Trace for a low buzz in one channel on AE3 preamp?

I have Audio Electronic Supply AE-3 Preamp. The previous owner replaced diode rectifiers with a rectifer tube.

I notice that the left channel has a slight buzz (hum). The buzz is consistant, regardless of the position of the volume on the preamp. As my amp has volume as well the buzz comes out only at the max volume on the amp (max. gain). The buzz is not that loud, on my 92 dB speaker, it can be heard only within a few feet. Don't hear it at the listening seat, unless really pay attention to hear it. The buzz exists with all other amps I have tried as well.

I open the pre and look for loosen soldering points, but don't see any. As it happens only on one channel, I think it's should be in the gaining circuit on that channel and don't think it's power supply circuit as both channel share the same power supply.

Where/how should I trace it for this problem?

Thanks a lot,

Try to move your preamp by a couple santimeters different directions. You're probably catching some AC or your left channel stays closer to the AC than the right one.
Another isolation question:
Is your preamp above poweramp or near? Try to separate them if that's the case.
You'll benefit from a high quality powercord as well as powercords "arround" your preamp from a different components.
My pre sits next to the power amp. My interconnect is only 1 meter long. I have tried severval custom made power cords and different outlet, but still buzzing. Anything else I could try?

Thanks a lot
In vast majority of cases poweramps MUST stay away from all other small signal components.

How long you're listening to AE3? Did you try swapping tubes?

You can try first to get some cheap longer RadioShack gold interconnects(BTW they're great!) for the sake of getting your preamp away from poweramp. Also make sure that poweramp is not standing too close to the AC-outlet. A strong and shielded powercord on your poweramp will be a great plus.

I have had it for about 6 months now, but used it occationally as my SE-1 amp had a volume as well. Tried pretty much everything.. tubes, powercord.. Might give a shot on longer interconnent, but I doubt it would solve the problem.. Hate to pay another $100 to have Cary fix it.. sigh..