Noisy SWR

[Daveomd]

Got an early 90s SWR SM400s and sometimes there is a relatively mild hum coming out of the cabs, even when there is nothing plugged in to the input jacks. It is there when the pre and master gains are turned all the way down but doesn't get any louder when I crank the volume way up. I run the send out into a separate power amp and the buzz is audible through the cabs from that as well. Any ideas? Haven't tried a new tube yet but I'm hoping that's all it is.

[JimmyM]

shouldn't you be sending your pre to the return if you want to defeat your amp's pre?

might have a grounding problem somewhere. since it does it with the separate pre, i doubt it's the tube, but you could always pop another one in there and check. either way, i doubt it's something really serious. might even be normal operation noise that's just there.

edit: sorry for the editing...i guess i didn't read clearly enough the first time and had to change my answer

[Daveomd]

Running 1 DB 112 cab with the SWR and a DB210 + DB112 with a QSC, using the SWR for the Pre.

[JimmyM]

i see...well then in that case it very well might be the tube.

[T-Bird]

Hi.

As You don't use the power section of the SWR, the most likely cause for hum, incorrectly biased MosFet's aren't most likely the culprit.

I haven't got the schematics at hand ATM, but if I remember correctly, there's separate filter caps for the pre and the power section. For an amp that old, the cap job is long overdue.

If the pre amp tube has sat in its socket for more than a couple of years, the pins are most likely oxidized, so wiggling it a bit may cure the problem. An 12AX7 is a cheap tube to replace though, so if You don't have one at hand, do buy one before you pop the lid open.

Regards
Sam