldering wires usually come in a special almost "fluo" pink or metallic green color, so as not to be mistaken with regular wire (you must have seen them zillions of times on low end stuff, such as wall warts, DVD transformers and the like), but now manufacturers offer them in "regular" colors, such as light brown or almost transparent.
I much prefer the regular Epoxy enamel ones, the kind you have to burn almost red hot and then scratch a lot .
The old "Synthetic" or "Oil" based high temperature enamels, used by classic Fenders and such, are excellent, of course.

[Prairie Dawg]

Sounds interesting. I've got a Marshall on the bench right now that got dropped out of a moving car and has now acquired a dead short in the B+ rail, and it appears to be in the choke.

[Ronsonic]

Of all the things in an amp that go wrong output transformers are among the easiest to test. Maybe it's because there's wires in, wires out and some mystery thing goes on inside it/ I've only changed two for cause in my entire career.

I'll disagree that they are easy to test. A shorted turn on the secondary can wreak unholy hell on it's function and be extremely difficult to find on the bench. The DCR of the secondaries is extremely low, well below the resolution of most bench gear. The primary sees high voltages and can arc internally, yet give any signs on a DC ohmmeter.

I suspect customers blame the OPT first is that it is the worst case, scariest scenario - kinda like the guy who has a medical symptom and figures it must be cancer.

[Gtr_tech]

I think the output transformer myth has been passed down from the days when ppl used to roast Marshall Superleads with Altair/Sholz power soak and similar power attenuators.

I replace one occasionally, but its not super common. The last 2 bad ones I had thru here were Mesa Rectumfriers with their cheesy undersized output transformers.