Mercury Magnetics in August VG magazine

[GibsonLover]

Did anyone read this article? They illustrated how a rusty transformer was actually better than a new one of the same type, due to the rust insulating the core laminations. I had always thought the rust would serve to short the laminations, and maybe that was the point of the article. Or, maybe the point was not to worry if you had rusty iron, but it has always been a deal breaker for me when buying a classic amp if the iron was rusty, thinking it was an indication of how it was handled and stored. Given an enviromnent to cause rusty iron, the speaker materials would not be in the best environment either. (unless it's good for speakers too..!) I feel better about the ones I've disassembled and painted anyway...Now MM can sell 'aged' transformers and get a premium for their stuff ;-)

[noelgrassy]

Yeah, logic dictates that rust on the laminations has effectively [or electronically, your call] connected them together. I have a rusty one out in my shed I'll measure it after I get it hooked up to a low voltage supply. This should be informative as I've got a non-rusted one that's essentially the same.

Gerald Weber would contradict the MM peoples on this one that's for sure.

[octal]

I suppose it's which sound you are trying to duplicate-

1.) what a brand new deluxe reverb would have sounded like in '65
2.) what a carefully stored '65 deluxe reverb will sound like today
3.) what a deluxe reverb which was abandoned in someone's damp crawlspace in 1965 1/2 when they discovered they weren't the next Eric Clapton and has been recently dragged out and thrown on e-bay will sound like.

* note- I'm not claiming that the DR and or EC are the best examples of their respective breeds; it's just an example, folks!

[Merlinb]

Rust is an insulator, so it does isolate the laminations. In fact, that is the traditional way to make a transformer. The laminations are stamped and deliberately oxidised (rusted) before stacking.
Of course, you are right when you say that a heavily rusted transformer implies that it may have been kept in damp conditions, unfavourable for the coil insulation.

Although based on MM's customer support and website, they appear to know nothing about tranformers, so no wonder they were surprised by the rust...

[Diablo]

Hmmm. I work for a very large steel company which makes electrical steel of all grades. My specialty is the manufacture and application of electrical steels. Most "good quality" transformers that you will find in tube amps will be made from something similar to M6 grain-oriented steel, and are sold with an insulating coating already applied at the steel mill in coil form. Typically, they will be stamped by the transformer manufacturer from slit mults into a stack of laminations and assembled into cores. Typically, they are not annealed after stamping. The coatings are similar to a varnish and are very insulating electrically.

Some lower quality transformers are made from cold rolled motor lamination steel (called semi-processed steel). These must be annealed after lamination stamping to get reasonable magnetic properties. During this anneal, a magnetite (blue oxide) coating is developed on the lamination which acts as an electrical insulator. However, this oxide coating is a very poor insulator compared to the varnish coatings on the grain-oriented steels. The magnetite coating has a Franklin value of around .95 amps. The varnish coating has a Franklin value less than .3 amps. High amps mean a poorer insulating coating between the laminations.

So, rust is not a benefit in any way because it is not a better insulator than the varnish on grain oriented. Is rust bad? Maybe, maybe not. If the lamination resistance decreases, the core loss rises, and the transformer gets hot. If your old transformer doesn't overheat, then don't worry about the rust or physical appearance. I wouldn't worry so much about the steel, but I would worry about the insulation on the old windings breaking down.

[Bruce / Mission Amps]

I have mentioned this before here on Ampage.... I've bought, used, seen and still have power transformers where the actual E-I laminations are welded at opposite ends. You can't possible have more of dead short across the laminations then that!!

I don't know if this is because the E I plates are not interleaved, staggered or what but, it doesn't make much difference to the transformer if the actual laminations are shorted or not.
I can only assume the welded laminations act like a one turn loop on the outside of the tranny and that could generate a small amount of heat due to some losses.
But I am not a magnetics engineer and don't know that much about the physics of all this stuff.

[Diablo]

I have mentioned this before here on Ampage.... I've bought, used, seen and still have power transformers where the actual E-I laminations are welded at opposite ends. You can't possible have more of dead short across the laminations then that!!

I don't know if this is because the E I plates are not interleaved, staggered or what but, it doesn't make much difference to the transformer if the actual laminations are shorted or not.
I can only assume the welded laminations act like a one turn loop on the outside of the tranny and that could generate a small amount of heat due to some losses.
But I am not a magnetics engineer and don't know that much about the physics of all this stuff.

Good observation Bruce, you're correct, the welds are short circuit paths for eddy currents between lams. Cores in the old days used to be bolted together to minimize hum from vibrations of the laminations. For ease of assembly, modern ones are often welded on the outside, and some are interlocked together - a large dimple in the steel fits into a dimple in the next lamination, male and female fit. The welds in cores should be made in an area of low flux density, so the core losses and heat don't rise too much.

[bob p]

Although based on MM's customer support and website, they appear to know nothing about tranformers, so no wonder they were surprised by the rust...

what more could you expect from a company that uses ads like this one?



I've had a lot of MM iron pass through my hands. I used to buy MM iron at end column pricing back in the days when I could buy MM iron cheaper than you could buy Hammond iron at retail. MM's prices have skyrocketed since they received some boutique recognition at the end user level, and their price increases have outpaced the competition because of it. The MM prices have become ridiculous. It seems that the days of buying the $75 MM plexi OT are long passed.

My luck with some of the Mercury stuff has been hit or miss -- some of the iron has been absolutely beautiful, while some of it has been of such a poor quality that I would have rejected the shipment if they would have made it easy for me to do so.

I've had tweed Fender OTs come to me with wire leads that have been spliced on; somebody cut a lead too short or damaged a lead, and instead of replacing the entire lead, they just spliced the lead and shrink wrapped the splice. I've had other transformers arrive with torn paper bobbin wraps, iron that was so poorly packaged that the teflon wires had cold-formed in shipping so that the insulation was gone and the leads were exposed, scratched end bells, and spray on insulation that was turned over on its side before it was allowed to dry so that it had bare spots that were caused by laying on the wires. Some of the workmanship was just shoddy.

As far as I was concerned, too much of the iron arrived in unsaleable condition. None of my customers were willing to pay a premium price for a MM "ToneClone" transformer, only to have it be objectionably flawed visually. The iron may have worked fine, but a lot of customers care what the iron that goes into their amp looks like -- especially when they're paying premium prices for it.

I talked with Andy Fuchs about my problems after I first reported them on Ampage quite some time ago. He told me that he NEVER had any of these problems, and that it sounded like I was dealing with a completely different company. My impression of the situation was that he buys in enough volume that they wouldn't dare send him a POS OT, and that buying in lots of 24-36 units, I was small potatoes and they were just using me to offload some of their less than prime specimens.

The final coup de grace for me was when MM published their wholesale price list on their web site for everyone to see. When that happened, everyone that bought a transformer from me expected to get single unit quantities at end column pricing.

IME these guys have made some really stupid moves in dealing with their wholesale clients.

Thinking about this makes me really miss OEI.