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Mesa 400+ w/ 2x15 Questions

Discussion in 'Amps and Cabs [BG]' started by Cerpin Taxt, Apr 14, 2010.

  1. Cerpin Taxt

    Cerpin Taxt

    Jun 12, 2005
    Hey folks,

    I've recently had some issues with my Mesa 400+ and my Mesa 2x15 cabinet. I've tried consulting technicians (concerning the 400+ anyway), but I'm really at a loss. I'd love to see if anyone here on the forums has any feedback regarding my issues.

    The Mesa 400:
    For the last couple of years, I've been playing an old Mesa 400+ that's really seen some time on the road. At one point, a transformer came loose inside the amp and bashed into some of the internal components. Naturally, I brought the amp in for repair. The tech re-mounted the transformer and cleaned up the internals. He tested the tubes and reported that they were all in decent condition.

    Here's the issue. If the amp is powered on for more than 30 or so minutes, it gets extremely hot. At this point, the sound begins to distort and eventually fades away completely. The tubes are burning to the touch despite the fan being on high.

    The different output jacks respond somewhat differently when it's in this state, but over a long enough time they will all stop working. I brought the amp back to the tech who fixed the transformer and he is stumped.

    I've tried using different cabs, bassses, outlets, power and speaker cables. Nothing seems to help. The only thing I've noticed is that if I open the top cover (which exposes the tubes to the cooler air), it takes longer for the problem to arise (but it still happens).

    The Mesa 2x15 Cab:
    The cab I'm having problems with is identical to this one:

    This cab has also been through some touring and has finally started to show it. The back panel of the amp has 4 1/4th inch jacks. On one side, the is an input and an output with an 8ohm label. On the other, there is another pair with a 4ohm label. Back when all of this stuff actually worked, I just took a speaker cable from the 400+ and went into the 4ohm input - no problems.

    Now, the 4ohm input only causes ONE of the 15"s so work. Similarly, the 8ohm input only powers the other one. If you plug into both, they both work. Now, I realize that plugging into both could be very bad - if one input is taking 8ohms and they other is taking 4, the result is an irregular ohm load and could hurt the amp (I think?). I tried it anyway just to try to diagnose the issue and it's the only way to get both speakers pumping.

    It's possible that the 400+ was played quite a big through only the 4ohm input even when it was in this state. If this input is broken (as it appears to be), perhaps this contributed to the 400+'s problems...?

    I've tried different amps and speaker cables - no luck. I checked the solder connections on the back panel and on the speakers - everything looks solid.

    If anyone has any ideas or suggestions concerning these two issues, I would truly appreciate it. I love both of these pieces of gear, and while i have great alternatives in the meantime, I hate seeing them collect dust. Thanks in advance!

    - P
  2. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    amp---take it to a tech. a different one if possible. not all techs are created equal.

    cab---one of the best arguments for matching cabs out there. not only will they have non-matching impedance, but the one with 4 ohms will get twice as much power as the 8 ohmer, so there will always be a volume imbalance. the 400+ should be able to take it, but it would drive me nuts with the volume imbalance.
  3. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    i agree that you should take it to a different tech, and one that is extremely familiar with high powered tube amps. to me, it sounds likely that the output tubes are not biased properly, and if they aren't, no amount of the fan's air blowing on it (or being in 'open 'cooler air) is going to solve that. and after the damage that you described, your amp should be completely gone thru thoroughly from the input jack to the output jack.
  4. if you have a multi meter isolate and check all the connections and wires for continuity.....if not switching the wires from the speaker that works to the one that does not will tell you,or run a wire from a different source into the speaker with alligator clips....preferably from something other than a high powered tube amp....if there is still no signal try gently pushing the speaker in and out near the edge ....any crunchy sound/feel is bad
  5. Head - take it in

    Cab - sounds like a loose connection to one of your speakers from the 4 ohm input jacks. You said that the speaker that does not work from the 4 ohm input does work when connected by itself through the 8 ohm jack correct ?

    Have you tried the other 8 ohm input ?

    If that works, then just connect to each of the 8 ohm inputs.
  6. Cerpin Taxt

    Cerpin Taxt

    Jun 12, 2005
    This is the strange thing. The 8ohm "side" of the back panel has an input and an output. (To be honest, I don't even know how you would use that output). The 8ohm input powers one speaker, the 4ohm input powers the other.

    Now, it's POSSIBLE that it's always been this way. This speaker is a hand me down and I haven't used it very much at all. Embarassing as it is to admit this, I may have just not noticed that only one speaker was working. This is doubtful, but possible.

    Unless I'm totally misunderstanding something fundamental about this cab, the 8ohm input should power both speakers at 8ohms, and the 4ohm input should power both at 4ohms. Since the cab is in a roadcase, going all the way inside is going to be... intense. Thanks to you guys, I feel that I know what to look for if it comes to that.

    As for the 400+... yeah, it looks like I need a new tech. The first guy is so perplexed that he wants it back to fix the problem for free, so I'm kinda trying to help him out by pointing him in the right direction. He did an amazing job cleaning up the amp and remounting the transformer, so I actually do have some confidence that he can follow through. I'm desperately trying to avoid having to retube it, and though this tech believed the tubes were ok the first time he looked at it, he could have been wrong. Given the cost of 12 power tubes and the fact that I have an SVT Classic as well, it's likely that I wont get around to re-tubing it for quite some time. :(
  7. You are mistaken.

    That cab is loaded with two 8 ohm speakers.

    The 4 ohm input is supposed to run both speakers.

    The 8 ohm input runs one.

    However, there should be TWO 8 ohm inputs (one for each speaker).

    Are you absolutely sure the second 8 ohm jack is for OUTPUT ?

    Have you tried connecting to the one you think is an output ?

    If not, try it by itself (don't make any other connections). If it powers the OTHER speaker, then that is how it is wired - 1 input for each speaker.
  8. billhilly66


    Aug 25, 2007
    Plano, TX
    I'd take the 400+ to a different tech and rewire the cab the way I needed it to be.
  9. Mo'Phat

    Mo'Phat Supporting Member

    Oct 1, 2003
    San Diego, CA, USA
    ...also, since nobody has said it yet, the 400+ through the RoadReady 2x15 (TL-606 based design) is the Fist of God rig.

    It should kill puppies in the next town over.
  10. Tim1


    Sep 9, 2005
    New Zealand
    Yep, used to run a 400+ into a RoadReady EV215,- it could cut through anything. in the end I got rid of the cab becuse the vOicing was a little too much in the upper mids for me and went to a Berg NV 610.
  11. Alex1984


    Jan 16, 2010
    I hope I don't get this wrong. In regards to the cab, I think it's similar to the wiring Marshall uses in their 1936 cabs.

    The design was supposed to allow stereo and mono operation. The 8 ohm speakers are wired in parallel, the 4 ohm plug uses this configuration and powers both using one input. The 8 ohm input likely has a switch which separates the 2 channels when that input is used, yielding 2 separate 8 ohm loads.

    So I believe it's simply a loose connection, and in the meanwhile, since both channels are isolated, no harm will be done from connecting both inputs at the same time.
  12. pulse


    Apr 10, 2007
    Berlin, Germany
    the cab is one of the best

    get it fixed by someone who understands it, but make sure they know!!!
  13. I'll post this in spite of John's post which is basically the same conclusion as I came to. The output tubes are running hot thus a problem with or incorrect setting of the negative bias supply. Tech time.

    The cabinet I feel is most likely dirty or corroded jacks. Phone plugs/sockets are not that suitable for speaker connections in the first place. To connect the speakers that are not directly connected, the unused socket bridges the wiring to the socket being used thus all the speakers are connected to the amp. This wiring scheme allows two different amps to power different speakers in the same cabinet. If this is something you never use I'd replace the entire speaker jack plate with one with speakcons and rewire the cabinet directly.

  14. mikeswals

    mikeswals Supporting Member

    Nov 18, 2002
    Seattle / Tacoma
    All four Mesa heads I've owned had this same problem (D180, 400, 400+). The preamp heater wire (solid aluminum wire runs along the inside side of the chassis) cracks or breaks loose from the circut board. It would work fine for a while when the amp was cooler, but sound would stop or fade in/out when it got to full temprerature. This drove me nuts the first head it happened to, I could see the power tubes were still glowing while I played, so I played it with the top lid removed and could see all the preamp tubes were shutting down...voila!
    Ever since then, every Mesa tube head I bought I discovered the same cracked heater wire at the board, and it's a simple 2 minute fix, and I always check it during my annual cleaning.

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