5751 preamp tubes vs. 12ax7 name tubes

Discussion in 'The Workbench' started by brett bradford, May 31, 2010.

  1. brett bradford

    brett bradford New Member

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    Can some knowlegable tech out there tell me if I have accurate info.
    I have read that the 5751 tube is basically a 12ax7 built for military and
    industrial application.I have also heard that the parameters are very
    closely matched on this tube and that the mutual conductance is a bit
    lower than on what would be perceived as a average 12ax7 tube.I am
    trying to knock some of the brilliance off my '87' jcm800 2204 and I have
    the notion that this might be a softer,high quality tube to accomplish this.
    Does this sound like a good idea and would there be any reason I might
    want to avoid this tube in a marshall,is damage to the circuit possible
    wit hthis tube.If someone thinks I am completely off with this,how about
    some ideas to take off some of the high treble from my amp.
    Thanks in advance for any help with my tone search...
    Brett Bradford.

    Marshall jcm800 2204
    matchless 2x10 lightening reverb
    bogner 2x12 cab,greenback g12m,g12h
    custom shop strat
    gibson howard roberts fusion
    martin om21
     
  2. MajorNut1967

    MajorNut1967 New Member

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    The 5751 is kind of an odd ball tube, most of them were JAN tubes. I personally don't like the way they sound in a Marshall, real bland. Now it won't damage your Marshall, but it does have a lower gain factor of about 70 vs the 12AX7 at 100. You can try it in your 2204 see if you like it?

    Then the reason your 2204 might be bright at lower levels is there is a Treble Peaking circuit in it, actually two in the amp, but the first one is the issue it can be real bright at low volumes.
     
  3. big dooley

    big dooley Well-Known Member VIP Member

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    JJ ecc83's may be a good choice for an overly bright sounding amp... they tend to sound darker...
    i tried a 5751 in the gain stages of my JVM as well, but eventually went back to 12ax7's... nowaday's i keep it in the PI...
     
  4. PaoloJM

    PaoloJM Well-Known Member

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    The lads have it spot on.


    I've personally found that the JAN 5751 works real well in V1 of draker sounding high gain amps. It brings a bit more definition (probably from the reduced gain) and reduces noise a little (again probably from the reduced gain) without siginificantly changing the amps tone.

    Wouldn't use it to tame the highs in a Marshall though.

    Two of my favourite mods for an overly brilliant 2203 style amp are:

    Insert a resistor of value 100k ish (to taste) in series with the bright cap to tame it a bit. This maintains a more balanced high end responce through the sweep of the gain control.

    Insert a resistor of value 100k ish (to taste) from the master volume pot wiper to the PI. This lowers the value of the high pass filter that the MV creates to around 72Hz even when the MV is at 1. This fills out the bottom end.

    This combination of taming highs and filling out the bottom will keep your amp from sounding too brilliant at bedroom volume while retaining it's true tone at higher volumes.

    Hope that helps.
     
  5. MajorNut1967

    MajorNut1967 New Member

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    Paolo has great suggestions, I take it a bit further. I actually remove the the Treble peaking circuit & replace the first 470k with a 68k. Take a gander at the photo.

    [​IMG]
     
  6. MartyStrat54

    MartyStrat54 Well-Known Member

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    I've never been a big fan of the 12AT7/5751. Neither tube is what I would call a singing tube of tone. To me, the 12AT7 sounds very bland and flat. I use high gain tubes in my PI so that leaves that position out. The 5751 is what I call a transparent tube. A big favorite with the HiFi crowd for use as V1. Yes, it is low noise, but it is of lower gain. When buying a military version of the 5751, it must be noted that the earlier tubes sound better than the later one's. Changes in the design made the tube more rugged, but it lost a lot of it's tone. I have heard 5751 that sound downright sad and the customer was wanting to know what was wrong. It's amazing how some people will buy NOS tubes without educating themselves or asking questions. It's a good way to get burned.

    I would go with the 12AX7's in the Marshall. Although the OE tubes are horrible, they are 12AX7's. There are several decent current production 12AX7's. Tung-Sol and Mullard RI's.
     

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