Altec SS amp transformer hum

[altecman]

Is it normal for the old Altec green face ss output transformer coupled amps to have some power transformer mechanical hum, this hum also transfers to the chassis. I've tried everything from tightening the clamping bolts to placing rubber grommets between the transformer and chassis and putting sheet rubber on the inside front panel. This reduced a fair bit of mechanical hum but is still audible at 20 feet when no music is playing.
Has anyone got a suggestion? Could the main filter cap cause this or do I need to totally isolate the transformer from the chassis. Need Help!!!

[bfish]

It's normal for all line frequency power xfmrs to have a bit of mechanical hum. When that bit becomes obnoxious, you've got a "****** ". (durn word filters...hum-mer)

Mechanical xfmr hum is caused by magnetostriction, where the iron or steel laminates minutely expand and contract as the magnetic domains are realigned. It's not caused or affected by other PS components, you could remove it from the amp and power it with a resistive load, and it'll still hum.

If you can't tolerate it, and tightening the lamination/cover bolts doesn't help, replace it.

[donaldpatten]

You could try potting the transformer with varnish.
Remove it, and suspend it in by its wires, in a coffee can of varnish. Completely immerse the transformer , cover the can and let it soak in. Later remove the transf and hang it out to dry (completely). Ideally the immersion would be in a vacuum to get the air out.

Don

[Little woof]

I was thinking along the same lines...but I was thinking of using Shellac..Not sure why, but years ago when I was working in electronic repair, I believe that's what was used. Course I could be recalling wrong..that seems to happen more lately. Thoughts anyone??

Dan

[altecman]

Thanks for all the responses, I find it is a bit strange that this problem is in both mono amps. I bought these amps off a gentleman who used to be a distributor years ago, he said that he only used this pair for home listening, not PA. Going by the general appearance, I must believe him as they are in mint condition. The power transformers have no visible lamination separation on the outside as the varnish is in perfect condition. I was only inquiring to see if transformer hum was a problem in early SS equipment, the amps in question are 1590C -200watts, they must have been very late versions as they are virtually identical internally and electrically to the later E version, basically a E version in a older green face skin.

[Todd W. White]

Atwater Kent, who made the worlds' first AC-line powered home radio receiver, the Model 40 (which I have, along with a Model 44), put their power transformers in a metal case and filled them with tar.

They lasted for years that way, but OH BROTHER - if they broke, they were HORRIBLE to fix...!