RM100 hum/buzzing issue

I purchased a used rm100 from a music-go-round and it has a constant hum or buzz.

I assumed it was a tube issue and wasn't too concerned about it. Life got busy and I am now past the time that the store allows returns.

Tonight I swapped all the tubes. Put in different power tubes and rebiased them. Swapped all the preamp tubes 1 by 1. The hum is still there.

There was a time or two shortly after I got the amp that after powering the amp up and hearing the hum, I shut it back down and started it up again, and the hum was gone.

This noise is definitely stronger depending on how much gain and volume you have dialed in. It almost sounds like a ground hum, but I have tried different outlets and different power cables to no avail.

So at this point, any suggestions would be great, otherwise this is going to be used as a fancy boat anchor.

Are there any dimmers or fluorescent lights nearby?

No, and all my other amps are fine through the same outlets.
If the **** thing wasn't so heavy I'd just ship it to Rob or Trace, but shipping both ways simply isn't worth it to me when these are going for under $500 as it is.

If you don't want to ship it and have a local tech you trust, email me for info that you can provide that should likely solve the issue. This is not a user serviceable issue and it requires a lot of skill to get to the source of the problem.

Thanks for the offer Rob.
I don't know of anyone locally who works on amps.

I'm debating if I should mess with shipping it back and forth or just try and offload it locally for cheap and take a loss.

If I did send it to you, are there any worthwhile mods to the amp you could do?
I am guessing I have the old P/D board because they are not as responsive as they are on my rm50.
The only other thing that comes to mind and probably isn't practical is a half power switch and a switch that would allow you to choose which pair of power tubes you were running.

Maybe you know some mods that would be worth it though.

if you could replace the power transformer with toroidal type than at least the shipping back cost would be much lower

So I just swapped in some 'less desirable' tubes because I was going to go to the local shop and see what they would give me for it as is.
I fired it up and it was humming as usual.
I shut it off and ran into the other room to grab a beer and when I fired it up again, the hum was gone and it sounded glorious.
So for now it stays.
But I would not feel comfortable gigging with this amp until the issue is resolved.

Good ole rm50 to the rescue again!

My Rm100 had an intermittant buzz for about a year. Fortunately it just stopped one day and has never returned.

When it was buzzing I could cycle the standby switch and the buzz would stop temporarily. Even the fasted flip of the switch would stop it.

Not sure if your's responds in a similar way but I gigged with that problem and was able to make it tolerable because I could stop the buzz between songs it needed.