Variac noise

Also possibly worth noting: when I power it on (amp plugged into it) it produces a very soft acceptable hum, but as the amp warms up over about 20 seconds, the hum starts taking on a buzzing quality.

-sorry, troubleshooting electronics is not something I’m good at…

So variac noise depends on load, as you report, once your amp warms up it hums more.

Unfortunately, hum isn’t very predictable. Several transformers from the same lot may all be the same, or there may be one that hums less and one that hums more.

Your friend may be lucky. Or he may have an amp that has a much lower load than yours. or his electrical system may be better. A couple of things to try:

1. Take your humming variac to his house and plug his amp into it and see if it hums.

2. borrow his variac for an evening and see if his variac hums at your house.

One thing people chase with transformer hum is DC offset. I’m surprised someone hasn’t already suggested that. I have never found DC offset to be the problem. But there is always a first time so maybe that is your problem. DC offset can be caused by unbalanced loads on your home system, but not likely.

You might find that a larger variac/transformer hums less since it will be less loaded. OTOH, sometimes a larger transformer hums more regardless of load.

A variac allows you to dial in any voltage. this is handy in a lab but is not needed in your case. there are many fixed transformers available that convert from 120v to 100v. The ones on amazon are easy to return if they hum. I would oversize the transformer by at least 100%.

Jerry

@ccogopher 

Again, variacs are not meant to be quiet. Your friend is lucky.

You could try a DC blocker on the AC line; clearly you're not interested in the method I outlined earlier, although it would solve the problem as well.

Not disinterested in your method. I’ve been trying to reach a tech friend who could try your idea. Also looking into a step down transformer as a possibility. 
I really appreciate your input as well as the others. 
thanks!

@ccogopher A stepdown transformer will be more expensive. Bucking transformers are fairly cheap, since the transformer does not have to be nearly so large.

For example, the Marantz 8 draws 190 Watts. It runs on 100V if the newer Japanese version. That means it draws 1.9Amps at 100V. So a transformer that puts out 20V at 2Amps should do the job. You can get one of those off of ebay, such as this one:

Hammond 20V transformer

The primary of the transformer is simply connected the the AC line to power the transformer. The secondary is put in series with the amplifier. The phase is important; one way you hook it up it will add 20V to the line voltage, which isn't what you want; the other way it will subtract 20V from the line voltage. This little transformer is only $18.00.

If it were me I'd get one with slightly more amperage (3-4A) since I don't like running transformers at or near capacity. But even so such a transformer is going to be inexpensive.

So this is a simple hookup. You could put the transformer in a box so it looks nice. Bud makes boxes that could do the job. Even with the box this is still cheaper than running a Variac or stepdown transformer that has the current capacity to do the job!

FWIW, your variac might be vibrating as much or as little as your friend's. Most of the time when you hear them, you're actually hearing something near them that is vibrating with the variac, rather than the variac itself. If that is so, silencing that bit (for example, a panel) can be inexpensive as well and since you already have the variac, might be a prudent way to solve this.