ARC VT-100 Crackling Sound

I've always found very good technical help from just calling ARC itself. see www.audioresearch.com for number.

I forgot to say, replace ALL 8 of the tubes of a kind (6550C / 6922) when you do this. If you don't the amp will sound imbalanced when you are done. I usually replace the tubes as a set (16).

The above advise is good... If you find one of the 6922s is bad, make sure you do (or have done) a calibration of the gain stage. This is different than setting the bias, which should also be done if there are any of the tubes replaced. At minimum, you will need a good digital volt meter (DVM) (3 1/2 digits or better) with appropriate leads, the procedure for gain stage alignment, a parts list, tube electronics knowledge, small flat-blade pot. adjustment tool, and about 2 hours time. To adjust the bias, you will only need a DVM, and the adjustment tool.
I have done this procedure, and it yielded good results both sonically, and it reduced the amount of heat the amp was producing.
As for the tubes, ARCs tubes are best, since they are properly selected and matched. I have used others, but make sure you have spares if you are calibrating your amp.
BTW, if this is amp is more than 3 years old, and you listen frequently, chances are you may want to replace all of the tubes and do the above alignments. E-MAIL me if you would like to discuss this further.

Try moving all output tubes from one channel to another (reverse them, that is). If the noise now comes from the opposite side you will have established a tube (or tubes) as the cause. I don't recall the VT100 tube layout, but move them one row at a time to try and isolate the problem tube, if that's actually the cause. This advice assumes that you have one or more extra tubes, (pretty much a must IMO). If it is a bad tube it can cause others in the same row to run hot, so I would test those too, just to be on the safe side. Any tubes glowing more than usual? Like most manufacturers of tube gear, ARC recommends that you get the tubes from them. For what it's worth, when I've had tubes go I have not had the noise go away as you describe. Anyway, this is an easy way to see if this is the problem. Good luck.