is my amp modded or is sumthing wrong

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Broadie, Feb 15, 2006.


  1. Broadie

    Broadie Member

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    Hi i have a marshall 100 watt super lead and it seems that it is very muddy and a guy who heard it said he thinks it may be hot rodded i know that there has been some mods done to it because it has a master vol. nob added to the back but that was all i knew of when i bought it, it sounds really muddy and chunky, i never played out of any other super leads besids this one, do you think thats normal or is it hot rodded or could there be somthing wrong like bad tubes or sumthing help would be much appreciated

    thanks a lot
     
  2. VintageJon

    VintageJon Member

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    Actually these are clear sounding and have a lot of headroom. I suspect a failed output tube or a bad master volume mod.

    Is the pot for the master a dual-stacked?

    What's the value?

    Is it "post-phase-inverter"? If so are there 4 caps used?

    Pics? Email if needed...

    -Jon
     
  3. Broadie

    Broadie Member

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    accualy i have no clue about the master mod it was there when i got it so i don't know whats going on with that whole thing / how can i tell if my tubes are bad or not. im pretty sure there all stock, i know the pre amp tubs are anyways so there probly pretty old / what tubes do you think would work best for an angus young real clear sound??

    thanks a lot
     
  4. VintageJon

    VintageJon Member

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    I really think you need to have a Pro look into it.

    Analysis of the Master Volume Mod and looking at the output on a 'scope into a dummy load will tell a lot in a short time. If you don't have a Pro Tech available then I don't know what to recommend- God help you in that event.

    -Jon
     
  5. Beer

    Beer Member

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    Can you open it up, take a picture (or two) of the circuit, and post it here? I'm brand new to amplifier gut tinkering (just built my first 50 watt clone), but could *probably* tell more about the amp by looking inside it.
     
  6. Gordon

    Gordon Silver Supporting Member

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    Photos would be real nice. These amps are very simple inside and we could probably tell a lot from a few pics. A couple of questions. What year is the amp, which channel are you using, and does the lead channel (channel 1) have a small capacitor across the volume pot?
     
  7. Broadie

    Broadie Member

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  8. Broadie

    Broadie Member

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  9. Beer

    Beer Member

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    Well, there are several things that jump right out. Obviously it has a master volume added. Also the two orange caps near the master volume (although you can't tell where they are attached, I bet others could guess as to what this is for and how they are connected, I'd be really interested in reading specifics). There is no "bright cap" on the bright volume pot. Here is a schematic for your era Marshall, I believe.

    http://www.drtube.com/schematics/marshall/1959-01-60-02.pdf

    Also, check out this site for a good "walk through" of the various components and the common modifications.

    http://marshall.redpt.com/clay/marshall_circuits_101.html


    Finally, you can try to figure out the resistor values in your amp by reading the colored rings.

    http://aikenamps.com/ResistorColorCode.htm
     
  10. Gordon

    Gordon Silver Supporting Member

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    Ok, you definetely have a non-stock mv circuit on there. that's what those two large orange drops are on there. You also do have brightness caps across your bright channel vol pot. On this amp they are on the board and there are two of them in parallel to arrive at the value that marshall wanted. I also noticed that you are channel jumping. So, here's my ideas. I always have found channel jumping to produce a muddy and inarticulate sound. I would try to play into the bright channel only. The mv circuit on there operates post driver and is intended to produce the most natural sounds. I have never tried it on any of my gear though so I can't comment. I also can't really follow all the wiring from the picture so I can't say if it's installed properly. But lets assume that it is. The bright channel has a 5000pf bright cap, which in my opinion is way too high. Is that why you were channel jumping? Because the bright channel alone sounded so shrill? Try just the bright channel, if it's too shrill have a tech remove the two brightness caps and replace them with a lower value. I like 180pf for a '69 plexi, but most guys put a 470pf on there, it's up to you and your taste. Once you've got it running and can see how it sounds you can decide if you want to keep the mv circuit on there, but be warned this is a brutally loud and agressive amp when it's running properly.

    Good luck,

    Gordon
     
  11. Gordon

    Gordon Silver Supporting Member

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    BTW if you do try just the bright channel and it is still dark and muddy the first place I would look is at the mv circuit. There is a silver mica cap located across the dual pot. It is probably there to phase cancel some high frequencies. It may be too large a value and may be the source of your muddiness. A tech should be able to determine if it's the source of your problem.
     
  12. Broadie

    Broadie Member

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    yeah when i play just out of the bright channel it sounds to thin and not enough low end but it still sounds muddy i think what im gunna do is take it in to a tech and have him restore to original specs i know it would be brutaly loud but i really would rather have it loud and not be able to play it at my house than have it quite and not sound right whats your opinion about that?? any tips you would throw in there like what would i be sure to tell the tech to do to it? i really have no clue at all about the inside of amps and circut boards or anything thanks a lot
     
  13. donnyjaguar

    donnyjaguar Member

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    Do all Marshalls have the tone controls backwards like this one?
     
  14. TubeAmpNut

    TubeAmpNut Member

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    This looks to only have the post phase-inverter master volume. Those big orange drops connect to the pins of V3 (PI tube) then through the dual ganged pot then to the caps blocking the bias voltage (the white mallory caps with the glue on them). It doesn't look to be a great mod job, but I've see a whole lot worse. It should take maybe 1 hour to fix and return to stock. I think you can reuse the white box caps too (with some cleaning up).

    Where do you live?

    BK
     
  15. TubeAmpNut

    TubeAmpNut Member

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    If you lived near DC, I'd offer to fix it for you.

    BK
     
  16. Broadie

    Broadie Member

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    no i don't i live up in New York thanks any way im sure i can find some one up here to do it how much should i expect it to cost
     
  17. Gordon

    Gordon Silver Supporting Member

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    Yeah man, I think that restoring it to original is a good idea. Like you say, it's better to sound good and be way too loud that like mud. BTW, the 5000pf brightness cap on the bright channel will make it sound SUPER thin and nasty at lower volumes. While the tech has it, it might be a good idea to have him replace it with a 180pf cap. That way you'll have a pleasant tone at lower volumes, and shouldn't have to resort to channel jumping (which I think always adds mud).
     
  18. John Phillips

    John Phillips Member

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    There's no point in restoring it to totally original IMO since the hole has been drilled for the MV pot anyway.

    You don't have to choose between too loud and muddy. It's perfectly possible to do it correctly so the MV works as intended and leaves the amp working properly. It's a post-PI MV which should retain the amp's natural tone very well. If it had been done right there is no need for the mica cap on the dual MV pot which Gordon pointed out - I'd start there as the source of the muddiness.

    I don't AT ALL like the positioning of the orange caps or the way the wires have been attached to the caps on the board and the caps secured with goop - this is just bad practice, and the position of the orange caps could be causing coupling and self-oscillation... which is maybe why the tech fitted the mica cap, to cure it by the principle of two wrongs making a right. Just bad workmanship - it can be done better.

    I agree about reducing the value of the bright caps on the Bright channel too - they're what makes it shrill when you just use that channel alone. The absolute maximum useful value there IMO is 1000pF, and even that's a lot. I have no idea what they were thinking with 5000.
     
  19. Gordon

    Gordon Silver Supporting Member

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    I couldn't agree with John more. It makes good sense to get the MV circuit done properly so that it really is useable. That was my concern with the silver mica cap on the pot. Probably trying to eliminate unwanted parasitics.
     
  20. Broadie

    Broadie Member

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    how much should i expect it to cost to have the MV mod. done correctly and the briteness caps changed? just so i have an idea so i don't get ripped off
     

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