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Shellac my tube amp transformers?

audiodon

audiodon

Addicted Member
Hey,
I've got a Fisher X-101-C that is a wonderful amp and is now fully restored with one exception . . . the power supply transformer has a mechanical hum of either the 60hz or 120hz variety. It does not get into the audio chain, but I can hear it nonetheless. Even with the amp in a case I can hear the buzz because in my setup the amp is four feet from where I sit.

I found this link on the web about how to fill in voids in the transformer.
http://home.att.net/~chimeraone/vintagetransrebuild.html

1. Has anybody done this?
2. Can you suggest any other references?
3. If you have done it, do you any experience with this procedure eliminating this specific problem?
4. Any suggestions for shellac?
audiodon
 
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Reading that site, I think the guy is
full of crap, WRT "limited lifetime" someone please tell me how long that is.

"Condensation" the average humidity level in an air conditioned home is 55%. that is not a lot of moisture.
 
ducati_EL34 said:
Reading that site, I think the guy is
full of crap, WRT "limited lifetime" someone please tell me how long that is.

"Condensation" the average humidity level in an air conditioned home is 55%. that is not a lot of moisture.
Click to expand...

I didn't make it through much of his page but I'll add that there was an interesting article in some high end gear magazine a few months back that measured some old pitiful looking output transformers comparing to pristine versions. The pitiful looking ones measured and sounded better. The article suggested that the naturally occurring oxidation between lamination layers created a benefit to that improved the sound quality.

Edit: sorry I was thinking ouput transformers and the OP question was about power transformers. I have heard that the hum can be from separating laminations. I wonder if it would be possible to use a clamp to temporarily tighten them up and test to see if that changes things :scratch2:
 
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I've tightened the laminations already. No change. It's inside. I can hear it best near where the wires come out.
 
Could it be that the older transformers contained pure copper? I know that today's penny has been cut with alloys and contains little copper.
 
Bump for the weekend crowd.
Can anyone offer some experience as to whether shellacking a power transformer fixed mechanical hum?

Bricktop, by tube diode, do you mean one of the rectifier diodes? There aren't many diodes in this box.

Also cross-posting into DIY.
 
Hey Don, is there a wooden wedge inside that TX? Sometimes they put a wedge in between the lams and windings to keep things snug. These wedges can dry out and shrink over time. Sometimes you can just tap them down a little and regain the fit. Is that SS rectified? BTW, I have one of those here, and could look if you want me too (if yours is all mounted up). I shellac TXs just to freshen them up and restore their factory ugliness.:yes:
 
Been wondering the same thing meself Don.

The KT-600 I picked up a while back has a humdinger of a PT. Apparently fairly common with this model.

I'm wondering though if a special grade of shellac is needed?

I've read that you can use the shellac that electric motor repair shops use and that you can take your tranny to them and have them dip it in their tank. Have to be careful to tape off anything you don't want coated though. Then take the tranny home and bake it in your own oven.

I'll see if I can track down the link to where I got that info.:D

Cheers

Lar
 
I'd think hard about baking a freshly dipped transformer at home. The fumes can cause some pretty severe, umm, side-effects, even with VERY good ventilation. I suppose it's possible, but not recommended, but at the very least, plan to throw away the cookie-sheet you sit it on (to save your oven). Besides, it might very well make your food smell or taste funny, possibly causing other types of problems. I know some of us bake old gear, to remove any moisture from transformers, but that's where the varnish or shellac is well cured, for several years. Big difference.
 
jaymanaa,
I'll pull the end bells and look for termite food.

Yes, this one just uses just two diodes for rectification. I've replaced them and done all the can caps and other electrolytics, as well as the coupling caps.
 
The most important thing to considered when using shellac is if it is fresh. Stale/old/rancid shellac will never fully cure and remain gummy. I use fresh shellac flakes and prepare them myself with denatured alcohol (that contains no water). A coat of fresh shellac will dry within twenty minutes. Whether wax or dewaxed is up to the individual. Myself, I prefer dewaxed because it will take any topcoat without a problem.
 
dBfreak said:
The most important thing to considered when using shellac is if it is fresh. Stale/old/rancid shellac will never fully cure and remain gummy.
Click to expand...

Mixed shellac will absorb moisture. If cured in the oven, that shouldn't be an issue. Have you actually had old shellac remain gummy, that was baked? The baking should draw the accumulated moisture out.
 
Goodwill_HiFi said:
Mixed shellac will absorb moisture. If cured in the oven, that shouldn't be an issue. Have you actually had old shellac remain gummy, that was baked? The baking should draw the accumulated moisture out.
Click to expand...

I keep mixed shellac in airtight containers (canning jars) in a cool and dark place (that goes for the flakes also). I've run into gummy shellac only while doing woodwork so I could not tell you if baking would be a fix. Mixed shellac is on average good for six months to a year when properly stored (my observation).
 
Today I unbolted the power transformer from the chassis, pulled off the end bells and discovered the buzz to be coming from inside the windings of the transformer. So, I marked where the leads go, took a picture and desoldered the transformer leads, and then removed the transformer.

Next I'm going to look for a company that rebuilds automobile starters and see if they can shellac it for me. The hum it just too pronounced to ignore. I've heard you can ignore mechanical hum, but it puts my laptop fan to shame volume-wise.
 
audiodon said:
Today I unbolted the power transformer from the chassis, pulled off the end bells and discovered the buzz to be coming from inside the windings of the transformer. So, I marked where the leads go, took a picture and desoldered the transformer leads, and then removed the transformer.

Next I'm going to look for a company that rebuilds automobile starters and see if they can shellac it for me. The hum it just too pronounced to ignore. I've heard you can ignore mechanical hum, but it puts my laptop fan to shame volume-wise.
Click to expand...

Most automotive rebuilders are centrally located big outfits now....... look for an industrial motor shop.
 
I rebuilt starters and alternators in the 80's. We never used shellac, better look for a transformer company, more of them around then people realize
 
Couple of freshly shellacked output trannys (Hock's dewaxed garnet flakes):
 

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