isolating a tube problem - ARC LS5 preamp

The easiest way, since you've got a noisy tube in one channel (which you almost certainly do), is to switch the tubes between the channels in pairs, one pair at a time, until the offending bits move -- and there's your bad apple. That said, I tend to agree that, since you can't know the age of any of the tubes and any (or many) of them could be loosing steam (whether they are being noisy about it or not), it might be a good idea to seriously consider re-tubing the whole thing. You may find your sound drastically improved across the boards, and the only way to know for sure is to try it. You’ll need new tubes eventually anyway, it’s always nice to have extra ones on hand, and, simply enough, that’s the worst that could happen by trying – you might end up with a few extra...