Mesa Bass 400+ help!!

Discussion in 'Amps and Cabs [BG]' started by Veejnas, Sep 22, 2004.

  1. Veejnas


    Aug 30, 2003
    Kansas City
    I'm new to this site and I'm hoping someone can help me out. I have a Mesa Boogie Bass 400+ and I recently experienced a loud buzz at rehearsal. It was intermittent but got worse with time. It would originally only happen after an hour or so of playing and then go away after I would flip over to standby for a few minutes. Eventually it progressively got worse so I figured it was time to retube it. Well, I retubed the power section with 12 JJ 6L6's. It worked fine at lower volumes but when I turned it up to a decent level, Same buzz, exact same problem as before. My bass can still be heard but the level is much lower and the buzz is predominant over the Bass guitar and it seems to come and go. The funny thing is iwhen it isn't buzzing, it sounds great, clean and plent of headroom. I took it into a local tube amp guru and he's scratching his head. He played it for 1 hour yesterday at a fairly high volume with no problems. Today it started buzzing after 15 minutes?????? He's thinking a grounding problem???? Any help or experience with a similar problem would be greatly appreciated.
  2. Passinwind

    Passinwind I Know Nothing Supporting Member

    Could be many things, like intermittent send/return jacks or control pots, bad solder joints in the power supply or elsewhere, a bad filter cap, etc. Your tech could be right about a grounding fault, for that matter. I recently had an amp on the bench that did just what you described. It was an intermittent standby switch, but it took a few stabs at it to figure that out. Finally, the customer mentioned that the standby had been widgy for quite a while. DOH!, thanks for mentioning that the first time...

    Just moving an amp from the shop to somewhere else can be enough to set off that sort of problem. Your tech needs to cycle the amp hard, with as much test equipment as possible monitoring various points in the circuit, while he plays with all the controls and in/out jacks. It's highly unlikely that anyone can really help you out online, unless you want a primer on repair methodology.


    -------------------------Charlie Escher
  3. Hiya!

    You said that you only replaced the power tubes, perhaps the problem is in the pre's?? I received my 400+ with a bad pre-amp tube, and it would buzz and rattle, and do it at random times, much like you described. (Especially when I went to show someone, it wouldn't do it... :scowl: )
    A tech found the problematic tube and replaced it, and nothing but good times followed :hyper:

    It's a simple solution you may have already covered and may have forgot to put it in your post, but...meh
  4. Rockbobmel

    Rockbobmel Supporting Member

    Your preamp tubes are (looking head on to the amp), from far right to left P1, P2, Phase Inverter, then Buffer tube.
    BTW, Welcome to TB., Bob
  5. Chef

    Chef Moderator Staff Member Supporting Member

    May 23, 2004
    Columbia MO
    Staff Reviewer; Bass Gear Magazine
    I've not had problems with my 400+ yet, but mesa customer service is reportedly quite good. Call and talk to their techs. If your local guy can't find it, mesa surely can, and they turn stuff around very fast from what I've heard.
    G'luck, mesa's rule :bassist:
  6. Veejnas


    Aug 30, 2003
    Kansas City
    Thanks for the feedback. I hope to have this thing back out and gigging soon.
  7. DubDubs


    Aug 23, 2004
    Los Angeles
    No, your 400+ is unrepairable so you should sell it to me at a really low price :D.

    Seriously do what everyone else is saying and get the preamp tubes checked.