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Marshall SC20H: not so good sound...please help!

Discussion in 'Marshall Amps' started by panoma, May 14, 2021.

  1. panoma

    panoma Member

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    Hi,
    hope some help from the experts:
    New SC20H arrived two weeks ago; my signal chain is very simple, Gibson LP Standard 60s 2019 bridge HB straight into the high sensitivity input of the SC20H paired with its SC212 cabinet, Preamp at noon, Master at 3; I am looking for 'solid' rock sound ala AC/DC but the sound has always a very strong high frequencies boost, too strong and I cannot reduce this with EQ knobs: Presence, Treble all the way down to zero (!) but no significant result; moreover some scratchy noise (with fading behaviour) is randomly heard especially on bi-chords on lower strings...

    As a test I have plugged the guitar directly to the RETURN input with effect-loop ON and the post-master sound of the power section is crystal clear, well balanced with no issues at all.

    I am wondering if I have some problem with the pre-amp valves........

    I am really disappointed :shrug: also because I have a CODE100H and I can get an almost decent JCM800 sound.....so there must be some problem..........

    thank you for any suggestion (before calling thomann customer service........)
     
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  2. pedecamp

    pedecamp Well-Known Member

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    Take a known good 12AX7 and try it in each slot until you find the bad tube, thats all you gotta do! Congrats on the new amp! :yesway:
     
  3. marshallmellowed

    marshallmellowed Well-Known Member

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    Could be that it's just brighter than you're used to, or you're using very bright speakers (V30's...). Beyond turning the Presence and Treble down, you could try an EQ in the loop, or try some darker sounding preamp tubes (JJ...).
     
  4. panoma

    panoma Member

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    The cabinet I am using is the SC212 with 2x 12" Celestion V-Type but I am not expecting such a super bright tone and also the scratchy sound is annoying
    Unfortunately I have not a good 12AX7 to swap the preamp slots; is there a quick manual way to check if the pre valves are properly set in place?
     
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  5. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    Brand new amp? You could take it back for another if its defective.

    800's are bright, but I don't think it should unbearable with the treble and presence at zero.

    Brand new speakers? They might need break in. The 800s were voiced for the G12-65.
     
  6. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    Just wait for them cool and pull them straight up and out and then push them straight down the same way. The V1 tube probably has a metal sleeve over it. You push and twist the metal sleeve to remove it or to put back on. You could tap lightly on the tube base with a pencil to test if the tube is microphonic. It will be like tapping on live pickup if that is the case. But if the tube is just plain bad that might not show anything. You really need a spare 12AX7 to find out if it's a defective preamp tube.
     
  7. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    Just a note...

    If playing tube amp's, you must have extra tubes hanging around. They are a necessary part of owning a tube amp.

    If something goes bad, you can plop one in, to get you through the show.

    It very well could be a pre-amp tube. I'd call Thomann anyway. If it's a tube, they should be glad to solve it that way, rather than going through all the hassle of return of merchandise.
     
  8. hpharley90

    hpharley90 Member

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    My suggestion is to call Thomann. its brand new call them...
     
  9. ken361

    ken361 Well-Known Member

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    volume at 3 isnt that great try 7
     
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  10. panoma

    panoma Member

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    master at 7 is impossible in my apartment :)

    thanks everybody for all the suggestions, tomorrow I will inspect the valves but I do not want to invalid the warranty: the amp is brand new so I will eventually call Thomann for service and see what they will tell me
    I will keep you posted

    Any other idea is well appreciated

    a big thank you from Italy!
     
  11. tce63

    tce63 Well-Known Member Gold Supporting Member

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    I have a Marshall 2061CX with V30:S and that sounds great with the SC20H, I have master at 8 and the pre amp at 4.

    I use a Weber Mini Mass 50 attuenator at home.

    Fantastic amp, Call Thomann, they have great support :applause:

    And HNAD

    Cheers :cheers:
     
  12. ken361

    ken361 Well-Known Member

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    change the v1 valve and try it then on to 2 and 3rd valve. If you dont have any try moving them around can't hurt anything.
     
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  13. CroTone

    CroTone Active Member

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    I had my JCM 800 Studio series head coming with a defective V1 preamp tube right out of the box.
    In the beginning, I thought that maybe some of the tubes got loose during the transport but it wasn't that. The culprit tube was just not working after I did some testing later on.
    Swapped V1 with another one, the amp was working just perfectly! Enjoy SC20 and don't give up...they are awesome little beasts! And I also use it with V30's...and play with a telecaster...not too trebly at all if you set your settings appropriately.
     
  14. ampeq

    ampeq Well-Known Member

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    Call the place you got it from and see if they will send a tube or 2 to see what is wrong. This will at least get a paper trail going and they will know it’s not right. If it does not work good with a new tube then you can send it back. If you can’t get a good sound out of it at low volume, it might not be a good fit for you.
     
  15. ken361

    ken361 Well-Known Member

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    I have not played low in awhile but it sounded pretty good today with my new Les Paul and the Hendrix curly cable its probably a bad tube lets hope
     
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  16. panoma

    panoma Member

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    I have taken the preamp valves out and reinserted them without swap; nothing changed. I might try the swap but a question: are all three the same valve model 12AX7/ECC83 ? Shall I swap them any combination I like? Why only the first from right has a shield cover? Sorry for the questions but I am pretty newbie in amp internals....and I do not want to void the warranty....

    Another question regarding the FX loop: is the Master volume BEFORE the SEND output ? Guitar plugged directly in the RETURN input does not seems to be modified in volume from Master knob...

    thank you....
     
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  17. marshallmellowed

    marshallmellowed Well-Known Member

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    The 1st tube is the 1st gain stage, so any noise picked up by that tube will be further amplified by the following gain stages, hence the tube is better shielded (metal cover). You can swap any of the 3 preamp tubes with the others. The 3rd tube is the phase inverter tube, the other 2 are preamp tubes. Yes, the master volume is located ahead of the FX loop.
     
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  18. Sapient

    Sapient   Silver Supporting Member Platinum Supporting Member

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    With the amp on. Tap the tubes lightly with a pencil. Do you hear a microphonic sort of sound? Or any odd sound over a different one?

    Not saying this is 100%, but it's sure worth a try.
     
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  19. Mystic38

    Mystic38 Well-Known Member

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    IT may be faulty (see all tube comments above) but..

    AC/DC tones are more associated with Plexi style amps than the JCM800.. which is a pretty bright amp.

    turning the master up may help with fullness, as would an EQ before the amp, in the loop or both, but if your goal is THAT sound, then I think you may be better off with a different amp.
     
  20. FleshOnGear

    FleshOnGear Well-Known Member

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    I have never owned a JCM800, so I’m no expert. But, maybe try plugging into the Low Sensitivity input and cranking the preamp. This will dial out the bright cap on the preamp control, and maybe give you a fuller sound, although with less gain. It’s worth a try.

    Based on what I’ve read so far, if the amp or tubes isn’t faulty, you might be better off with an SV20 with a passive volume box in the effects loop.
     
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