Mesa Bass 400 (Non-Plus) Problem/Questions

Discussion in 'Amps and Cabs [BG]' started by tbz, Jul 25, 2013.

  1. tbz


    Jun 28, 2013
    I was practicing with one of my bands a few years ago, with my Mesa Boogie Bass 400 (non plus) head, when the speaker cable blew out the back jack.

    My instinct was to power it down, so I spun around and lunged for the power button, but ended up hitting Standby instead. There was a small blue electrical spark and Standby ended up fusing into place.

    At the time I was quite low on funds and had just purchased an Eden WT-550, and was unaware of the resale value on the 400 so I quietly put it in its case and stored it. Fast forward to now, I'm playing a lot more and am interested in getting it back working again, and wanted to ask for input from anyone with experience repairing tube amps.

    At this point the amp will power on, and all of the power and pre-amp tubes glow. Unfortunately, since Standby is fused into the Off position the amp just can't come out of standby. I've taken the bottom cover off and looked at the area around the Standby switch and there does not appear to be any scoring or burn marks around the switch.

    Does this sound, at "first glance" like I just need to replace the Standby switch?

    How likely is it that the initial scenario, speaker jack flys out of the head while the amp is being played through and then Standby is hit, could have caused serious damage? Which components in particular could have been damaged?

    Lastly, just asking because it has been forever since I used this head. Are the paired speaker outputs independent or parallel? Specifically, if I'm running 2 8ohm cabs, do I use the 2 4 ohm outputs, or the 2 8 ohm outputs. I'm assuming the 4 ohm, but I'd hate to break this thing once I get it fixed....

    In closing, I'd appreciate any advice as, due to my lack of experience with this stuff, I'm going to have to pay someone a few bucks to do this and it would be great to walk into that armed with at least some knowledge beforehand.
  2. chaosMK


    May 26, 2005
    Albuquerque, NM
    Too much hip thrust
    The amp is worth taking to a tech. Fortunately these old school designs are pretty straightforward to repair.
  3. tbz


    Jun 28, 2013
    Bump. In case anyone online during daytime hours has any input ;-)
  4. B-string

    B-string Supporting Member

    Replacing the standby switch is first, or a tech can jump it temporary for testing. Un loading a tube amp can be destructive but with no way to apply high voltage there is no way to know if you got lucky. The output tubes would likely be the first damaged and would need to be tested for shorts, OT 'could' be a remote possibility. Chances are the damage is minimal, but just replacing the switch and not checking for possible trouble could wind up very expensive.
  5. tbz


    Jun 28, 2013
    Thank you for that! I've read some of your other posts and per my understanding you've fixed a tube amp or two in your time
    You and the gentleman that PM'd me both seem to agree that I'm risking additional damage if I just swap the switch. So it's off to the shop!
  6. B-string

    B-string Supporting Member

    Haha, yes more than a couple :D
    Good plan going to a tech with it, could just well save you some major expense. The amp is definitely worth some professional care. Let us know how things turn out!

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