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GK MBF500 - Scratchy sound

Discussion in 'Amps [BG]' started by loopee, Dec 9, 2013.

  1. My fusion has been making scratchy sounds lately, it seems to go away after a little while, possibly because the tubes have warmed up?

    I've never had a tube amp before....does that mean one of the tubes is going?
  2. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    Groom Lake, NV
    Independent Contractor to Bass San Diego
  3. you're a man of few words Munjibunga.....lol
  4. Mystic Michael

    Mystic Michael Hip No Ties Gold Supporting Member

    Apr 1, 2004
    New York, NY
    What kind of scratchy sounds? Have you ruled out dirty pots?

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  6. jnewmark

    jnewmark Supporting Member

    Aug 31, 2006
    Stax 1966
    Play guitar.
    Might be the old Effects Loop thing. Take a 1/4 cable jack and run it in and out of the Send and Return connections a few times and see if that clears it up ( with the amp off, of course ).
  7. Yes, not the pots....haven't even used them.....just playing and getting scratchy sounds and now it doesn't go away anymore.....the head was used when I bought it though apparently used little and it's gone out on one gig in the last 1-2 years that I've had it and just occasionally used (with headphone) in my den......so could this be the tubes?
  8. Mystic Michael

    Mystic Michael Hip No Ties Gold Supporting Member

    Apr 1, 2004
    New York, NY
    Say what? You've never used the pots? You've never turned the volume of your amp up or down? You've never adjusted the EQ? :confused:

    I'm not saying that your amp has dirty pots. Apparently it doesn't. But if it did, they could very well be introducing noise into your signal, whether you were turning knobs at the moment, or not.

  9. LOL.....of course I've turned the pots.....lol.....I just meant that I hadn't turned the pots when the noise started happening....lol....turning the pots didn't seem to make a difference with the scratchy sound....

    So, I've tried the Send/Return thing....still scratchy....tried a different cab.....still scratchy....tried different cables, basses, etc....still scratchy....

    Would the pots, whether turning them or not, actually create a scratchy sound?
  10. OP: If you have an active bass, try replacing the batteries, and check any petals too.
  11. B-string

    B-string Gold Supporting Member

    Possible especially if they don't get turned or sit in one position for a long time unpowered.
    Vacuum tubes made these days are not as stable as they "used" to be. Seems as though if they can pass somewhere around 100 hours without failure you can reasonably expect long service life. IME new tubes generally fail if they are going to within 1/2 hour to 60 hours of use. If they continue past 100 hours it is reasonable to expect several thousand hours from preamp tubes before they weaken (barring mechanical damage).
  12. been using passive basses only....(didn't even try using the active one) and all the pedals are on a One Spot.....I did plug my bass in the return input and lo and behold....no scratchy sound.....so it is in the preamp....the tubes, right?

    Well, I've barely got 30 hours on that head so these tubes were not the "good" ones then.....several thousand hours? Really? Hope I luck out on the new ones then.....

    Should I get the same ones? I've never opened up an amp before, are they hard to get too?
  13. so a few days ago I decided to look inside my head...lol...make that the MBF500.....to see what tubes were in there and possibly go buy some new tubes the next day...but I couldn't see a name on them and really know nothing about tubes so I just put the top back on and figured I'd take it to a nearby music shop.....

    I plugged in my bass the next day and NO scratchy sound....plugged in again a few hours later....NO scratchy sound....plugged in the next day....NO scratchy sound....plugged in the following day....NO scratchy sound....took it to a Xmas Jam Party over the weekend.....it was ON for more than 7 hours and NO scratchy sound....and it sounded fantastic (with one of my Tots)......2 other bass players plugged in their basses and they sounded killer (1 Music Man Stringray, 1 Fender J and my custom passive Fast Bass with SGD pups).....I'm very happy and I guess it just needed some attention (all I did was take the cover off but I treated it very gently).......go figure
  14. GretschBassist


    Nov 11, 2012
    The MBF uses JJ 12AX7 (ECC83) Tubes (Link)

    Also, it is recommended to allow tubes to heat up for 10-15 minutes but scratchy doesn't sound like it's a cold tube issue. As this is the only user-maintainable thing and doesn't cost a fortune, I would always try swapping the tubes first. Next step would be an amp doctor having a look at it and then a witch doctor having a look at the guy who sold you the thing ;-)
  15. Thanks for the link for the tubes.....somehow that's not what I remember seeing in there but I might be wrong.....

    When I used the amp and the scratchy sound started it had been sitting in my warm den for weeks so the tubes should not have been cold, (unless you mean cold because they hadn't been on for a couple of days).......

    In any case it's working now and if the scratchy sound comes back I'll just order the tubes and have someone else put them in (while I watch..so I can do it next time)...

    I'm sure the previous owner has nothing to do with it (he seems like a stand up person here on TB).....tubes are tubes, nothing much we can do about it.....