How robust, durable and long lasting are output and power transformers?

I’ve been thinking about long term plans with my amplifiers, and what I should have on hand for the future, other than, of course, tubes.

Potentiometers, resistors and capacitors, wiring, tube sockets and miscellaneous hardware look like they’ll be available for a good stretch.

But what about the power transformer and output transformers?

Power transformers can get noisy and vibrate, that I’ve experienced. But do they eventually burn out?

What about output transformers? I’ve read of copper leads becoming brittle with age or wire insulation getting dry and splitting or crumbling.

Are these things us tube aficionados to be concerned about?

What about a tube going rogue, shorting or red-plating? What’s at stake here outside of the potential of popping and frying some caps and resistors? What’s the threat to power transformers and output transformers? How about chokes?

I have a few Dynaco ST70s, stock and modified. Two use the old cloth lead transformers. One has a noisy vibrating power transformer. Currently Pacific Audio Magnetics are winding modern copies of these output transformers for a fair price. Power transformers aren’t available yet but are coming. Places like Tubes4Hifi and Dynakit Audio Parts are still working on sourcing a new winder for their transformer offerings.

Is it good insurance to grab some spare transformers? Maybe chokes?

What’s the likelihood of power and output transformers failing from age? What about risk from damage from a tube failing?

Haven thoughts, stories, anecdotes and opinions welcome.
 

MrEd

Senior Nobody
Site Supporter
The power transformers on the ST70 were notorious for being under sized and stressed. Smart to upgrade.
The OPT's are really good transformers and should be kept in my opinion if they have not be damage by overload some how.
 
The power transformers on the ST70 were notorious for being under sized and stressed. Smart to upgrade.
The OPT's are really good transformers and should be kept in my opinion if they have not be damage by overload some how.
I know the power transformers were a bit under-specs especially considering these modern times we’re living in. So plans are to upgrade those- getting spares may be in the future too.

The output transformers- I’m keeping the originals in use- it’s more a consideration of putting a few of the newer ones in a box for insurance in case the originals ever fail.

In your opinion, are transformer failures something to be wary of?
 

MrEd

Senior Nobody
Site Supporter
I know the power transformers were a bit under-specs especially considering these modern times we’re living in. So plans are to upgrade those- getting spares may be in the future too.

The output transformers- I’m keeping the originals in use- it’s more a consideration of putting a few of the newer ones in a box for insurance in case the originals ever fail.

In your opinion, are transformer failures something to be wary of?
No opt's don't normally fail.
 

Salectric

Site Supporter
Site Supporter
The ST70 weak points are the power transformer and choke. If you want total assurance of reliable use for the rest of your years, I can see getting a spare PT and choke. In fact, if you can find a replacement choke with a higher current rating that fits in the ST 70 chassis, I would replace it now. I don’t recall reading of any chokes failing in the 70, but you could get better performance with a more conservatively rated choke. Dyna’s choke is fine for an EL34 mono amp. The problem is using it with the current draw of a stereo amp.
 

paul_b

Active Member
Site Supporter
Transformers for an ST-70 are likely to be available for a long time. Old transformers can experience failures for a number of reasons, and even things like the papers that were used inside of older transformers can cause shorts over time. Thankfully these degenerative processes have been studied and we have superior materials today compared to the past, so the limit to transformer lifetime currently is more likely to be tied to catastrophic failure events that proper fusing is likely to disrupt quickly.
 

airdronian

Radar Member
Transformers for an ST-70 are likely to be available for a long time. Old transformers can experience failures for a number of reasons, and even things like the papers that were used inside of older transformers can cause shorts over time. Thankfully these degenerative processes have been studied and we have superior materials today compared to the past, so the limit to transformer lifetime currently is more likely to be tied to catastrophic failure events that proper fusing is likely to disrupt quickly.
Papers like this ?

TrannyPapers.jpg
 

paul_b

Active Member
Site Supporter
You can't look at the paper and know whether it's going to be a problem or not, but that is a good visual showing a paper bobbin, paper layers mixed in with the windings, and a paper cover on the outside of the wind.
 
How long do you plan to live? ;-) Well-made transformers, power or ouput, should last a lifetime and more. I have Stancor, Peerless, Acrosound, Dynaco, Eico and UTC output transformers that are 50-70 years old and measure as good as the day they were made. Same with good power trannies.

There were a few amps of yore that experience problems--the Heathkit W5 nortoriously contained underspec'd parts that caused failures in both the original (Peerless 16309) and power trannies. OTOH, I have a bunch of 16309s in good condition, and with proper care and handling I expect they will last quite a while. Most failures in vintage amps were due to catastrophic issues, most of them brought about by careless users.

I did have an Acro TO-330 that developed a short to the case. Quite rare--and rather unpleasant, too!

I would expect a Hammond, Edcor, Electrprint or similar modern tranny would last a very long time. Frankly, I wouldn't worry about it. However, a buzzing power tranny is, IMO, unacceptable and should not be tolerated. If it's current manufacture, return it immediately. Also, buy the best tubes you can afford from a reputable dealer to avoid meltdowns. Most OPTs will withstand a good deal of punishment from failed tubes, but there's a limit. ;-)
 
Just a slightly different angle. It depends upon:

1. the design of the transformer,

2. the amount of cushion the designer used, and

3. protection of the transformer under catastrophic conditions.

-----

1. If the transformer designer happens to be sloppy with inferior materials, moisture could eventually cause failure.

2. If the circuit designer allows for ample current leeway, the transformers can last virtually forever. Some of my
designs have been going easily for 30 years without a failure.

3. Without ample protection (from fuse), a transformer could be overloaded due to catastrophic failure of another
part of the circuit. There are two ways of protection.

A. Proper fusing in the high voltage circuit, and using the proper number of fuses in series so as not to allow
internal arcing of the fuse(s). I actually worked on a customer's amplifier with a melted fuse holder from
internal fuse arcing. The 1 fuse was rated 240 vac but had some 600+ dc volts across it when it blew, causing
the fuse holder to melt.

B. A short in a tube would generally cause a surge/peak current of the filter capacitors through the OPT, and
within a very very very short time, the main power fuse should blow, protecting both the power transformer
and the OPT. (Kept simple for general audience understanding.)

Hope this helps.

pos
 
Last edited:
Just a slightly different angle. It depends upon:

1. the design of the transformer,

2. the amount of cushion the designer used, and

3. protection of the transformer under catastrophic conditions.

-----

1. If the transformer designer happens to be sloppy with inferior materials, moisture could eventually cause failure

2. If the circuit designer allows for ample current leeway, the transformers can last virtually forever. Some of my
designs have been going easily for 30 years without a failure.

3. Without ample protection (from fuse), a transformer could be overloaded due to catastrophic failure of another
part of the circuit. There are two ways of protection.

A. proper fusing for the circuit

B. Designing so if a catastrophic condition does occur, such as a short to ground, maximum current would be
limited to less than the transformer rating. For output transformers (OPT), a short of a tube would generally
cause a surge/peak current of the filter capacitors through the OPT, and within a very very short time, the
power fuse blowing. The OPT should be protected. (Kept simple for general audience understanding.)

Hope this helps.

pos
Thank you- it does. Much appreciated.
 
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