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Mesa Boogie 400+

Discussion in 'Amps and Cabs [BG]' started by Classickbass, Oct 23, 2010.

  1. Classickbass


    Aug 15, 2010
    Kingston, TN
    Good morning all. I will get right into it, I have a Mesa Boogie 400+ that I am going to have the Psycho mod done to but I have a few questions:

    First, does anyone have the technical schematic to this head?

    Secondly, will a small amount of cathode resistance aid in overall balance and control?

    Lastly, am I correct in thinking that it is a TWO bias pot adjuster?

    Thanks to all who can offer some help and/or insight.:help:

  2. rnilson


    Mar 8, 2008
    Kansas City, MO
    call mesa they should give you the schematic. it will have all your answers.
  3. IIRC the 400 does not have bias pots though it is a popular modification. I'm not sure what you mean by cathode resistance.
  4. anderbass


    Dec 20, 2005
    Phoenix. Az.
    In case any one's wondering, info regarding the "Psycho mod" the OP's referring to can be found in this thread: http://www.talkbass.com/forum/showthread.php?t=387202&highlight=anderbass

    1 - I dont know of any 400+ schematics currently posted on-line but I'll second rnilson's suggestion of calling Mesa for a set: http://www.mesaboogie.com/US/info.html

    2 - Yeah I'm not sure what you meant by Cathode resistance either but on a semi-related note here's a thread about my 400+ Cathode-lift mod you might enjoy: http://www.talkbass.com/forum/showthread.php?t=238662&highlight=

    3 - My amp tech installed a dual bias pot (pitched below) in my 400+, but said he typically goes with the single pot mod in D-180, M180, M190 and 400 Mesa models.

  5. Classickbass


    Aug 15, 2010
    Kingston, TN
    Thanks anderbass. I actually copied and pasted that photo to the tech. He is the one who mentioned the cathode resistance, he may have said cathode lift. It was a long conversation with a lot of technical jargon. I had the contact information for Mesa from their website, whom I will call monday, I was just hoping that someone here had a pdf of it.

    My tech and I are going to do before and after recordings on this project so I will post sound clips as soon as they are available.

    Thanks again to all!!!!!!!!!!
  6. billhilly66


    Aug 25, 2007
    Plano, TX
    I've got the schematics. Send me a pm with your email and I'll get them to you.
  7. Classickbass


    Aug 15, 2010
    Kingston, TN
    PM sent. Thank you!!!
  8. The problem with lifting the cathodes is that you play hell with the amp/cabinet matching. IMO it's not something that should be done in a bass amp.
  9. dave2


    Feb 10, 2007
    I would like to join that Google group too. I would like more info on the 400+ head. For example I wonder why my Channel 1 is not as loud or as fat sounding as Channel 2 on my 400+? Channel 2 sounds absolutely wonderful! Channel 1 not nearly as good.

    I will join if I can.


  10. Alex1984


    Jan 16, 2010
    It's clearly stated in the manual it's 2 different preamps: Ch. 1 goes for a more clean, low-gain, clear sound that can be tweaked for stuff like funk, Ch. 2 goes for a high-gain grindy sound, so the difference is expected, nothing wrong at all.
  11. yeah, hence the old y cord trick. Run your bass into both channels and you can get a more solid foundation under the grindy.
  12. dave2


    Feb 10, 2007
    OK thanks maybe I should read the manual in this case. If there are 2 preamps and my preamp tubes have not been changed lately perhaps the tube or tubes for Channel 1 is going wrong. It doesn't sound that clear or clean. Channel 1 used to sound better.

    Does anyone know which preamp tubes go with which preamp? Its in the manual I guess.

    Thanks all.

  13. Actually its V1 for both. Thats the 12ax7 that is closest to the input jacks. EAch 12ax7 has two sides (dual triode!) that is it has two valves. One side is for #1, the other side for #2. Yes it is possible to wear out one side of a tube. I have an old RCA tube that is great on one side (channel #1 on a Bass 400) and is all piddly and lame on the #2 channel side. Skip teh Mesa preamp tubes. Pretty much anything else sounds better. Esp JJ tesla, EHX and NOS tubes like RCA, Mullard, Valco etc.
  14. dave2


    Feb 10, 2007
    Thanks Calaverasgrande. I'll start looking around at changing the preamp tubes. Or do I just need to change V1?

    What would be a reasonable price to pay for a JJ tesla 12ax7? Or a set of 4?

    Also I'll give the google group site a look at now - thanks again for that.

  15. preamp tubes are usually $10 or so. They can be had for as little as $7 and expect to pay as much as $20 though.
    You dont have to buy PREAMP tubes in sets. EAch one has a different job so its not as crucial to match them.
    For instance you may have one tube as the preamp front end. Another tube for the eq, and a tube that splits the phase of the signal for the power tubes.
    The general wisdom is that the tube that makes the most difference in the amps sound is the first tube. The one that gets the bass from guitar level to line level(approx). This is a lot of gain so lame tubes will show there colors more here than they would in a position that is low gain.
  16. dave2


    Feb 10, 2007
    Can you tell me the main funtion of each of the 400+ preamp tubes? I take it that the first tube works with the incoming bass signal and prepares the signal for use by the amplifier.

    Would the tubes be disussed in the manual? I will have a look later on.

    Thanks for all the info.

  17. This is absolutely normal for a 400+. Changing tubes will not have much of an effect, as this is how the amp is designed. To get the same perceived volume, one must turn the CH1 volume up significantly higher than CH2. You can think of them as being "active" (CH1) and "passive" (CH2) inputs if it makes it simpler than "Low" and "high" gain. However, they do sound a bit different even ignoring the volume difference. I always preferred CH2, only used CH1 when blended with 2 via an external splitter. I never found Channel 2 to be high gain in the sense of being overdriven, like guitarists would use the term "high gain;" higher than 1, louder, but I didn't find it to be a cornucopia of distorted sounds. IMO the 400+ is the wrong Boogie bass head if that's your primary interest.

    Download the .pdf of the manual for yourself at Mesa's web site. That will probably be clearer than trying to go by various people's recollections of something they once read in that manual. I include myself in this -- I read it when I had a 400+, but I'm not going to do it now.
  18. Classickbass


    Aug 15, 2010
    Kingston, TN
    Can anyone tell me what is the difference between a JJ 6L6 and a Winged C 6L6 tone wise? I know the Winged C's cost more and I guess that is what I am getting at, is the tone difference worth the extra cost?
  19. No experience with the Winged C. Have had a brand new "matched set" of JJ power tubes in my 400+ "especially selected" by Eurotubes for the 400+, professionally installed during a basic service including bias check. Tech confirmed that the tubes were a match for the stock bias setting. No noticeable hiss, hum, or other such bad behavior. However, the amp sounded a little thin; I actually preferred the sound it had with the old Sovteks I had previously had in it, so I put those back in. That's just my opinion, I know some people love the JJs.

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