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JJ 5751 and Valve 1

Discussion in 'The Workbench' started by Uncle Fester, Feb 9, 2021.

  1. Uncle Fester

    Uncle Fester New Member

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    Hi All

    I buy a little homemade head and dude keeps his Tubes
    and sends a list of Favorite tubes.
    Yeah these Muthers are pricey with a NOS Tun Sol Rectifier being the Cheapest.

    Anyway I finalle settle on the GEC 6V6's at power, and the 3 Pre amp tubes ar an interesting sort of bishes
    Valve 3 Black Plate RCA NOS
    Valve 2 12AX7
    Valve 1 12Ax7

    Notes
    V1 If less Gain is desired use 12AY7.
    V1 If more Gain is desired us 5751
    (The 5751 is a Substitute for the 12AX7, but has Higher Gain)

    With a Trio of Tung Sol 12ax7's and the GECs good frikin grief
    waaay too much Gain.
    I sub in the 12AY7 and the gain has dramatically dropped off.
    This points the finger at the Power Tubes.
    Out Go the Gecs and in go EH 6V6.

    Gain is still a a bit anemic with the 12AX7 so I order a JJ off Amazon $19.95 (NOS RCAs are $40 a pop on these)
    With the 5751 in place I get a beautiful sweeping Gain,
    and I have ordered a Tung Sol 5751 for $32 to see where this goes.
    If I like then I will chase down a NOS 5751 which NOS are selling between $42 & $50 per Tube.

    Have you tried the 5751 in your 800 or earlier Marshall?
    If so I would love to hear your thoughts on this.
     
  2. AlvisX

    AlvisX Garage Guerrilla Gold Supporting Member

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    Ive used 5751's in V1 since about 1992 when I bought all the GE 5751's from an electronics store that was ridding themselves of tubes .They're great in the cascade amps ....to get them into more "classic" Marshall territory
     
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  3. IOSEPHVS

    IOSEPHVS Well-Known Member

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    5751 does not have more gain than 12AX7.

    5751 has mu of 70. 12AX7 has mu of 100. There is also the CV4004 with mu of 90.
     
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  4. mickeydg5

    mickeydg5 Well-Known Member

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    At the specific voltage ranges and typical operation used in most musical amplifier circuits the 5751 and 12AX7 exhibit practically the same characteristics.

    It is more or less a question of tube quality and personal opinion at that point.
     
  5. proxy

    proxy Well-Known Member

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    Do you actually need more gain?
    What is it you are trying to do with 5751?
    I find JCM800 little bit too thin sounding.
    If I was in your shoes I would stick 12AU7 in V1 then hear what is that like
    Keep in mind your amp will take pedals much better with 12AU7.
    Keep us posted
    Keep rocking ! ! !
     
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  6. BUTTONTY

    BUTTONTY New Member

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    I tried rolling a 5751 around in a peavey 6505 amp and thought it made it very dead sounding! Just tried this yesterday actually, got done tubes to try from the tube store. Lots of ppl saying online they replaced a v1 and PI tube with this and it opened the amp up. Anyone have actual luck with them? I was buying into it after reading that, but my ears/ amp had much different results. The gain didn't become more usable as puerile have written before. There was less gain, but still got muddy at around the same adjustment level on the pot.. and harsh sounding for my amp anyway...
     
  7. MarshallDog

    MarshallDog Well-Known Member Gold Supporting Member

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    Said a different way a 5751 has 30% less gain the. A 12AX7 does. They are rather interchangeable with the 12AX7s and putting one in V1 will have a noticeable lower gain.

    12AY7s have 55% less gain than a 12AX7 but the tone will be affected as they are a bit different than the 12AX7s!
     
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  8. mickeydg5

    mickeydg5 Well-Known Member

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    If you did not test the tube then you have no idea of what it is capable of in a circuit.
    Not all 5751 are equal.
    A low quality 5751 will not sound good.

    Please understand a crappy 12AX7 can have less output than a good 12AU7.

    Nothing lives at its potential forever.
     
  9. XTRXTR

    XTRXTR Active Member

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    5751 uses 50 mA more current on the heater with a mu 70 than a 12AX7 with a mu 100. 12AT7 has a mu 60 with a 250V but can get to 70 mu with lower B+ of 200V. Plate, Grid, and Cathode resistance values and surrounding circuit design also makes a difference.
     

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