Old Marshall: Bright Cap and Channel Jumping

Discussion in 'Amps and Cabs' started by dtirer, Jan 14, 2011.

  1. drbob1

    drbob1 Silver Supporting Member

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    I've got a stock 71 100w Superlead with RCA EL34s? I jumper but I'll try the y cord and see if it's better. Right now, no problems with excessive treble, no way I'd mod this amp, it's creamy and crunchy and touch sensitive...

    OTOH, my JCM800s sounded terrible and was improved markedly by lifting the bright cap.

    Part of the difference may be that the Superlead has to be played loud to sound great, vs the 800 which can sound decent at less than crushing levels... The higher you turn the bright channel the less the bright cap matters.
     
  2. FourT6and2

    FourT6and2 Supporting Member

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    Pretty much. That, and different years of the 1959 NMV 100 watt circuit came with small differences. Different value bright caps, different negative feedback circuits, different presence cap values and with/without the V2a .68 cap. All of that will affect (partly) how bright the amp is.
     
  3. illini

    illini Member

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    I have owned a few 800's, and always unsolder the bright cap to see how it sounds. On some I have liked it. Others got way to dark. My current 2204 sounds best with it in place. To many variables to assume the amp will always sound better with the bright cap clipped.
     
  4. drewl

    drewl Member

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    It's all Hendrix' fault....he wanted the amps brighter!
    Anyway, if it's an old 4 holer, leave the cap.
    It's the master volume amps with the circuit changed that are usually way too bright and need the cap changed/removed.

    And instead of a Y cord, or jumpering, use an A/B box....instant two channel amp :aok
     
  5. Stu Blue

    Stu Blue Member

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    Sorry you're wrong. Most hear it as a tone change, the "unused normal" control sounds like a variable depth control when you're in the bright channel, and the "unused bright" sounds like a variable bright control for the normal control. Techs may well argue about it (when they look at the Fender bassman circuit rather than the actual Marshall one), the most recent explanation from Dave_C on TGP with help from Germino was that the unused channel alters the drive to V2 (tone section).

    Don't clip the bright cap, it's there on every amp for a good reason, ie: the quieter you are the brighter you need to be to be heard. Instead dime the treble and bass like JIMI did and sort out your guitar balance-setup, minutely wrong and you get loud, harsh top and inaudible (on stage), muddy bottom... (or over dirty top and clear bass).
     
  6. jezzzz2003

    jezzzz2003 Member

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    Well glad you asked! ;)

    Marshall decided to use te same value cap as the famous germanium boosters used at the time to give everyone the sound they wanted.

    For what it's worth I like the stock cap in my reissue and have thought about removing it but I don't want to mess with the great tone I can get leaving it in. Anyone that says it's too bright even with the treble channel off must sound like they're playing with a mattress over their spkr cab.

    All in all it gives more of a harder rock tone than the late 60's plexi's.

     
  7. Stu Blue

    Stu Blue Member

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    If you've got a real Marshall 4 holer, try going in the top normal channel, zero the normal vol (silence) and then wind up the bright channel vol... loud bright sound. I must have done that to over 40 4 hole Marshalls, (and owned 5). The other way round, in the bright channel there is no sound with that channel zeroed, but , especially when you have the amp at stage level, or up close to where it changes character (gets much dirtier) winding the normal vol while you are in the bright channel makes a very noticable low end change.(with a strat, humbucker people mostly only hear fuzz anyway:phones).
     
  8. FourT6and2

    FourT6and2 Supporting Member

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    Mine is a Metro Super Lead not a real Marshall. But from what I can tell, they're the same. But I still hear no change in sound. But whatever, I've sort of lost interest anyway. :):phones
     
  9. MRscratch

    MRscratch Member

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    a/b/y. three channel amp
     
  10. dewman

    dewman Gold Supporting Member

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    I still say clip those bright caps....
     
  11. Steve73

    Steve73 Member

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    Yeah, on my '74 JMP, turning the unused volume knob definitely changes the tone of the used channel. You can hear and feel it.
     
  12. Wheatburst

    Wheatburst Member

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    I just found this thread and after reading it I went over and clipped the bright cap on my 74 Super Lead 100 4 hole, and it made a huge difference.
    The bright channel was unusable till a clipped the bright cap.
    I'm going to check my 79 JMP and clip the bright cap on that one as well.
     
  13. Geetarpicker

    Geetarpicker Member

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    I went the opposite of what you expect with my vintage Marshall. I have a '68 Superbass 100 plexi, and I'm running a 3500pf bright cap! The bigger the bright cap the more the cap both adds highs, and mids. However when you go really big like a 3500pf it's adding highs and mids, some upper bass, and effectively just limits the deep bass. This low end roll off is just enough to keep the low end tight without having to turn the bass knob way down or off, which can be typical of a JTM45 or Superbass 100 which share the same preamp.

    I agree that the bigger the bright cap the more effective highs it seems to add. However if you go really big you simply get more of a low end rolloff, which on some amps may be helpful.
     
  14. Dave_C

    Dave_C Member

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    Yes, it will. See the left side of the JTM45 schematic I redrew below. That's the equivalent input circuit of the Bright channel with nothing plugged into the Normal channel. The Normal channel volume pot is still in the circuit and controls the amount of gain fed into V2. It's NOT a tone change, but strictly a gain change. So, to get the most gain out of the bright channel, you also need to max out the normal channel. I confirmed this with Greg Germino. (The converse is true too. When plugged into the Normal channel, the Bright Volume controls the gain into V2, even with nothing plugged into it.) BTW, in the Super Lead circuit that 270K resister is a higher value, can't remember what it is, so the effect is not as great, but it's still there.

    [​IMG]
     
  15. FourT6and2

    FourT6and2 Supporting Member

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    Cool. On paper it works that way. But I honestly don't HEAR any difference. I've tried it over and over. I hear no change in tone nor gain. No big deal, really.
     
  16. dmczern

    dmczern Member

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    I just bought a 75 50W "4 holer". Here is my ignorant question: Is channel 1 the "bright" channel?? :hide2 Is it still a good idea to jump 1 to 2? :hide2
     
  17. Steve73

    Steve73 Member

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    Yeah, channel 1 is the bright channel and channel 2 is the normal channel. Jumping the channels is a personal preference, some guys like it, some don't. I want my 50 watter darker sounding, so I use the Normal channel input only. I do have the channel 1 volume on about about 3 though because even though I have nothing plugged into Channel 1, it changes how the second channel sounds and responds.
     
  18. Dave_C

    Dave_C Member

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    I'll try to look up the schematic for your amp. The later super lead circuits apparently were not as noticeable in this regard because I think they changed a few resistor values so that the unused channel volume pot made up a much smaller percentage of the resistance in that voltage divider feeding V2 and, therefore, didn't create as much of a noticeable difference. I'll check it out.
     
  19. dmczern

    dmczern Member

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    Thanks Dave C!
     
  20. jrigg

    jrigg Gold Supporting Member

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    I clipped the bright cap on my '81 JCM800 but I like it on my '74 50 watter. Whether the bright cap sounds good or not depends on the value of the cap and also on what level you are running the volume control. On my '74, the bright cap really thins out the sound unless the bright channel volume is 6-7. I adds a really nice edge to the sound at that level.
     

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