SWR amp hums

Discussion in 'Amps and Cabs [BG]' started by fighterfire, Oct 24, 2003.

  1. I used to have a SWR Bass 350 the chrome version. I was happy with that. I got it from a member here at TB for a good deal in like new condition. Sometime recently, I thought that I would like to have a SM-500 or SM-400S for the extra power and more versatile EQ. I went in to a new shop where I live and there was a SM-500 that did not look too bad. I worked a deal with that shop for my Bass 350, Ampeg B-115(older 70s to 80s?) and Fender Bassman 25 plus a little cash and took home the SM-500 with 30 day warranty. Before I traded the Bass 350 it had a hum which I thought was the preamp tube. I don't recall that it hummed that much when I first got it.

    I tried out the SM-500 in store and thought it sounded good and did not hear noticeable hum. During the past week it has bothered me more. The hum occurs with and without anything connected at the imput. I tried two different preamp tubes and the hum is definately not as loud now but still iritating. The hum gets louder as the volume or gain is turned up. It is present at the effects send. I connected a patch cable to the effects return with no signal going through it and dialed the effects blend completely right and the hum went away, all that remained was a very quiet hiss that to me seems normal. I do not know if hum is present at the XLR but I assume it probably is. None of the pots or sliders seem noisy. The hum is annoying even at a low volume. I tried changing speaker cables and tried all of the various outputs. I tried a different power cord. I tried it in various rooms and circuits in my house. I unplugged almost every other electric device in my house. Nothing stopped or decreased the hum.

    To me this is not normal and I really think that this amp should be very quiet. I would like to know what others think and what their experience has been. I still have 20 days on the warranty. I can get the Bass 350 back if he still has it but it hums too. I have done all that I would want to do moneywise with the tubes. I don't want to spend $ for a tech to look at it. I think that the only thing that I have not tried is taking the amp to another place entirely and see if it hums there.

    Should my amp hum this much?
  2. Nope. If the amp did not hum at the store, you need to try it at a friend's house (preferably in another neighborhood) to totally eliminate the noisy power angle.
  3. I did not notice the hum at the store, can't say for sure that it was not there. I am leaving here in a few to try it at another location.
  4. ostrzoskrzydly


    Jul 1, 2003
    SWRs are very sensitive to power outlet hum. get a power conditioner.
  5. Well, I tried the amp at another location across town. Same thing, hums like crazy. Any other thoughts?
  6. Mcrelly


    Jun 16, 2003
    Minnesota, USA
    ground loop? have you tried a cheater to lift the ground on the plug?

    I have a 350 and I've noticed that it hums badly with my RBI running through the effects loop. I tried shorting (bad term) the send right back to the return with a short instrument cable and left the blend to the right and the hum was not there. I suspected my new tube. when I run the RBI I'm essentially preamping TWICE, so I try to either bypass the swr preamp and just come in through the return, or keep one of my gains low.

    did you say your guitar was connected? shielding?

    is it low freq hum (groud loop?)or high freq buzz (shielding)?
  7. RevGroove

    RevGroove Commercial User

    Jul 21, 2002
    Burlington ON Canada
    Acct Serv. Man.: Long & McQuade; MTD Kingston Basses Intl Emerging Artist; Bartolini Emerging Artist
    I had that problem with a 350x I tried out for a couple of days this week...which I decided against purchasing...but I found in my basement, there was a really nasty buzz, at the rehearsal spot, there wasn't...at the store, there isn't...I know the wiring in my basement was done crappy, so I assume it's bad power or a bad ground...as said above, a power conditioner would likely go a long way to cleaning up the noise...
  8. natebass


    Sep 6, 2001
    Bremerton, WA
    I had a capacitor go bad; made noise like mad.
  9. Thanks for all of your responses.

    I did play with the effects loop and found that the only way to eliminate the sound was to connect one end of a patch cable to the effects return and leave the other end disconnected(preamp bypassed). When the effects blend was dialed all the way right, no hum came through.

    It does it when something is connected at the imput and when nothing is connected.

    It seems that some of the hum is low freq but most of it is about 1K.

    I don't want to buy a power conditioner. I'm not saying that it is a bad idea to use one but something is just not right with this amp. My house is only a few years old and there are no other problems with the wiring or ohter devices. I have tried the amp in another building which we use for rehearsal. It does pop a little when touching the chassis. So far I think it is possibly a ground issue or it could have some bad caps. I do not feel that I can trust that this new shop has someone to do repairs that really knows what they are doing. I am now leaning towards taking it back and getting my 350 back plus some cash for the other items I traded. The 350 hummed too so I can try to see what is going on with it and at least I have money to fix it. Otherwise I can sell it and combined with the cash back from my ohter items I would have quite a bit to work with to get something else.
  10. When I put a new tube in my SM400S (same as SM500), it hummed like crazy - I wound up going thru 4 different tubes before I finally found one that didn't hum. Try using a 12AX7EH tube. They are awesome.

  11. I think that's your best bet.
  12. I have tried two new tubes and I think that is as far as I want to go. The tube that was in it was a GT ECC83. I have tried a Sovtek 12AX7WC and a JJ ECC83S. There is not much else available locally.
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