BeëlzeM

I own my JCM 800 50 watt head for probably 18 years now and in those years I used it quite a lot and it has been gigged with.
My power transformer looks a bit rusty and brown. Can't really remember when this started to happen or whether it was on there when I bought it second handed. I am wondering for a while now whether it would be good to replace it or if it's not really neccesary, maybe it's just cosmetic? It's not that bad, but it certainly does not look new.

Marshall Amps

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erndawg

Surface rust, even scaling, is common on transformers and chokes over time and dependent on ambient humidity. Although it may not look so good, it is generally not a problem.
If the rust penetrates past the surface, and gets between the iron laminates, the magnetic efficiency will start to become compromised. This can happen on transformers where the laminates are not tight - generally due to poor manufacturing techniques.
Over time, as the rusting spreads between the laminates (below the surface), the magnetic permeability becomes lower which results in lower inductance of the transformer or choke. If it becomes severe enough, you can start blowing fuses.
Bottom line, don't worry about it. It's common, and generally not an issue. Some overly worrisome types apply a very light layer of Rustoleum. But I'm not sure if it does anything more than cosmetic.
Good luck - erndawg

MajorWatt

Yes, rust is not a transformers friend. A couple of amp restorations and one of my own amps had this beginning to become an issue. I took a brass wire brush and knocked the scale/rust off the laminates, then got some rust neutralizer and brushed it onto the surface. Problem solved.

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BeëlzeM

Okay guys thanks for the info. What should I notice when there's too much rust on there? Less power or something?

Might clean it up though and put on some rust prevention for the future.

erndawg

If the Power Transformer becomes much less inductive, you'll start blowing the Mains fuse. If the Output Transformer becomes much less inductive, you'll start blowing the HT fuse.

luekemeyer

Realistically, I think that it would take some significant rust to affect the impedance of the transformer (loss of metal). Read below:

From another post somewhere else: "As a coincidence, it turns out that I recently had a similiar discussion about rusty transformers over lunch with a friend who is a retired design engineer from Motorola's tube R&D department. We talked about surface rust, and he made it very clear that surface rust was not at all bad on a transformer, and that you shouldn't let surface rust bother you. He emphatically stressed that you should not try to remove surface rust, for the same reasons mentioned in the FAQ on the MM site."

From MM:

Transformer Rust -- Friend or Foe?

Question: I heard that rust is bad for my transformers. I’m being told that rust causes an increase in eddy current losses which will heat up and further damage my transformer. Rusty transformers will also hurt my tone, so I must remove the rust or replace the transformers to make my amp operate properly and extend its life.

Answer: Nothing could be further from the truth. While iron possesses the necessary magnetic properties, it is also electrically conductive. Eddy currents are circulating currents in the core induced by a magnetic field emitted by the energized primary winding of the transformer. These electric currents are undesirable. The core is acting much like a shorted secondary winding. They are power-robbing losses that convert your guitar playing (input power) into heat that beats up your transformer. This effect is certainly not helping your tone!

On the other hand, rust (iron oxide) is a nonconductor of electric current. The high resistivity of rust reduces eddy current losses. Since iron is also a conductor of electric current and the goal here is to minimize eddy currents, which is why the construction of the core is made up of a stack of thin plates of iron (laminations) which are insulated from each other by an oxide (rust-like) coating. This confines the eddy currents and prevents them from circulating between the laminations. The higher resistance created results in a reduction of power losses within the core.

What’s this mean in plain English? Back in the early years, manufacturers of laminations intentionally exposed their iron to moist air to encourage rust to naturally form on the surfaces as an insulator! Like a fine patina on an antique bronze the rust on your transformer could be viewed as badge of honor. Assuming that your transformer doesn't have other problems like coil damage from excessive moisture exposure (common with paper bobbins), you can actually expect some improvements in output transformer tone and lower losses in power transformers as rust develops.

If you are still bothered by the cosmetic aspect of rust, never scrape off the rust to the point of exposing bare metal because you will bring back those nasty eddy currents. Instead, brush off the loose particles and re-paint the area with varnish. If you want to send your transformer to Mercury, we can re-dip it in varnish and bake it for a small fee. Transformer evaluations are always no charge providing you cover the shipping costs to and from Mercury.

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BeëlzeM

That was an interesting read. I think I might leave it to what it is and replace it when I notice things get worse. Thanks.

erndawg

Luekemeyer: From your reply "Back in the early years, manufacturers of laminations intentionally exposed their iron to moist air to encourage rust to naturally form on the surfaces as an insulator!"

That may have been a true statement about 60 - 70 years ago, but since then laminations have been made with high permeability silicon steel, AKA 'electrical steel'. They have quite different properties magnetically, electrically, and chemically.

I'm not sure how much weight I'd put on an anecdotal luncheon conversation with a retiree. Manufacturing processes and techniques have come a long way and have been continually evolving.

As I mentioned in my post, surface rust and scale are generally not a problem. If the oxidation gets past the surface of the laminates due to loose stacking of the laminates, or from aggressive oxidation, then core losses will increase. This will result in a 'shorted turn' effect. Some techs may recall the 'shorted turn' effect that was the demise of so many earlier Matchless transformers before Jamison took over. I saw many of those failed transformers.

luekemeyer

You are correct about the aggressive oxidation and agree with what you are saying. I just think someone should consider everything before replacing a transformer due to a little harmless rust.
Regards.

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JCM2000 DSL 100 (2003)
1960BV (1995)
MC412 Cab (2008)

Insanity: doing the same thing over and over again and expecting different results.
Albert Einstein