Noise in SWR head

[brianmharrison]

Hi all-
I have a buzz/humming noise that has appeared in my SWR Bass 350. I have replaced the preamp tube with a JJ tube, changed basses, made sure everything was connected right, changed outlets to a different breaker but it is still there. It is even there when I just have the amp on but no cable plugged in. It's not super loud but I can definitely tell it is there and I know this head is supposed to be dead quiet (except for the exhaust fan). Here is another thing I noticed. When I plug an instrument cable into the active/passive jack the noise gets quieter, but if nothing is plugged in or if it is plugged in to the active jack, the noise stays the same.

Here is my path:

P-Bass>SWR prefender black face 350 head > >1/4 shielded speaker cable>4 ohm Avatar B2x10 Neo

Any ideas? or do I need to take it to a tech?
Thanks for your help!

[dukeisdog]

How is the tweeter set on the Avatar? It may be reproducing noise from the amp. Back off all of your high frequencies.

[brianmharrison]

@Duke - tweeter is a quarter of the way up. I tried backing off of the highs, but I can still hear a good amount of noise through the 10's, so it is being reproduced by the 10's and the tweeter. Also, I have all eq set flat on the amp and I am using a passive bass.

[B-string]

Ah....what do you mean "shielded speaker cable"??????

[brianmharrison]

meaning a speaker cable is running from the head to the cab and not a standard instrument cable...

[brianmharrison]

bump...

[Munjibunga]

Originally Posted by brianmharrison

meaning a speaker cable is running from the head to the cab and not a standard instrument cable...

Very few speaker cables are shielded. What brand is it?

[brianmharrison]

oh my bad...maybe it is not shielded then...sorry about the confusion. I just know it is a speaker cable and not an instrument cable so my thinking is that would not be what is producing noise. Any ideas what is causing the noise?

[brianmharrison]

I guess I will take it to a tech unless someone knows something I can try...

[dukeisdog]

Originally Posted by brianmharrison

I guess I will take it to a tech unless someone knows something I can try...

Id say it's tech time. Nobody can know what the problem is without looking at the amp, and it's dangerous to open the amp up and look around if you don't know what to do.

[B-string]

It might just be a "shorting" input jack that is dirty and no longer shorts when it should or the power supply caps maybe drying out. Just two possibilities in a list.

[JimmyM]

Try cleaning the jacks on the back of the amp. Sometimes effects loops can get corroded if not used, and stuff like that can happen. Best to use a contact cleaner, but if you don't have any, just take a cable and stick the plug in and out of each jack rapidly a few times, and that should knock enough corrosion off to see if that's the problem.

[Plays_For_Dog]

Sounds like a grounding issue to me. That's a tech fix. Someone with know-how will have to shoot your grounds to make sure they have a good Ohm reading. You can also get interference when a power feeder is too close to a low voltage or grounding wire (Inductance).

[brianmharrison]

Thanks for the replies. I will take it to a tech.

[Ben Esparza]

Originally Posted by brianmharrison

Thanks for the replies. I will take it to a tech.

Before you take it to a tech, make sure it's unplugged and remove the top. Buy some Deoxit contact cleaner and get to work. Place some rolled paper towels (like a hand rolled cigarette) under the front control bank and spray Deoxit into each pot while working the pots back and forth. Also work the push/pull pots the same way. While keeping the paper towels handy, unplug any and all connectors (lightly, patiently) and spray Deoxit into those connections as well. Do it one connector at a time and plug/unplug several times to kind of scrape and sand the metal connector tabs. This is also important for the preamp tube connection. Do the same to the back connectors and make sure everything is connected firmly. The paper towels are to sop up any excess Deoxit.

If you run across a loose wire, connection or something looks fried or worn, THEN take it to the tech for repair. Personally, I've repaired more issues similar to the one you're describing by doing this. YMMV.

[JazzV]

I just went through this on my 550X. I would get an obvious humming sound with nothing plugged in the input jack, and it would get louder as I turned up the master volume. I did some looking on the net, and found that there was a chance it was the jack itself. On the 550X, the input jacks have several spring-loaded(?) strips of metal (three, IIRC) that are supposed to contact an underlying ground when a plug is not in. When a plug is inserted, these move away from the contact point. What had happened on mine is that one of the strips wasn't making contact without a plug inserted, thus preventing proper grounding and producing the hum. It was only about a 1/32" gap, but that was enough. If you want to test this, you have to open up the amp. With the amp unplugged and no cable plugged in, use a soft tipped object (a q-tip works great) to press each of the strips in succession. If there's a gap, that strip will need adjustment. If the gap is small, an easy way to adjust it is to then insert your instrument cable and then using a Phillips screwdriver, gently press down on the top of the free end of the strip. Remove the cable and check for contact again. It may take a couple of tries, but once the strip is bent sufficiently to make contact when there's no cable plugged in, the hum should vanish.

[brianmharrison]

@ Jazz and Ben - Cool! Thanks for the tips. I will try these this week to see if it fixes the noise!

[brianmharrison]

JazzV - I tried adjusting the strips on the inputs but no dice. The hum is still there. I am just going to put it back together and take it to an amp tech.