Mechanical buzz coming from the chassis

It appears that the hum is due to the bottom plate on the chassis casing vibrating against the side wall. I placed something underneatht the amp to pressure on the bottom cover and this has eliminating the hum.

I guess I could also loosen the screws and place a bit of blue tack between the two surfaces and then tighten everything again.

What Viridian said. They generally all hum, but some more than others. My tube amp is totally silent, but it is the exception.

Double check your bias also.A tube starting to short can cause this too.If this amp is easy to bias,you can try swapping the tubes between the two amps.If its hard,it may cause a problem that's not there.

I have the amps sitting on 1/4" thick cork mats. This helps, but regardless the hum is not audible from my listening position.

The other strange thing is that only one amp hums but both amps had relatively loose screws.

Also, soft mats and tightnening screws will eliminate the symptoms but do not address the root cause.

I had the same problem with my Quicksilvers. I called and was told if the amps were on a shelf I should put carpet underneath amps which solved the problem. I bought a set of isoblocks (cork blocks)and put them under the feet of the amps and they work great. I think that should have been mentioned in the owner's manual.

This is a matter of degree - so how close does your ear need to be to hear this ?

I checked the screws on the power transformer, they were loose. I tightned them and the hum is nearly gone. I really have to get my ear in close to hear it now.

Do I need to be concerned that there is an underlying problem with the transformer or incoming power? I have a dedicated 20amp line for the system so there is not much I can do to improve the power.

May be that a bolt on the toroid is loose and needs to be tightened. Or could be DC on the line. If it is DC on the line, you can get a PS Audio Humbuster or an equivalent from Channel Islands Audio. The Humbuster fixed my prob in my last apt. 30 day trial for both products, so if DC on the line isn't the prob then you can return the product with no worries.

Sounds like tha amp is acting normal during operation if you have to place your ear that close to hear transformer hum. If it still bothers do what newbee suggest and also placing the amps of something soft if on hard surface. Placing them on a rubber mat does help.

Sounds like transformer vibrations/hum. If you can, try mass loading the case, or the transformer with something heavy, lead shot in a bag for example. This might help a lot. BTW, if it is transformer hum it could be a problem with the transformer itself, or a problem with the electrical current feed.