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Thread: Marshall dsl 15 h mod help

  1. #1
    Rod
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    Marshall dsl 15 h mod help

    Hi everyone:

    After the following mods(schematic attached):
    - C58 changed to 470K
    -Removed R80
    -Removed C76
    -I tried to remove also the C75, but removing this capacitor the amp starts squealing, so have to put it back again.

    Now i just want to add a little more gain, but it seems that every change i have tried the amp starts squealing, i have tried to raise R34 to 220K, or adding a capacitor of 0.68uf in parallel with R48.
    Can someone tell me what to change to avoid the squealing? Why the squealing happers, to much gain, or a lot of high frequeancy's?
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  2. #2
    Member torcamaniac's Avatar
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    I think you mixed up C58 and R80.
    C58 is a capacitor and R80 a resistor.
    Put them back the way they were, and start experimenting with gain by raising preamp tube plate resistors (R16) or lowering cathode rasistor (R14) and play with its bypass cap values.
    If you have removed cap C58 and now have only resistors there than I would say that the squell could be coming from there at some gain settings on gain pot VR3 and also because you removed C75/C76 but I am not sure as I am not familiar with DSL's...
    I am sure some of the more experienced can give you more advice.

  3. #3
    Stray Cap DrGonz78's Avatar
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    Quote Originally Posted by torcamaniac View Post
    I think you mixed up C58 and R80.
    C58 is a capacitor and R80 a resistor.
    Put them back the way they were, and start experimenting with gain by raising preamp tube plate resistors (R16) or lowering cathode rasistor (R14) and play with its bypass cap values.
    If you have removed cap C58 and now have only resistors there than I would say that the squell could be coming from there at some gain settings on gain pot VR3 and also because you removed C75/C76 but I am not sure as I am not familiar with DSL's...
    I am sure some of the more experienced can give you more advice.
    I think you meant "lowering cathode resistor (R40)". Looking at the schematic R14 is the plate resistor for V1A.
    When the going gets weird... The weird turn pro!

  4. #4
    Supporting Member loudthud's Avatar
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    One thing that sticks out about the schematic is that there are no grid stoppers. Adding them might be problematic if the whole thing is on a PCB with PCB mounted tube sockets.

    That said, oscillation is probably layout related and tracking down the exact cause won't be easy. You are going to need a scope and have to carve up the PCB possibly adding coax to bypass some of the traces. The best plan of action to increase overall gain is to make small increases in each stage and not do it all in one place.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

  5. #5
    Member torcamaniac's Avatar
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    Quote Originally Posted by DrGonz78 View Post
    I think you meant "lowering cathode resistor (R40)". Looking at the schematic R14 is the plate resistor for V1A.
    Yep maybe I wrote false resistor numbers from the schematic but I meant increasing plates vs decreasing cathode for gain. Is a good starting point.
    DrGonz78 likes this.

  6. #6
    Rod
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    HI thanks for your help guy's

    The values of the mods i have posted here are wrong, the rigth values are the following:

    - C58 changed to 470Pf
    -Removed R80
    -Removed C76
    -C27 changed to 33K
    -C36 changed to 33K
    - R51 removed
    -I tried to remove also the C75, but removing this capacitor the amp starts squealing, so have to put it back again.


    The problem is the same: now i just want to add a little more gain, but it seems that every change i have tried the amp starts squealing, i have tried to raise R34 to 220K, the amp start squealing so i undone the this mod, after this i tried to add a capacitor of 0.68uf in parallel with R48, the same problem with the squeal i have to undone his mod also.
    Can someone tell me what to change to avoid the squealing? Why the squealing happers, to much gain, or a lot of high frequeancy's?
    Last edited by Rod; 01-13-2017 at 10:05 PM.

  7. #7
    Supporting Member Steve A.'s Avatar
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    I have a Marshall DSL-15H, too, and I want less gain from the OD channel and plan to do it by replicating the lower gain OD mode from the DSL-40C. If I wanted more gain I'd use a stomp box, either a clean boost or a low gain OD.

    Someone asked if it was a PCB amp... yes it is. These days it is boards with SMT components that scare me...

    Steve A.

    P.S. I had a DSL-40C combo amp very briefly - I had to return it because it was so darned heavy. Bill Moore recently posted a link to an eBay seller who builds cabinets, including a head cabinet for a DSL-40C for around $150.

  8. #8
    Member torcamaniac's Avatar
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    Quote Originally Posted by Rod View Post
    HI thanks for your help guy's

    The values of the mods i have posted here are wrong, the rigth values are the following:

    - C58 changed to 470Pf
    -Removed R80
    -Removed C76
    -C27 changed to 33K
    -C36 changed to 33K
    - R51 removed
    -I tried to remove also the C75, but removing this capacitor the amp starts squealing, so have to put it back again.


    The problem is the same: now i just want to add a little more gain, but it seems that every change i have tried the amp starts squealing, i have tried to raise R34 to 220K, the amp start squealing so i undone the this mod, after this i tried to add a capacitor of 0.68uf in parallel with R48, the same problem with the squeal i have to undone his mod also.
    Can someone tell me what to change to avoid the squealing? Why the squealing happers, to much gain, or a lot of high frequeancy's?
    Hi Rod,
    not sure if you've read my first post.
    Like I said, put everything back the way it was.
    You want more gain, so start by increasing plate resistors vs decreaing cathode resistors on V1 and V2 tubes.
    Removing and changing caps and resistors around your pot won't do much in what you are after and as you've experienced yourself would introduce more problems and noise.
    Your gain is in the tube stages, so start there.
    If you are only looking for more distortion, than use a boost pedal to drive the tubes harder.

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