Modify the BBE BMAX input stage...

Discussion in 'Amps and Cabs [BG]' started by tarnationsauce, Dec 8, 2005.

  1. tarnationsauce


    Dec 7, 2005
    Hi, does anybody know how to modify the input stage of the BBE BMAX so that it is not so sensitive? I know I can send it in and they'll mod it for free. But I don't have that kind of time, and I'm not at all scared to add a resistor, or wahtever needs to be done.
    I've got the gain knob up maybe a 16th of a turn and it clips, annoying. Otherwise a superb sounding preamp.
  2. doctec


    Mar 22, 2005
    Beaverton, Oregon
    I've got a BMAX-T and I have to hit it pretty hard to get it to distort. I'm driving mine with a Yamaha TRB5-P that has active electronics and (I think) a pretty hot output. I usually set the volume between 3 and 5 (assuming the knob goes to 10) and bring up the master to get the level I want. Curious what you're plugging in to yours...

    I'd be interested in getting schematics to help me implement a mod for the bass control on mine. I get all the bass I can use with the control set at about 3.

    Later model Fender Bassman amps had an extra cap in the tone circuit that added another pole (EE speak) in the response -- which rolled out some of the extreme low end and made the bass control more linear. I think my BMAX-T might benefit from that change...

    Having said all that, I'm generally quite happy with the BBE. I've got a SWR IOD and a Presonus Eureka that I use occasionally, but I'm playing through the BMAX at all my gigs now.
  3. sorry to sound possibly condescending...
    have you set the preamp pad? i.e. are you using a "low input" for an active bass, or hit the "active" setting for the input?
    maybe the signal you're sending is too hot...

    also, what is you EQ like? really cranked?

    just food for thought..
  4. C-5KO


    Mar 9, 2005
    Toronto, Canada
    Here's a quote, straight from Memo at BBE:

    For the BMAXT you have 4 component changes

    1) R7 gets changed from a 46.4k resistor to a 68.1k
    2) R10 changes from a 46.k to 21.5k resistor
    3) C8 is removed completely
    4) C9 is removed completely

    For the BMAX

    1) R7 changes from a 46.4k to a 68.1k resistor
    2) R10 changes from a 46.4k to a 21.5k resistor
    3) R12 changes from a 6.81k to a 14.7k resistor
    4) R14 changes from a 5.62k to a 10.0k resistor

    Of course, I would recommend talking to somebody at BBE first - and I take no responsibility for whatever may happen to your pre :D .
  5. OBBM


    Jan 26, 2005
    Surrey, UK
    I also changed the tube to a Philips JAN 5751. It has less gain than the stock tube.
  6. toobster

    toobster Guest

    Jan 22, 2006
    Doctec: Read the manual. Bass and Treble set at 2 are supposed to be flattish. Since the passive mid is cut only, full up would be flat for that one.

    I have a BMAX and a BMAXT. The BMAX needed the mod and got it. On the BMAX the input gain has to be kept so low to avoid that farty distortion that there were noise issues from cranking the master.
    The BMAXT not so much. I like way the T sounds from the factory. You can alter the gain structure by changing the tube. The high gain actually makes the T MORE flexible. I too rolled in a Philips 5751.
    Nice sounding tube. The 12ax7 in my ultra-rare 2nd generation Peavey VTA400 got replaced by one too. My old Mesa D-180 got them too. 5751's seem very bass friendly.
  7. Chef

    Chef In Memoriam

    May 23, 2004
    Columbia MO
    Staff Reviewer; Bass Gear Magazine
    I had the input done at bee on my bmax. It took about a week with priority shipping there. They turned it around fast, it now I can set the volumes where ever I like, instead of at "nothing."
    There is no input pad to set, there are inputs for both passive and active on the front...
  8. Joe Beets

    Joe Beets Guest

    Nov 21, 2004
    I stuck in a 12AU7 tube and with everything set flat I could turn the gain almost half way up. It also helps if you get a QSC PLX power amp as they take a high input signal. Or just dump it and switch over to Aguilar like I did.
  9. andrewd


    Sep 5, 2003
    when i had my BMAX (solid state) i seem to remember having the gain at halfway up or maybe even full. no clipping in sight.
  10. spego


    Dec 1, 2008
    Hazleton PA
    I'm sorry for opening such an old thread up.....but have a question.

    The resistors listed above.....are they 1/4 watt or 1/2 watt?

  11. spego


    Dec 1, 2008
    Hazleton PA
    I got the info I needed from Memo at BBE. He also told me the above locations were incorrect......posting the correct locations below in case anyone ever searches this and needs to know what the mods are for the SS BMAX.

    per Memo's email:
    The designator numbers are incorrect, however the value changes are right on the money. So let me give you the correct designator number, so you can perform the mod correctly.

    R3 ([DEL]R7 regulates the amount of voltage feeding the transistor Q1[/DEL]) - changes from a 46.4k to 68.1k

    R8 ([DEL]R10 regulates the amount of high end for the bright switch[/DEL]) - changes from a 46.4k to 21.5k

    R12- changes from a 6.81k to 14.7K

    R14- changes from a 5.62k to 10.0k
  12. Arjank


    Oct 9, 2007
    Above Amsterdam
    I do have the Bmax-T and do not have such severe issues with the sensitivity of the input. Though I must admit that with my basses I do not set it past "10" 'o clock.
    Nice to have these modification values though ;)
  13. spego


    Dec 1, 2008
    Hazleton PA
    Yeah the BMAX-T are probably different.....this one I have is the BMAX (the SS version). This one you couldn't get the gain past like one or the master was super sensative. Now its MUCH better.
  14. vlad335

    vlad335 Supporting Member

    Apr 14, 2003
    Latrobe, PA
    Sorry to bump an old thread but did anyone perform this mod? I just acquired a BmaxT and love it but it does have a great deal of gain. It came with a 12AT7 installed and I immediately swapped it out for a 12AX7... Whoa. Too much! Now I know why the 12AT7 was in there. Swapped it back in.

    Never heard of this issue until experiencing it first hand. I did contact BBE last week and they never replied. Looks like I am on my own... I can solder, just want to verify the component values to get this right. I can do it but just may take it to my tech for piece of mind.

    Are we talking 1/2 or 1 watt resistors?

    How about swapping in a 12AU7? The way I understand it, the gain of a 12AX7 is 100 and on this scale a 12AT7 would be 60. The 12AU7 would be 20 gain compared to a 12AX7. Anyone try rolling a 12AU7 in this preamp? Would this solve the problem or make things wonky?

    Much appreciate any help provided.
  15. spego


    Dec 1, 2008
    Hazleton PA
    I've owned both the SS and tube versions. I put a 12au7 in my T........worked great.....I had the same issue as you.
  16. vlad335

    vlad335 Supporting Member

    Apr 14, 2003
    Latrobe, PA
    Cool. Thanks for the quick reply. I will purchase one and give it a try.
  17. Troph


    Apr 14, 2011
    Kirkland, WA
    I have a BMAX (solid-state) purchased last year, and I have not noticed a super-hot gain stage, and certainly no "farting" of the output. So has BBE "fixed" the problem over the last few years, or were people here just running high-output 18V onboard preamps or something (which I don't happen to have)?
  18. spego


    Dec 1, 2008
    Hazleton PA
    I was told it was fixed on newer versions.
  19. spankdaplank


    Jan 19, 2003
    By the way, BBE customer service is A++. From my experiences.
    Just sayin'.....
  20. vlad335

    vlad335 Supporting Member

    Apr 14, 2003
    Latrobe, PA
    Heard back from BBE. There was a mix up with email.

    As above the mod for the BMAX-T is:

    1. R7 changes from a 46.4k to a 68.1k resistor
    2. R10 changes from a 46.4k to a 21.5k resistor

    C8 and C9 are removed completely.

    Resistors are 1/4 Watt, 1%.
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