Is this Output Transformer Shot...?

[acorkos]

Marshall TSL100 head....with low output. All voltages meter to spec. Tried new power tubes and preamp tubes, tightened all tube sockets. Cleaned effects loop jacks. All output jacks are working as they should. All resistors in phase inverter/power amp meter to spec. Effects Send jacks to another amp = normal preamp operation. Signal into Effects Return jacks...NO SOUND. I was able to trace a signal to the grids of the power tubes....but still this low output. Is the output tranny gone? How do I check to verify before I purchase a replacement?

[defaced]

Run through this and post up the results. Output Transformers

[52 Bill]

The only sure way that I know to prove a bad output transformer is by substitution.

I have a Fender Concert output transformer that I keep around just to sub in, when I can't find anything else wrong. It only takes a few minutes to wire it in and it will prove the old one good or bad without question.

At least this way you know before you order up a replacement.

[Enzo]

yep, I have an old Fender Bassman OT nearby for the same reason. It is my universal test OT. It doesn;t matter if the impedances are wrong, we have ZERO sound, and all we want to know is if a different transformer works or not. If the original transformer doesnlt work, and some other wone does work, then you have your answer, even if it sounds crummy.


This is mysterious to me - what does all the output jacks work as they should except the output is low mean? Low output doesn;t sound to me like "as it should."

[acorkos]

This is mysterious to me - what does all the output jacks work as they should except the output is low mean? Low output doesn;t sound to me like "as it should."

what i meant to say was that low output is evident from any of the output jacks...but all the jacks look to be functioning normally - nothing fried, shorted or sprung open.

[nashvillebill]

You do have a little bit of sound coming from the output jacks, though? That pretty much rules out a bad OT, I would think. Did you check plate voltage?

[Enzo]

Bad OT is almost always the last thing on my list, but they do fail once in a while. If he has a couple shorted turns, the thing would still try to function, but would be severely loaded down by the turns. Go over to Geofex.com and find and make a tranwsformer tester you find there. Very simple thing. It will test for shorted turns.

[acorkos]

i swapped in a bandmaster output transformer....the amp works loud and proud. so, the original OT was damaged. but how does this happen? the owner said the speaker cab and cable he was using when the amp got quiet are working fine with his loaner amp head....? maybe i need to check out his cab and meter it.

[J M Fahey]

It's relatively common on late model Marshall OTs ( and many others) because they use self soldering/autosoldering wire enamel, the kind which is designed to evaporate at 120ºC, and forms a very thin layer to boot.
Good for small cheap PSUs where scrapping enamel to solder would be a mess, but it has nothing to do inside a Tube amp OT. Oh well.
It gets much easier to puncture with cabinet connection mistakes.
That wire is usually color coded green or fluo/metallic red/pink while old fashioned wire used to be some shade of brown and had to be burnt with a lighter, then scratched to remove the still sticku charred enamel, worlds apart.
I often rewind them, and the classic symptom is that the amp "works" but went to stadium beast to bedroom puppy.
Your friend's cabinet works, no doubt, I wouldn't trust the speaker cable he used that day (or even if he plugged it correctly)
Sorry.
PS: I still have (and use as a paperweight) an output transformer from a Mesa Bass amp, the one which uses 6x6L6, a monster, perforated and using green fluo wire.
Yet they had used all the Mojo stuff: kraft paper bobbin, a small piece of wood (probably organically grown) to fill a gap between bobbin and core, etc.
If they only had used PE or Formvar or at least Epoxy wire !!!!!!!!!!!!!!!!!

[nashvillebill]

Wow, I'm more used to industrial transformers, motor windings, etc so it's kinda weird for me to think about OT windings just randomly shorting under normal service.

[J M Fahey]

Way to go!
That's the "boring, engineerish" way of thinking, making things strong and to last.
I use motor winding grade , 180ºC rated wire.
It even costs about the same !!!
But I have to waste all of a minute per soldered connection to burn enamel, scrape it, wrap it around the terminal and solder. Irreparable economic losses !!! Huge industrial empires have fallen !!!
[ironic mode off]

[Enzo]

Most things fail under normal service. people always want to know how this was done to their amp, but the reality is that most amp failures are not due to something someone "did" to them. I didnt have it turned past 5, I know the speaker was the right impedance, we never play loud, I never drive past the speed limit, I don;t kick my children...

[R.G.]

The only sure way that I know to prove a bad output transformer is by substitution.

Respectfully, but for completeness; if a winding is really open - it's bad. If a winding is actually solidly shorted to another winding it's not supposed to be connected to - it's bad. If a transformer really does fail the internal inductive-kickback test - it's bad. It has to be. It cannot work properly with these flaws.

Because transformers are slippery things to measure, you need to be very, very sure you're measuring correctly, but these cases are sure ways to prove it's bad if you do the test correctly.

There are subtle flaws that these tests might not show up, like an internal arc-over at high voltage and temperature, but an open winding is a dead giveaway - the OT's dead. An internal short is a dead OT; historically, the inability to find an internal short before the inductive kickback test led to the idea that there could be flaws you could not find externally and the only way to prove "bad" is by subbing.

It is certainly comforting but it proves the reverse of the OT being bad - it proves the amp is otherwise good, so the OT is judged bad by elimination.

If there was a bad/open/cracked joint that you didn't notice or "repaired" by the process of subbing in another one, the OT you removed is still good but you couldn't tell. That's a far fetched illustration, but I have seen it happen, albeit not on OTs. Having a replacement work in the amp proves that a replacement will make the amp work, not that the OT removed was bad.

Sorry for the sophistry, but until one finds an actual flaw in the OT removed, it is not certain it was bad.

It may be the best/cheapest for the customer to do a replacement test though, trading off bench time against the price of a new OT.

[J M Fahey]

Most things fail under normal service. people always want to know how this was done to their amp, but the reality is that most amp failures are not due to something someone "did" to them.

+1
And adding: many kids drive me crazy insisting obsessively:
"put a "good" part there, so it's *sure* it will never fail again"
" please repair it well"
"what must I do so it never fails again"
"it failed because "they" use cheap parts, didn't it?"
"it failed because it's made in China, didn't it?"
"I bet the vintage ones never fail"
"are *you* sure it will never fail again ... do you swear?"
plus an Argentine/Brazilian special:
"the good ones go to USA/Europe/Japan, we get the cr*p of the cr*p"
and its derivative:
"I will buy one in Miami/L.A./N.Y./EBay to make sure I get one of the good ones."

[acorkos]

i got a replacement dagnall OT and installed it today. i asked the customer to bring in his speaker cab and cable. the cable had a seriously bent tip, and the jacks on the cab (1960A) were not metering impedence consistently. i pulled off the jack plate and found a few broken solder joints on the jacks and spade connectors. like the amp fell off the cab and jerked on the cable, or some such scenario. i resoldered everything on the jack plate PCB and ordered the customer to throw out the speaker cable and buy a new one.