Do you trust 50+ year old transformers?

[x3workshop]

Looking for some opinions here on the reliability, durability and safety of the power transformers used in MC30's & MC240's (and 225s & 275s).

I have 5 units from that period and have sent one 207-A PT to be tested/rewound because it was getting warmer than I liked with an indication that there was some overheating at one point in its life prior to my acquisition. It failed a hi pot test so I'm glad I sent it out.

So, given that these amps have seen thousands(?) of on/off/heat cycles surely the insulating coatings have become less pliable and may be flaking, and that line voltages are higher now than when they were first produced, would you recommend rewinds as a matter of course if funds allow?

I recall when I was 12 & 13 working for my dad hand winding very large PTs (one was 200lbs!) for large transmitters we coated each wind layer with additional insulation because, as he said, "they need to last more than 50 years." We also took great care on the first few wind layers so as not to kink the wires too sharply for a certain gauge.

I'm not looking for the 'Mine works fine' anecdotal responses, but more of a professional level determination here. (Terry?) I would assume that OPTs aren't quite as frail in their old age, but feel free to correct me if I'm wrong.

Thanks!

 

[CountD]

This is a fantastic question. I will be following this one to see where it goes.

 

[twiiii]

You correct there are a lot of high voltages flowing around inside the amps and though I only experienced 1 or two power transformer failures they were both in the first few months of operation. MC2105 and I have forgotten the other. It could be a problem, but even the disco amps that were on 12 hours a day 7 days a week for 10's of years at least running full power for 8 of those hours never failed. That said I never had power supply issues either even when the outputs fried and the driver cards smoked. Don't remember loosing a autoformer or output transformer either. Can't say it won't happen. Call Steve or Ryan at Audio Classics, they'll have a better answer. Its a shame Frank Gow isn't around to answer that Question.. His dad was President of McIntosh for many years and Frank sold Mac for many many years and grew up with the stuff . We talked about the subject briefly, too. But I don't remember him ever saying there was an issue with anything but the extremely rare power transformer failure.

 

[playdrv4me]

This question about the durability of power transformers over long periods comes up here in some form or fashion every so often, and it seems the determination is almost always that even for as long as these amps have existed, transformer failures are very very remote. And apparently remote enough to not warrant rebuilding them on the same schedule as all the other time worn parts.

 

[twiiii]

When tubes were the big thing, one of the big selling points of Mcintosh over Marantz and other amp companies were the transformers. The fact they were made at the factory in house to very tight specifications and the distortion measurements proved that countless times at Mac clinics, Mac transformers were the BEST. Iss ues with autoformer was always there until the quad balanced amps came along. The sound, current capability, ultra low distortion and the great signal to noise was a direct result of the circuit design which was guaranteed by using the autoformer. Pretty much shutting up the direct coupled crowd. So I wouldn't worry about buying a Senior Classic Mac. Whether tube or SS, they all need to have the power supplies recapped and maybe the signal caps, too. Noisy resistors are going to be more an issue in the phono stages than transformers. Just like anything an eye to details and a thorough inspection are necessary. Take the bottom off and look for leaks. Listen for buzzes and hums. One of my 207's is a fraction noisier from the transformer, but in all other respects they have been working well for over 6 years. My 2505 is still going strong after 46 years. MPI-4 more than 40 years and the MR 80 over 35 years, etc etc.

 

[1moreamp]

Humor time:
maybe someone should post a picture of a substation transformer blowing up LOL LOL LOL

Oh maybe me. oh but can we post links out to U tube maybe ? They got lots of transformers blowing up lol

An engineering moment.. brought to you by our sponsor "Tranny's R US" ..lol.lol lol..

I think if a transformer is over heating to begin with its not due to insulation breakdown and if so then it should be arcing down inside and blowing fuses right and left.
Overheating could be due to core magnetic issues and core de-lamination heat related issues. < this is where the little slices of the iron pull apart due to glue failure etc...They also make noise then also, along with over heating..because the magnetic's are all screwed up now...Oh and PF loading issues most of all...

Nothing like a Hi-Pot test with a megger to figure out what the breakdown voltage is to test while in the box so to speak..

Wheres my Biddle meter when i need it lol...lets slap 15Kv against it and see what it does lol lol lol..

Were you Hi-pot testing to ground? Then yes most UL rating require a 2500 volt safety margin, but most of this old Mac gear is not UL approved anyway, so where does that leave us? Plus 600 volt enamel wring will not hold up to much above 2500 plus volts of testing. So how did your tranny get tested and to what voltage level did it fail is my question????

Hi-Pot testers can make a spark plug short out thru its ceramic insulator if used incorrectly is part of my point...

It only needed 600 volt insulation to begin with in most cases this is all you will find even in industrial motor winding situations for magnetic wire breakdown of insulation...
So depending on what kind of Hi-pot testing your doing anything above the typical 600 breakdown of varnish enamel insulation is what would be expected.
All magnetic wire has a voltage rating, and its coating will match thickness with those expected engineering needs, and you will not want to break or kink that enamel coating while bending such wiring around angles cause it will expose bare cooper wire which obviates the whole insulation idea and leave potential arc over points if voltage is high enough..and I say IF.....

I would tend to think...Unless you have a winding against winding layer design then those corners would be a pinch point to watch out for, along with any abrasion between the windings due to lose windings to begin with...But these make noise also...buzzing etc...

I can make just about any transformer breakdown under high voltage stress it all depends on HOW high of a break down you were testing to, to begin with and what was expected from the manufacture....Sealed up transformers get hot no matter what. Excess heat could be cause by many factors both mechanically and electronically speaking....

I would tend to be suspect that the metal slices used in the core had separated from excessive heat and age because the glue used to hold them tightly together had failed. < due to burn up mind you >
And anything else like voltage arc over well that would have been tripping fuses and what not because it would have been so intense power draw wise, and you would have heard it in the audio of the amp also IMHO.......
So I don't think you had enamel breakdown IMHO and if you did anything above 600 Volt testing would have shown yes it was breaking down.
But I don't know of any Mac amps having anything near or above that voltage in them??? Especially those little 250 and 275 amps...

Now once the winding have seen cheery red heat well then that is a whole other story my friend since all the enamel would be burnt off to begin with.. Now how did you burn off all that insulation LOL...

I have seen a 18 inch whole blown into a 12 foot tall transformer before, but it was shorted out externally to do that and it had 112KV input and 12KV output not 600 volts like your talking about....and the arc down exploded the 1/4 inch plate metal casing like it was nothing.. but you don't have anything like this in a Mac amp...

PS I did a lot more then amps in my life so I am just pulling on some of what i have seen..by the way PG&E wants to know the secret to forever lasting transformers cause at their level its an issue...lol...

 

[CountD]

You guys need to realize something. Mc has never made their power transformers. I have read it in several places, and Gordon Gow and Mr. Mc never wanted to deal with certification of UL and the rest of the nonsense, so power transformers were always farmed out to other companies. Autoformers were made in house, therefore they are Mc. I think the only power trans that Mc ever made were the 15W1,20W2,50W1/2, and the MI200. The rest of the consumer line were always outsourced to companies that complied with UL.

 

[1moreamp]

You guys need to realize something. Mc has never made their power transformers. I have read it in several places, and Gordon Gow and Mr. Mc never wanted to deal with certification of UL and the rest of the nonsense, so power transformers were always farmed out to other companies. Autoformers were made in house, therefore they are Mc. I think the only power trans that Mc ever made were the 15W1,20W2,50W1/2, and the MI200. The rest of the consumer line were always outsourced to companies that complied with UL.

I can believe that fact.

Custom output transformers, especially patented ones like Mac had were well worth doing in house. With all the rest being farmed out like most other amp makers worldwide and even car amp makers did not wind toroids in house, they always had someone else do it. It was just cheaper and less headaches in most cases...So very good point , thanks:thmbsp:

Oh about the UL thing, well that is something the entire appliance must comply to not just power transformers. older amps like the MC2205 i just did don't have three prong grounded plugs so anything that old and no three prong plug is non-compliant anyway for the whole device not just the tranny..not nit picking, just extending full knowledge to all.. Plus the third prong would require the entire internal grounding scheme to be redone since the chassis is common ground with the secondary of the power transformers ground and even the RCAs which are pressed into the chassis. I know i thought about floating the RCAs and hard wiring all of the little ground lugs across the amp above the chassis and grounding the chassis with a third prong plug... Just too much work and liability if i did it though... But this amp had seem a line surge and it would have been less damaged if it had been grounded to current standards which I think Mac does comply with. The preamp was damaged also thru the RCAs due to the old grounding scheme. There was a good reason for three prong earth grounded chassis and UL approval. It keeps people from being electrocuted and fires from happening also..
Heck none of my 70's gear had three prong plugs on them, its just the way things were back then.. Its almost mandatory nowadays though...

 

[dshoaf]

Just to be clear, I believe Mac wound their own Autoformers, according to Roger Russell.

Scroll down to the 1957 pictures here and see the winding machines in action.

The power transformers may have been different since I can find no detail on their winding history.

Cheers,

David

 

[Pio1980]

The tube Macs with solid state rectifiers ran doubler rectifiers, which ran the power xformer at a much lower output voltage than the b+, presumably because of the limitations of the "new" silicon rectifiers. This would reduce the high voltage stress considerably in an otherwise well designed well made power transformer. Others like Harman Cardon Citation used doubler power supplies for their tube amps.

Sent from my SM-N900V using Tapatalk

 

[62caddy]

Yup - McIntosh makes at least some transformers in house. I even met the man who pots them when I took the tour who's also AK member Jimimac.

Cannot say whether they make autoformers or PT or both - the distinction never crossed my mind - perhaps he might chime in...

Trying to predict failure might be challenging - even for a scientist.

If even the hot running PTs (ie MX 110, MAC 1500, etc) rarely have issues, it would seem intuitive that the much cooler running PTs would pose even far less potential for failure.

Suffice to say catastrophic OPT/PT failure is apparently sufficiently rare that few seem to worry about it as it barely ever gets mention here - or elsewhere.

 

[Beowulf62]

The power transformers on my tube amplifiers were wound to specification by Hammond in Guelph, Ontario and the cases were oil-filled. That was in 1958. I've never had any concerns with heating, though that has to do as well with derating. Many years ago, one of my amplifiers suddenly started to sound like a machine gun...and the source of the sound was physical. I could not isolate the problem and since the noise was coming from the power transformer, I pulled the unit and shipped it to Hammond for testing. It was removed from the case, inspected and tested...no problem. I reinstalled it and finally discovered that one half of the diode stack in the full wave rectifier was dead. Now I know what a power transformer sounds like when only one half of the secondary is loaded.

 

[x3workshop]

You correct there are a lot of high voltages flowing around inside the amps and though I only experienced 1 or two power transformer failures they were both in the first few months of operation. MC2105 and I have forgotten the other. It could be a problem, but even the disco amps that were on 12 hours a day 7 days a week for 10's of years at least running full power for 8 of those hours never failed. That said I never had power supply issues either even when the outputs fried and the driver cards smoked. Don't remember loosing a autoformer or output transformer either. Can't say it won't happen.

I know that insulating lacquers used by wire manufacturers improved dramatically over the years so I would think there shouldn't be much of a problem with units made after the 60's. Mid 50's thru the 60's might be different.

This question about the durability of power transformers over long periods comes up here in some form or fashion every so often, and it seems the determination is almost always that even for as long as these amps have existed, transformer failures are very very remote. And apparently remote enough to not warrant rebuilding them on the same schedule as all the other time worn parts.

10 years can make a difference. I'll do a search for those threads. Thanks.

When tubes were the big thing, one of the big selling points of Mcintosh over Marantz and other amp companies were the transformers. The fact they were made at the factory in house to very tight specifications and the distortion measurements proved that countless times at Mac clinics, Mac transformers were the BEST. Iss ues with autoformer was always there until the quad balanced amps came along. The sound, current capability, ultra low distortion and the great signal to noise was a direct result of the circuit design which was guaranteed by using the autoformer. Pretty much shutting up the direct coupled crowd. So I wouldn't worry about buying a Senior Classic Mac. Whether tube or SS, they all need to have the power supplies recapped and maybe the signal caps, too. Noisy resistors are going to be more an issue in the phono stages than transformers. Just like anything an eye to details and a thorough inspection are necessary.

Agreed.

... Overheating could be due to core magnetic issues and core de-lamination heat related issues. < this is where the little slices of the iron pull apart due to glue failure etc...They also make noise then also, along with over heating..because the magnetic's are all screwed up now...Oh and PF loading issues most of all...

Laminates can become loose due to vibration if they are bolted together, but seeing as Mac xfrmrs are tar potted as well, I doubt this is the case.

Were you Hi-pot testing to ground? Then yes most UL rating require a 2500 volt safety margin, but most of this old Mac gear is not UL approved anyway, so where does that leave us? Plus 600 volt enamel wring will not hold up to much above 2500 plus volts of testing. So how did your tranny get tested and to what voltage level did it fail is my question???? Hi-Pot testers can make a spark plug short out thru its ceramic insulator if used incorrectly is part of my point...

I sent it to Gary at TRS. Since Terry D uses and recommended him, I'm confident he didn't 'over test' it.

Now once the winding have seen cheery red heat well then that is a whole other story my friend since all the enamel would be burnt off to begin with.. Now how did you burn off all that insulation LOL...

As I mentioned, I believe it was stressed before I acquired the amp.

 

[x3workshop]

All good info here. Thanks guys.

 

[Sam Cogley]

I have several 1950s power transformers which still work fine, and a handful which have failed. If it works, and it doesn't show signs of problems, I'll leave it alone.

 

[apachef1]

The key to keeping the PT's from overheating, breaking down the winding insulation, and eventually shorting out/open coil, keep all the components in the circuit within their operating limits. Running a 50 year old amplifier with 50 year old capacitors way out of value can ad stress to the PT resulting with the overheat condition. Running an amplifier in this manner for any length of time will lead to a PT failure later on due to winding insulation breakdown.

If a power tubes shorts out during operation, this could potentially cause damage to the PT. Check power tubes with a tube tester on some sort of regular schedule. Any unusual noise develops, check the power tubes ASAP. watch for arcing during start up. A power tube can short out at any time new, old, or in between. More or less be proactive. 9-10 times Power tubes will give some for-warning something is not rite. Just passing on my experience with power tubes.

 

[Hyperion]

I think you can trust 50 year old transformers - I used to work at a place where they were made and vacuum impregnated. After manufacture they were tested very thoroughly indeed, every now and then they would pick one to be tested to destruction.

These were transformers used in DC-DC inverters so they had to be pretty tough.

But in the end I suppose it depends who made them.

 

[x3workshop]

The key to keeping the PT's from overheating, breaking down the winding insulation, and eventually shorting out/open coil, keep all the components in the circuit within their operating limits. Running a 50 year old amplifier with 50 year old capacitors way out of value can ad stress to the PT resulting with the overheat condition. Running an amplifier in this manner for any length of time will lead to a PT failure later on due to winding insulation breakdown.

I've entirely rebuilt my amps (PS/caps/resistors) so that isn't so much of a concern for me.

If a power tubes shorts out during operation, this could potentially cause damage to the PT. Check power tubes with a tube tester on some sort of regular schedule. Any unusual noise develops, check the power tubes ASAP. watch for arcing during start up. A power tube can short out at any time new, old, or in between. More or less be proactive. 9-10 times Power tubes will give some for-warning something is not rite. Just passing on my experience with power tubes.

This is good maintenance advice. I test every 8 weeks unless I'm running them a lot.

 

[x3workshop]

I think you can trust 50 year old transformers - I used to work at a place where they were made and vacuum impregnated. After manufacture they were tested very thoroughly indeed, every now and then they would pick one to be tested to destruction.

These were transformers used in DC-DC inverters so they had to be pretty tough.

But in the end I suppose it depends who made them.

Yes, one of the units I wound when I worked for my dad failed a test in a catastrophic way. Never seen so many sparks and I'll never forget the sound of that explosion!

Of course it couldn't have been due to my bad winding. I was a pro at age 13!

 

[W9TR]

I would not recommend rewinds as a matter of course. Nor would I trust a 50 YO transformer to be ok based on all the info you've presented here. If you have several, measure the excitation current to get an idea of the variation and hi pot the ones that seem to be drawing more current than they should. There is so much we don't know about the used gear we buy that it is better to be safe than assume something is ok and have it fail on you in what could be an expensive and spectacular way.
Tom