Author Topic: Protecting power transformers - help  (Read 1069 times)

Motorola Minion

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Protecting power transformers - help
« on: June 08, 2016, 01:24:07 PM »
        Why do they sometimes fail even after risky rectifier tubes and vintage capacitors have been replaced?


      Is it the higher line voltages (125 volts AC) some of us have? Can line-traveling lightning or other transient voltage open a primary winding as a V.H.F.-member recently experienced? Will adding Y2 safety capacitors from each line of mains to ground provide action similar to an MOV or do we need line surge-protection also where plugged in? Will an inrush limiter (thermistor) help?

       

    Is it the over-sized fuses/circuit breakers, if even used, that are failing to protect them? The oversizing may have been done to eliminate nuisance-fails when unit is switched off then on quickly, causing a cemf-induced surge in the primary winding.

Have some been in dampness for too long, requiring a drying-out procedure before expecting insulation* to perform and prevent winding flash-over? Can we re-create or re-flow the varnishing into the windings to mitigate this?

* A Zenith tube amp has a 720-volt :o center-tapped secondary to develop the 375 voltage to the OPTs.

Or to drop our primary winding current at least 10% and run cooler, by eliminating heater loading by 5Y3, 5U4 and 6CA4 tubes - an opting for solid state silicon rectifier diodes and chokes to drop the excess voltage (like the field coils of our really old Maggies)
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Dave

Larry H

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Re: Protecting power transformers - help
« Reply #1 on: June 08, 2016, 01:49:48 PM »
I've been fortunate that I've never had a power transformer fail.  Not even a customer's set with power transformer has been bad.

The key is to ALWAYS replace all the electrolytics in any power transformer operated amp. 
--Larry

electra225

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Re: Protecting power transformers - help
« Reply #2 on: June 08, 2016, 03:13:10 PM »
I agree with Larry 100%. 

Zenith built its reputation on TV.  They used transformers in TV sets that you could weld with.  Their TV sets were among the best ever built.  I believe they gave their audio equipment the short shrift when it came to engineering. 
If it ain't broke, call me.  I can break it....

voxACthirtee

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Re: Protecting power transformers - help
« Reply #3 on: June 09, 2016, 10:20:18 AM »
From all the ones in Magnavox and most other consoles that i've personally seen, they really made them undersized, so there may be no hope. If its been abused it will probably just fail.

What makes these and most electronics fail is heat, so if its been ridden hard in its life, generating a lot of heat, then there's not much we can do.

That being said, the largest draw on all these console transformers is the heaters.
The DC required to run the plates/grids/cathodes will ALWAYS fall in the milli-amp range, which even at high voltages won't really tax a reasonably well made transformer that much.

A transformer running TWO 5U4GB rectifiers at 3 AMPS each, might get pretty warm a lot of the time though.

Since this is "hifi" and you don't really want rectifier/power supply "sag", i would wholeheartedly recommend putting in some sort of solid state rectifier.
These old consoles came with tube rectifiers because thats all they had at the time.
Saves lots of work on the transformer and would bump up the voltages a bit.
They make plug in "copies" so you can replace a 5U4 with a silicon equivalent if you want to have the same voltages.
That would take the 5v - 2 to 6 amp load off the transformer
(5y3 is 2 amps, 5u4 is 3 amps each, 6ca4 is 1 amp, 5ar4 is 2 amp)

And i'm sure you've heard guys rolling different rectifier tubes in their amps telling you how one tube makes their amp sound "dark" and one makes it sound "airy".
That may be true, to them, but it probably means more about the imperfections in whatever rectifier they are using. Solid state rectifiers are just fine for hifi.
You'll note that Mcintosh and many others used solid state rectifiers with tube stuff and their stuff sounds just fine.

So after you take that load off the transformer, you should fuse the AC, and if you have room, fuse the DC after rectifier and before the caps.
This should save the power transformer from catastrophic failure due to tube failure or shorts or cap failure. But old is old. These measures might be great, but if its been abused for the 40 or 50+ years before you got it, well, nothing was made to last forever.

I've gotten stuff with bad transformers, but i've never had one smoke.
I've built stuff with cheap Chinese transformers that i was riding REAL hard to specifically test for durability that failed. But they just "stopped" and blew the fuse.
No smoke, just immediate failure, fuse pop, removal and garbage can.

Motorola Minion- the changing line voltage would matter more to the tubes.
A power transformer is an unintelligent, unfeeling step up/step down transformer.
It doesn't care that the voltages might be 5-15 volts higher. it will execute its winding ratios either way, whether it 1:3 for the high voltage secondary or 1:.05 for the heaters. its the current that would effect the transformer. Aberrant spikes and transients that would effect other electronics would probably effect it as well, so a surge suppressor is probably good for anything electronic.

As far as reworking or adding all that other stuff......i'd just fuse them. Then if they go, buy new. I'm not at all in the camp that a "vintage" power transformer effects the sound. Electrons are indiscriminate. A badly designed power supply would effect the sound more than just a "vintage" transformer. I've built amps with old ones, new ones, torroidals, etc. and to me its the circuit/tubes/caps/output transformers that effect the sound, not the power transformer. But hey, that's just me.

electra225

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Re: Protecting power transformers - help
« Reply #4 on: June 09, 2016, 10:40:52 AM »
Heat has been mentioned as killing transformers.  Most of my Magnavox power transformers run hot enough to fry eggs.  The cabinets are poorly ventilated.  Rectifiers set too close to the transformer.  Apparently, heat was not a consideration by the engineers, (or the bean counters!).  Those old stereos were not designed to be running 50 or 60 years after they were built.  They were designed to last from 5 to 10 years, then something better would come along.  Heat, age, abuse all take a toll on transformers.  I'm not sure I advocate that Magnavox transformers were undersized.  Perhaps from an audiophile standpoint.  I have never had one fail yet.  Replacing a tube rectifier with solid state is a waste of time in my opinion.  Use what it was designed with and make sure everything else is okay downstream and you will be just fine.
If it ain't broke, call me.  I can break it....

hermitcrab

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Re: Protecting power transformers - help
« Reply #5 on: June 09, 2016, 06:59:56 PM »
I just had my Zenith Transformer fail last Saturday, was not the secondaries , but the primary fried... was playing fine then volume dropped to nothing.... I cranked the volume and it was still audible, but very low, figuring it was a tube I looked in the back and could hear the tell-tail sounds of cooking transformer.... pulled the plug but the damage was done...why did it fry? no idea, I am thinking since this is in my basement that over the years moisture infiltrated the winding's and caused this to happen...fortunately I had a generic hammond lying around from another project, had to fabricate some metal angles to mount it to the chassis, but is playing again.
Elton

AlexanderMartin

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Re: Protecting power transformers - help
« Reply #6 on: July 15, 2016, 03:39:46 AM »
Could always considering fuses if we're talking power trannies.

Motorola Minion

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Re: Protecting power transformers - help
« Reply #7 on: July 21, 2016, 01:23:38 PM »
Yes, fuses! I amp using 3AG (fast-blo) 2A fuses. My 1960 Symphony with amp-175 has been fitted with a the belt and suspenders of line protection IMHO. First -a fuse (had none), two 2.7 ohm 10 watt resistors to drop my normal 125V down to 120V, two Y2 safety caps from line to chassis and a CL-80 inrush limiter. I left the 5U4 in for now because it is MORE responsible for that smell of warm vintage electronics that those four light-weight p-p 6V6's :P. This console needs to stand tall in THAT department just for style points.

If my transformer has a ton of hours and the insulation has already been compromised like in an overworked motor, just waiting for the last straw to let out some magic smoke, I tried my best as a conservator.
Tubes - Magical - Tubes

Dave

bastardbus

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Re: Protecting power transformers - help
« Reply #8 on: August 12, 2016, 01:35:23 AM »
Seems to me the transformer or circuit design has a lot to do with it.  I have seen the same radios-phonos have bad transformers.  I have a GE G60 radio from 1930s.  Bought and recapped it.  Played great.  Was listening to it for weeks and all of a sudden one night she got quiet.   I turned it off and checked it out and the PT was really hot.  Turned it back on and could hear that sizzling sound round back and the PT would get REAL hot again and started to stink.  The PT fried but everything else was still fine.  Installed a replacement and she is back working again without issue.  Since then every G60 I see has had the PT replaced.  Hmmmm.....

The only other bad PT I have come across was in an RCA SHF-4 I bought.  Had a buddy grab it for me and bring it home.  Said he tested it for me and no sound.  Got it hear, plugged it in the dim bulb tester and BRIGHT!  uh oh...did checking and found a shorted rectifier tube and cooked PT.  Got hot as hades and made that same sizzling sound and stinky smell as the GE G60.   Not sure but my guess is the shorted rectifier tube took it out.

TC Chris

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Re: Protecting power transformers - help
« Reply #9 on: August 15, 2016, 11:42:08 PM »
I couldn't find a GE G60 schematic or any other info, but I have an E62 (1930s, and close in model info, at least) that had a dead power transformer too.  I had assumed that the filter cap had taken it out, but maybe it's a standard GE problem from those years.  I had an old transformer with the right high voltage and put it in.  It runs hot--probably should add a separate rectifier filament transformer to take some of the load off.

Chris Campbell