Old rusty transformers

[Randall]

Just came upon a '67 Bassman head that has been abused, modded, been wet, and the head cab is full of ants. Figured I'd take it in as a project for when I'm bored. In the process of clipping out the mods and cleaning it up, I tested the transformers with the neon lamp test. Both PT and OT tested good. I'm not sure how to test a choke for shorted windings.

Question is, how secure would you feel about proceeding with a full restore on a chassis that has a fair amount of surface rust? Does the exterior appearance give an accurate assessment of how the interior shape is?

I'd like to strip this down and build it back up, but I'd hate to do it only to have a PT die on me.

[The Dude]

Can we see it?

I'm just curious how bad it is. Pictures please?

Without seeing it, I'm thinking a little wire brushing/steel wool and some rust inhibitor? I doubt the rust has penetrated the actual windings. You could remove the bells and see how far in it's gotten.

[Leo_Gnardo]

Transformers that have been soaked, all you can do is let them dry & hope for the best. Slow low heat can help - if you were still in Maine I'd say take the bells off & park 'em on a house radiator for a month or two in the winter. I've had transformers revive just fine that way. You'll have to improvise something similar in your current tropical location.

I had a Bassman I bought from a guy in North Carolina, it was the rustiest thing I'd ever seen. Guess it was used to play shag music on the beach from the time it was new ('67 or so) 'til the week before I bought it. On rebuilding it, just made sure the grounding points were clean shiny metal & didn't sweat the rust. Ended up being one of the best sounding Bassmen I ever owned & eventually a customer bought it & has had no problems with it.

Choke - they ordinarily measure about 100 ohms thru the coil. It's rare that the coil develops a short winding to winding. On rare occasions I've had a choke develop a high resistance thru the coil, of course then it's scrap. Also if you measure any resistance between the coil & frame, I wouldn't trust that choke. Cheap insurance to buy a new one instead of trusting one that shows any problems.

If you feel like doing a lot of work to create a neat looking build, you could strip the chassis & have it sandblasted. An auto body shop might be the place to go. Problem is, the cleaned up steel is likely to rust again in short order. Might do to let 'em paint it, leaving some tape on spots where you know you'll need access to bare metal.

[Randall]

Sandblasting is exactly what I have in mind. My neighbor is a sand etching glass artist, maybe he can do it. Then I will paint it. Also going to build a new cab and cover it with something cool. Maybe build something cool in the bass channel, but what I don't know.

I didn't think of pulling the bell covers. I could have those blasted and painted, too. Here are the pics:

[Chuck H]

I'm going to differ from the theme so far and say DON'T sandblast and DON'T wire brush. The lams are ideally isolated. Rust doesn't conduct well and any coating left on the lams doesn't rust, so... If the transformers are working then you have the best case scenario now. wire brushing or sandblasting could conceivably cause more inter laminate contact than you have now. Better to just use a stiff plastic or natural bristle brush to remove any very loose scale and then fully coat the lams with rust inhibitive paint. If no oxygen gets to the lams they won't rust any further. You will need to remove the end bells to fully coat the laminates. You don't actually want to use a high heat paint because those products don't cully cure until they reach temps much hotter than the transformers will ever see. Just use normal rust inhibitive spray paint. A few coats. It will stink for a while whenever you warm up the amp, but that's the brakes.

[Justin Thomas]

Doubly so given this amp is probably living in 100% saltwater humidity, right?

Justin

[Mick Bailey]

Your amp is nowhere near as bad as some of the amps I've restored and should come back just fine. The worst one was a Traynor combo that had sat in a shed with a leaking roof for decades. The chassis was nearly through on one corner and the transformers were severely corroded.

I've never blasted transformers, despite having a grit blaster. I remove scaling rust from the laminates using a brass scraper and gently clean with a Scotchbrite pad. Just enough to get rid of the rust without abrading the laminates. I then treat with Kurust, then paint. The best paint I've ever found for this job (and many more) is some military stuff that was used to paint field artillery. I've never been able to locate it again - it cleans off in methanol and is super-thin. On bare steel it etches and is not removable once cured. I'm down to my last 1/2 pint. I guess I'll end up using satin black auto rattle cans once that's gone The end bells I just clean up with abrasive and then paint. If they're galvanized I remove the rusty patches then use special etch primer for zinc and then respray to match the original finish.

Even the most severely rusted and beat-up amps seem to come back as near to 100% reliable as you could ever expect and will probably still outlast new stuff built today. The one thing I do carry out is an insulation and earth leakage test on old power transformers before beginning work. You don't want to spend time getting a good finish on a transformer only to find it's useless. Don't know about elsewhere, but here in the UK many venues require a current PAT (portable appliance test) certificate. A bad PT will fail the test and come back to you.

[Pdavis68]

Yeah, I'm with Chuck. Don't wire brush or sandblast the laminations... Sergio Hamernik at Mercury Magnetics argues that rust on the laminations actually improves performance of the transformers...

[Mick Bailey]

Well, the rust between the laminations serves a purpose. Good thing, because it would be pretty difficult to remove other than by chemical or electrolytic means. On the other hand, I don't believe that external rust serves any purpose and in a full restoration customers don't expect to see rusty iron. The same with my Hi-Fi builds where transformers are on show - they need to look good.

On one of my recent AC30 restorations the chassis restored nicely but the transformers let it down. I managed to match the original grey on the end-bells and got a decent finish where they'd rusted - primed with 'Acid #8' etch primer and airbrushed with the final colour coat. The laminations got a very light rub with a pan scourer, then treated with Kurust and a final brush over with 'transformer varnish' to look sympathetically old but not brand-new.

[bob p]

Sergio Hamernik at Mercury Magnetics argues that rust on the laminations actually improves performance of the transformers...

Don't believe everything you hear from Sergio Hamernik.

Sergio Hamernik sold me a brand-new OT that had a spliced/shrinkwrapped lead and when I complained he tried to BS me with some nonsense about how the silver solder that he used to make the splice contained some sort of magic mojo that would improve the tone relative to a transformer that had an unspliced lead.

The reality is that Sergio Hamernik was just trying to bullshit his way out of a tight spot when I called him on selling poorly refurbished returned merchandise as new merchandise. Someone at Mercury Magnetics was too lazy to remove the endbell and replace the short lead with a proper lead. They just spliced the lead and put off the shoddily repaired merchandise onto a customer.

[olddawg]

Your amp is nowhere near as bad as some of the amps I've restored and should come back just fine. The worst one was a Traynor combo that had sat in a shed with a leaking roof for decades. The chassis was nearly through on one corner and the transformers were severely corroded.

I've never blasted transformers, despite having a grit blaster. I remove scaling rust from the laminates using a brass scraper and gently clean with a Scotchbrite pad. Just enough to get rid of the rust without abrading the laminates. I then treat with Kurust, then paint. The best paint I've ever found for this job (and many more) is some military stuff that was used to paint field artillery. I've never been able to locate it again - it cleans off in methanol and is super-thin. On bare steel it etches and is not removable once cured. I'm down to my last 1/2 pint. I guess I'll end up using satin black auto rattle cans once that's gone The end bells I just clean up with abrasive and then paint. If they're galvanized I remove the rusty patches then use special etch primer for zinc and then respray to match the original finish.

Even the most severely rusted and beat-up amps seem to come back as near to 100% reliable as you could ever expect and will probably still outlast new stuff built today. The one thing I do carry out is an insulation and earth leakage test on old power transformers before beginning work. You don't want to spend time getting a good finish on a transformer only to find it's useless. Don't know about elsewhere, but here in the UK many venues require a current PAT (portable appliance test) certificate. A bad PT will fail the test and come back to you.

I looked at the pictures and agree. I live in the San Diego area most of the time and near the beach. From the pictures it doesn't look bad at all. You can get a reproduction front panel if you want and it bothers you. WD 40 can be your friend if you don't over do it (the stuff tends to creep). You can seal problem areas with Rustoleum paint. I use a 1960 Ampeg Jet that looks much worse than this. Lol.

[bob p]

Can anyone explain exactly what Sergio Hamernik is telling us in this excerpt from that article?

"We did an A/B comparison on
identical transformers. One was
left outdoors for seven years,
exposed to the weather, cobwebs...
you name it! The second one, from
the same production—same
machine winder, etc.—was put
away pristine, without a speck of
rust, for the same seven years.

We put both on the bench and subjected
them to a battery of true A/B tests,
showing them the same input voltages.
And we intentionally left the
output leads unconnected so we could
measure each transformer’s performance
without outside influences."

Without outside influences? Like an actual load?

In other words, his experiment amounted to measuring open circuit voltage and and he never actually examined the transformer's performance under load, or at least he's not talking about it's performance under load. What kind of test is that?

"The rusty transformer ran at a
lower temperature because it drew
less power. Bear in mind that each
transformer started life drawing the
same power! At the very least, the
results show that rust didn’t harm
the performance of the transformer.
And they imply it actually helped a
little."

The rusty transformer drew less power under idle no-load conditions than the clean transformer. Ultimately, he equates that with an improvement in tone, though he doesn't bother to explain any sort of proposed mechanism for the improvement:

"If there’s any shift at all, it
shifts toward helping the tone, not
hurting it. The same holds true for
output transformers. We’ve had
more than a few rusty output transformers
in amps with the most
amazing tone."

.

That paper seems to make carefully worded statements that are intended to lure the reader into drawing invalid conclusions. Although Serigo said that they conducted "a battery true of A/B tests," the only test result that he discloses are at idle, without actually disclosing in-circuit under-load test results. How much of a temperature change was there? Was it significant? Why were all of the other results in their "battery of true A/B tests" left undisclosed?

[Pdavis68]

I assumed he meant that that was one of the "battery of tests" as opposed to the setup for all the tests. I would assume they did some tests in an actual amp.

[bob p]

Without making assumptions, I don't see any data that allows any objective conclusions to be drawn. All that I see is mojo marketing hype.

those articles are written the way they are written on purpose -- to lure people into believing in mojo.

[Pdavis68]

I'm not defending them because I don't know squat about MM other than their transformers seem to be ridiculously priced... I understand you have a bone to pick with them, and that's fine...

But I don't see how selling people on the idea of rusty transformers being better than non-rusty transformers, helps them business-wise. In fact, I'd expect more of a, "if it's rusty, then what you really need is a brand new big fat Mercury Magnetics transformer to replace it with..."