Changing out your preamp tubes on Marshall 1987 from 12AX7/ECC83 to lower gain tubes...

Discussion in 'The Workbench' started by Brayshall1987, Feb 22, 2021.

  1. Brayshall1987

    Brayshall1987 New Member

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    So, I own a Marshall 1987X. I originally purchased it from eBay for only $750 but you get what you pay for… The output transformer was shot.
    I sent it off to David Bray who modified it… Incredible amplifier! David is truly a guru! But, sometimes? The amp can have too much gain for me!
    I’m not a high gain player.
    So I did a little bit of research and I went with a trio of 5751 preamp tubes.
    Tung-Sol. From the get-go the amp sounds much much better to me!
    I just wanna make sure and I’m doubting that it could cause any issues but…
    By changing out from the typical preamp tubes on a Marshall… Will tubes like the 5751s be all right in the amplifier? It won’t cause any damage?
    Thanks see anyone and everyone who responds… And, please excuse me if there is another thread on this issue.

    Also… Can anyone please explain to me how I can create a signature? Thanks again…
     
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  2. fitz288

    fitz288 Apprentice Fermi's Paradox Analyst Silver Supporting Member

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    Nope, changing preamp tube values from stock won't hurt anything.
    I did a similar thing to my DSL20, put in an ECC802 JJ in V1 to tame the gain, but I'm going to switch it again after getting different speakers.
    Also swapped out all three preamp tubes in my Origin20 to up the gain, and I have an 832 coming to try for a little more clean on my Haze clean channel.

    Click in your user name in the upper right, then click signature from the drop down.
     
    Last edited: Feb 22, 2021
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  3. MonstersOfTheMidway

    MonstersOfTheMidway Well-Known Member

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    @OP:

    At times, some people have chosen to substitute certain preamp tube types for another preamp tube type so long an appropriate substitute preamp tube type is selected.

    In your case, substituting a 5751 for a 12AX7 is usually/generally considered acceptable as far as electrical specs/tolerances go. Like you, most people make this kind of substitution to lower the gain factor and make an amp more "manageable." Change in other aspects of tone characteristics could be a benefit as well.

    I've substituted other preamp tube types in my amp. My amp was designed for 12AX7/ECC-83 preamp tube types, but I've also used 5751 and 7025 types-all without suffering any malfunction/damage to the amp. I've also substitute different power tube types, but that also requires a bit of research before making the substitution.

    As long as you are making an acceptable substitution preamp tube type, your amp should be OK.
     
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  4. South Park

    South Park Well-Known Member

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    Greetings some of these tubes are built to military specks and break up a little bit different . My SLO 50 clone has tubes made for a air raid siren
     
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  5. pedecamp

    pedecamp Well-Known Member

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    I had a high gain Peavey and put 5751's in it, it barely made a difference, get something else.

    [​IMG]
     
  6. tallcoolone

    tallcoolone Well-Known Member

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    Had a couple Bray’s—that’s just not what they were meant to do.
     
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  7. Chris-in-LA

    Chris-in-LA Well-Known Member

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    You might try a 12ay7 in the V1 position, half the gain of a 12ax7.
     
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  8. Brayshall1987

    Brayshall1987 New Member

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    Hey, thank you! I just wanted to make sure because I love that amplifier! Honestly? It’s a totally different beast now! I’m really liking it but I might go for even less gain preamp tubes.
     
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  9. Brayshall1987

    Brayshall1987 New Member

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  10. Brayshall1987

    Brayshall1987 New Member

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    Yeah I know… The amp is a killer ramp! It’s a distortion generating beast but the new tubes? They actually make a cool difference! I’m liking it! Thanks again for your input…
     
  11. Brayshall1987

    Brayshall1987 New Member

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    Hey everyone that commented on this post. Thank you so much! I really appreciate it. I love the chart too!This is easier than going post my post and thinking you so here I am… Thinking each and everyone of you. Actually I’m having fun with us! I should’ve gotten a mixture that I wanted to just go straight trio. Right now? I’m loving it! The amp? I love that critter! She’s a mighty fine beast! Thanks again to everybody.
     
  12. XTRXTR

    XTRXTR Active Member

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    The 5751 runs 50ma more current per tube for the heater x3=150 ma more in total. The PT heater winding could potentially get warmer or even hot depending on the 6.3 VAC winding rating. The 5751 is a high mu tube at 70 vs 100 mu for 12AX7. You should try a 12AT7 at 60 mu but with the same current on the heater as a 12AX7. It will be slightly less gain than the 5751 but you might have more control. If its not enough gain you can jumper your inputs and dial in something that might be just right. Anyone have the 1987x PT D2479 heater current rating?
     
    Last edited: Feb 23, 2021
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  13. JP2036

    JP2036 Well-Known Member

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    Have you considered just "turning down the gain" on the amp ?
     
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  14. tallcoolone

    tallcoolone Well-Known Member

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    Not really an option on a Bray modded Marshall. Volume = gain
     
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  15. JP2036

    JP2036 Well-Known Member

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    If it has a MV its an option which id bet it does.
    Nobody mods a non master volume amp to be non master volume.
    And he's not just "reducing gain" he's completely changing the mission statement this amp was augmented for.
    Try buying a Peavey instead.
     
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  16. tallcoolone

    tallcoolone Well-Known Member

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    I take it you have never played through a Bray? I’ve owned two Bray modded 1987x amps and I can assure you that the headroom after the mod is almost nil, regardless of whether or not he installed a MV.

    I 100% agree that a preamp tube swap is a waste of time—not what that amp is supposed to do.

    Not sure about the Peavey comment? You throwin shade lol?
     
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  17. Brayshall1987

    Brayshall1987 New Member

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    Yeah... I've played through a Bray... I mentioned in the post that I own a Bray Modded 1987x. It's the amp I am trying to tame. The 5751 tubes worked wonders! The amp is tighter now..
     
  18. Brayshall1987

    Brayshall1987 New Member

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    Yes it has a master volume… At this point in time? I wish you didn’t. My Marshall SP20H? It’s screening loud and it doesn’t have a master volume on it. I run it with a BOSS WAZA TAE. It sounds incredible through it! Thank you so much for your suggestions.
     
  19. tallcoolone

    tallcoolone Well-Known Member

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    Well...I wasn’t talking to you, so... If swapping out 12ax7s for 5751 tubes worked wonders in your Bray 1987x then one or more of the 12ax7s in there were completely blown.
     
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