Tech help !!!....1987 amp clone

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by louis, Aug 10, 2018 at 10:59 PM.


  1. louis

    louis Supporting Member

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    Hi all , need advise please!
    I went and try a Marshall 1987 clone today and while testing the amp , there was some popcorn noise ,...so I swapped V1 in V2 and the noise stopped but when I was touching the glass on V1 , there was a ground hum which I think is not normal so that’s why I’m asking !!

    Tx !
     
  2. gldtp99

    gldtp99 Member

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    All bets are off on a clone.

    Who built it ? Mr. Fixit ?

    Could be assembled improperly.

    Could have poor lead dress.

    Could have a poor grounding scheme.

    Could have poor solder joint(s).

    It might just be an unstable preamp tube.

    Or cobwebs.

    Have you seen any cobs running around the area ?
     
    Steppin' Wolfe and big mike like this.
  3. pdf64

    pdf64 Member

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    It’s best to turn the amp off when swapping tubes.
    Does the amp him when the tube envelope is not being touched?
     
  4. gldtp99

    gldtp99 Member

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    One day a customer (a TGP member) brought me a 1987 clone that was built by a company that had many fanboys here on TGP.

    Many threads about this model of amp built by this company singing the praises of this clone built by this one guy.

    The customer said the amp had never sounded "right".

    It had been sent back to the builder to check out and fix the problem----- the builder checked it out, found no problem, and even played a show with the amp----- amp was sent back to the owner.

    The customer played the amp in my shop and I agreed----- it didn't sound right at all.

    It sounded "Weird" is the best way I can describe it----- I'd never run across this sound from a real 1987 or a clone before.

    I checked all tubes and found no problem.

    I looked inside and saw that the overall build was done quite well------ decent quality parts----- good soldering/wiring job---- again, no obvious problem.

    I drained the caps and began measuring components.

    I found that the Treble pot measured @25k (should be @250k)----- I replaced the pot with the proper value and tested the amp.

    It still sounded weird.

    I measured the other pots and found that the Mid pot was 250k (should be 25k)---- I put in the proper value Mid pot and we tested the amp.

    Now the amp sounded like a 1987 clone should sound----- pretty good, actually.

    The original builder had mixed up the values of the Treble and Mid pots (250k vs 25k) when the amp was built.

    And when the amp was shipped back to him for repair ----- he had failed to find and fix the build mistake.

    He had even played a show with the amp and declared it "ok".

    Good assembler----- not a great troubleshooter.

    These are the things one can run into when dealing with clones.

    Everything needs to be checked ----- it can't be assumed that the amp ever operated properly.
     
    Steppin' Wolfe and pdf64 like this.
  5. louis

    louis Supporting Member

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    No it did not !
    I know the amp and it sounded pretty good a few years ago but it’s the first time I hear some hum when touching the glass!

    Here’s the amp !
    [​IMG]

    [​IMG]
     
  6. louis

    louis Supporting Member

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    Good story GT !!!.......thank’s for sharing!!
     
  7. big mike

    big mike Fixed Bias Moderator Staff Member

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    That’s not an 87 clone.
    It’s got a shared Cathode
    And bass spec tone stack
     
  8. louis

    louis Supporting Member

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    Thank’s Mike !!!
     
  9. pdf64

    pdf64 Member

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    I think it’s based on a ‘black flag’ JTM50 era 1987 but it’s clearly not a clone of anything (eg 47nF coupling caps to power tubes, fx loop, master vol, some sort of NFB pot).

    I don’t think the hum indicates a problem; under the same conditions most amps might do the same. A tube can would probably have stopped it.
     
    Last edited: Aug 11, 2018 at 2:02 PM
  10. big mike

    big mike Fixed Bias Moderator Staff Member

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    Yeah someone went 1/2 way between lead and bass on the pi caps
     
  11. MKB

    MKB Silver Supporting Member

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    IME it's normal to get hum on V1 if you put your hand near it; it's got a lot of gain after it and noise can be picked up by an unshielded tube (that's why it has a shield base, and original Marshall V1's were most always shielded).
     
  12. louis

    louis Supporting Member

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    Thank’s !!!......very nice to know!!;)
     

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