Mesa 400 pulsing noise?

Discussion in 'Amps and Cabs [BG]' started by Captain_joe6, Sep 18, 2006.

  1. Hey all. I just got my awesome mesa 400 (not the plus) and mesa 1x15 hooked up today to test it out, and after some tweaking to resolve most issues, i have a couple left that I'll throw to you guys to see if there are any solutions. This is my first and only experience with any tube amplifier, so if something is characteristic of the amplifier that I am misinterpreting as a problem, just let me know.

    Firstly, there is a low pulsing noise comming from the speaker, about once per second. Its not loud, just annoying. My thought is that is is something to do with the fan because changing the fan from low to high speed makes the noise go away. This would be acceptable, but then there's the added noise of the fan to deal with.

    Second, there is some sort of hum comming through that sounds a lot like a grounding problem, but with a slightly lower pitch, along with quite a bit of hiss.

    And finally,on the grounding issues subject, plugging in my jazz bass causes some sort of grounding hum. I know its ground-related because when I touch something metal on the bass or cable, the sound goes away. While this may seem to be an issue with the bass, it is not because the hum is not present when plugged into my little solid-state peavey. The humis also present, but not as loud, when I plug my P-bass in. This problem is the same for both channels.

    So, thanks for reading, thanks for replying, thanks for taking the time to help me out. If there are any more points of information anyone needs to point me toward a culprit, feel free to ask.
  2. sunbeast

    sunbeast Supporting Member

    Jul 19, 2006
    Denver, CO
    I have a Mesa 400+ with a ground filter switch, I imagine the 400 has it as well. Just look on the back near the fan switch (at least thats where mine is), it should be a 3 position switch. You are supposed to set it to whatever postition gives you the least noise; if they are all about equal, set it in the center position. This may or may not help, as there are many things that can cause hum/ hiss that could contribute. You might want to try a different power outlet first to make sure that the one you are using is properly grounded.

    As far as the problem with the Jazz Bass, does the problem go away when you touch the strings/ bridge? There will almost always be some noise when you aren't touching the strings, which is grounded once you touch the strings. If touching the strings does not have the same effect on the noise as touching a knob or cable, it means there is no ground connection attaching your bridge/ strings to ground. Unscrew your bridge and there should be a small wire touching it that comes from the control cavity, this wire (if it is there at all) connects to the back of a knob or some other grounded part of the bass to ground your strings.

    Again, the noise could be coming from many sources- these are just a few suggestions!

    Karl Zickrick
  3. Yep, tried the ground filter switch and got the results you proposed: equal in all positions. And as far as the jazz,the bridge is grounded properly, I just didn't use the strings as a metallic example. I will try a different outlet, and I just spent some time with some steel wool cleaning some gunk off the power plug, hopefully it will help. Beyond that, any ideas on the pulsing? I thought maybe I have a power tube going microphonic, and the sound comming from the fan blowing right on them, but does that happen to power tubes like that? I've only ever heard of it happening to preamp tubes and then its only high-pitched. This is more akin to the low pulsating fan noises often seen in movies -- woop-woop-woop --
  4. And as a follow up - changing outlets seemed to have no impact, but the one that I tried may have been on the same circuit. I'll try another one later. The pulsing is still going on the sameas before and seems to get worse the longer it has been on. I'm led more to think it may be the tubes because they all look to be the original Mesas, or at least they are all Mesa-branded. The channel and master volumes have no effect on it. And as far as the ground buzz, I discovered that it is present on both the jazz and the P-basses, but it is inaudible with the master volume set below the 5-6 mark, but above that it is quite loud and only eliminated by touching something metallic on the bass or the cable. Just thought I'd let ya'll know.
  5. Tualatin


    Feb 7, 2005
    A couple weeks ago, a capacitor came unsolderd from the circut board, which bypassed any guitar signal, and created a square-wave that was sent through my whole rig. It sucked. Now this DOES NOT sound like your problem, but You can either retube it and hope for the best, or take it to a good tech, and get it back perfect.
  6. If it aint under warranty-take it to a good tech who knows about tube amps-usually it aint much to fix& TF not much $$$$
  7. danomite64


    Nov 16, 2004
    Tampa, Florida
    heh, if it's a 400, I think it's safe to say it's no longer under warranty.
  8. Yeah, when you see things like "Mesa Boogie - 1985" printed on the circuit board, you kinda figure that.

    On the plus side, Portland is home to Inner Sound Audio Repair Service, which seems to be a highly regarded and is factory-licensed by pretty much everyone. Guess the 400 is going for a little trip...
  9. danomite64


    Nov 16, 2004
    Tampa, Florida
    My '86 400 gives me the buzz sometimes; the worst was when I did an outdoor gig off of a generator.....remember, flat for this amp is bass & treble on "0" and mid on "10", so anything above "0" is going to accentuate the buzz, and tweeters are definitely going to make it heard as well.
  10. I thought flat on the 400 was Bass 2 - Mids 10 - Treble 2. That seems to be what I'm hearing round the old TB watercooler.

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