Do-it- yourself preamp tube replacement

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by realbluesoldkid, Jun 1, 2015.

  1. realbluesoldkid

    realbluesoldkid Member

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    Hallo ,in order to improve the tone I'm gonna change the preamp tube (12BH7) of my Blackstar HT-5 ; could I do this work without any risk ? I have any notion about electronics. Thank you
     
  2. guller

    guller Member

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    plug and play

    Edit: Did this say 12AX7 earlier or did I read that wrong?
     
    Last edited: Jun 1, 2015
  3. abnerfm

    abnerfm Member

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    Be careful not to shrink the tube's pins while trying to fit them in their slots. Otherwise, no problem, just replace it
     
  4. Silent Sound

    Silent Sound Member

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    That 12BH7 isn't a preamp tube. It's a power tube. It's also a fixed bias amp, so if you replace that tube, you'll need to replace it with another 12BH7 and you'll need to have the amp biased.
     
  5. zenas

    zenas Member

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    Maybe I'm missing something but I believe the 12bh7 is a twin triode preamp tube.
    Or is it used as a low wattage power tube in this amp?
     
  6. Silent Sound

    Silent Sound Member

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    In the HT5, it's used as a power tube in a push pull configuration. There's only two tubes in that amp, the 12BH7, and a 12AX7 which is the preamp tube. It appears to have some solid state stages in the preamp and is run by a solid state phase inverter as well.
     
  7. EADGBE

    EADGBE Member

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    Turn the amp off and unplug it from the wall before you change tubes.
     
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  8. DaveKS

    DaveKS Member

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    The 12ax7 is the one you need to focus on. It's where you'll get most improvement of tone, try a old Sylvania 5751 if you want to tame gain a bit.

    The 12bh7 is power tube, there are bias and bias balance adjustments for this tube. Best to start with a semi-balanced, pro tested tube and not some iffy used tube that's anybodies guess how balanced the triodes are.
     
  9. jay42

    jay42 Member

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    :hide2 going to get ugly...munch munch...driver tube really, as used in Ampeg designs. There are a number of low wattage amplifiers which use 12xx7 tubes for output. A few can drive some current, and the 12bh7 is one.
     
  10. realbluesoldkid

    realbluesoldkid Member

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    I apologize for the dummy question but I never opened an amp. Where's located the preamp tube in this combo ? I wouldn't go wrong
     
  11. DaveKS

    DaveKS Member

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    There's only 2. One that's taller is the 12bh7 power tube, should be labeled. Shorter one is preamp, should be labeled 12ax7 or ecc83.



     
  12. realbluesoldkid

    realbluesoldkid Member

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  13. J M Fahey

    J M Fahey Member

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    I would leave the amp as is and not mess with it, any "improvement" will be subtle if any, doubly so in an amp which is so SS as this one.

    What are you seeking and what do you think the "improvement" will be?
     
  14. realbluesoldkid

    realbluesoldkid Member

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    A lot of people (in this forum also) agree with the non-fantastic character of stock Blackstar HT-5 cone and the preamp tube ; what I need is an improvement of tone in terms of clean and more fat and hot sounds too . Maybe that's the right way and I wanna try , I can't follow an infinite series of effects that 'til now didn't resolve my 'problem'...
     
  15. J M Fahey

    J M Fahey Member

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    Cool.
    What I'm afraid of (and that's an understatement) is that the "harsh and cold" sound you imply is more in the project itself than "in the parts" , if you get what I mean.

    A tube change will be most felt in a simple amp, say a Champ, a Marshall 18W, an old Fender or any other simple, all tube, little EQ design, and will be lost, swamped, in a more "processed" one, even more one with significative SS elements.

    Anyway, it's your amp, you do with it what you wish.

    Good luck :)

    PS: report results, an MP3 or YT video would be even better.

    PS2: a speaker change will change tone much more than any tube swap, try plugging it in a 2x12" or 4x12" or at least into a *real good" single 12"
     
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  16. realbluesoldkid

    realbluesoldkid Member

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    Ok
     

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