Marshall Dsl 401 3 Preamp Tubes Not Glowing

Discussion in 'The Workbench' started by Kanakanaka77, Apr 20, 2019.

  1. Kanakanaka77

    Kanakanaka77 New Member

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    I've read quite a few threads about this issue but will need some help on the next steps.

    Amp has almost no sound coming from it unless it's dimed.
    Replaced tubes and notice that 3 of the preamp tubes aren't showing any glow.
    Tested tubes and they are good.
    Tested bridge rectifier and readings show between 400 - 600 which I'm told is good.
    Could anyone help point me in the right direction. I should mention that I'm not too savy from an electronics perspective but am trying to learn.

    Thanks!
     
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  2. Jethro Rocker

    Jethro Rocker Well-Known Member

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    How did you test the tubes? How do you know they are good? It seems like the heater is not working on the pre tubes. Whether it is part of the heater circuit or a tube problem can't tell. Do you have other pre amp tubes? Do you have another amp to test them in? Another guitar and cord to narrow it down.
    Speaker wire Good? No shorts in the plug?
     
  3. JohnH

    JohnH Well-Known Member

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    The classic cause of that problem on this amp is the bridge rectifier getting really hot and over time, cracking/melting its solder joints. This cuts off heater supply to the first 3 preamp tubes. The fix is to replace the BR with one with longer legs and a heatsink, and to reflow all solder joints. Its not difficult except that getting access to the board underside is a PITA
     
  4. Jethro Rocker

    Jethro Rocker Well-Known Member

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    Thanks. Good to know someone knows about this exact problem.
     
  5. JohnH

    JohnH Well-Known Member

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  6. Jon C

    Jon C Well-Known Member

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    Welcome to the MAF @Kanakanaka77 !!
    @JohnH is the DSL401 guru here ……
    The Bridge Rectifiers in the 401 get ridiculously HOT !!
    Especially the early years of production ……
    I’m more of a hack but as you can see from this pic …… mine stays nice and cool !!
    [​IMG]
     
  7. Kanakanaka77

    Kanakanaka77 New Member

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    Perhaps I didn't test them properly.
    I tested each one by plugging it into the only socket that had a glowing preamp tube and they glowed there. Would that be an indication that there wasn't an issue with the tube?
    Yes to testing with different guitar and cable although the same guitar/cable works in other amps.

    Here's a better question...
    Should all of the pre amp tubes in this amp be glowing?
     
  8. Kanakanaka77

    Kanakanaka77 New Member

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    Thanks for the replies everyone.
    However, if the legs are reading 400-600 on the bridge rectifier wouldn't that indicate that it was good?
     
  9. Jon C

    Jon C Well-Known Member

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    Tube heater power has been compromised by the Heat from BR …… your tubes are probably fine …… This a Circuit Board issue most likely ……
     
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  10. Kanakanaka77

    Kanakanaka77 New Member

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    Thanks for the replies everyone.
    However, if the legs are reading 400-600 on the bridge rectifier wouldn't that indicate that it was good?
     
  11. Jon C

    Jon C Well-Known Member

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    Not if the joints or traces are damaged …… Test at the pins……
     
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  12. JohnH

    JohnH Well-Known Member

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    yep, the BR could be getting supply, and be working, but the traces that take it onward to the tubes could be disconnected. Reflowing the joints may help it.
     
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  13. ampmadscientist

    ampmadscientist Well-Known Member

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    Aloha Kanakanaka, welcome to the forum. :cheers:
    The bridge rectifier breaks down because the filter capacitors on the output of the rectifier go bad from age / heat.
    This causes the BR to run too hot and it is over-current condition.

    To fix this the BR and the filter capacitors need to be replaced. (the BR is usually still working) but the stale filter capacitors are loading the voltage down.

    To test the bridge rectifier (properly) it needs to be disconnected from the circuit.
    But instead of testing it, usually just put a fresh one in when the capacitors are replaced.
    The circuit tracks may need to be repaired because of the over-heated conditions.
    Then the whole DC filament power supply starts fresh again...

    Also: the circuit tracks on the board are damaged easily by soldering. This is why it may be a good idea to hire a experienced professional who uses the better soldering equipment.
    2. It is possible to use a higher grade filter capacitor which lasts longer.

    When the filament rectifier and filter caps are replaced (check fuses also), the filaments of the preamp tubes will be working again. The sound will usually work again.

    Are you located on Oahu? Might be able to point you to a technician if you need one...

    dslfilament.png
     
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  14. ampmadscientist

    ampmadscientist Well-Known Member

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    The BR needs to be removed from the other components in the circuit if you want to test it.
    The other components in the circuit including the tubes will interfere with the readings.
    But usually we just replace that puppy and start w/ a freshy.
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    upload_2019-4-21_5-53-23.jpeg Here we are again, back at that thermally compromised rectifier crossroads...where liquid nitrogen looks more and more appealing.
     
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  16. RLW59

    RLW59 Well-Known Member

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    The glow comes from the heater filament in the tube. The tube has to be hot to work properly.

    In some cases, due to your viewing angle and the physical construction of the tube, you may not see the glow. In those cases, the important questions are "does the tube get as hot as properly working tubes do, and is the amp working properly?"

    (Since you can see the glow when you test the tubes in the other sockets, and since the amp sounds bad, chances are the tubes aren't getting hot when plugged into the first 3 sockets, and the problem is with the heater power supply circuit.)
     
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  17. RLW59

    RLW59 Well-Known Member

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    I've never dealt with heater failure before.

    He says "the amp has almost no sound coming from it unless it's dimed".

    I've been under the impression that a cold tube is an open circuit, and the amp shouldn't pass the signal at all. And when I've read about people dealing with heater failure, this is the first time I can recall that the amp worked at all.

    So why does his amp make any sound? (Other than power amp hiss.)

    Are the filaments getting some power? Enough for the tubes to conduct, but not enough to work properly?

    Or is there some parallel path sending the guitar signal directy to the PI?
     
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  18. ampmadscientist

    ampmadscientist Well-Known Member

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    Because the cathode grid plate have inter-electrode capacitance between them, a very small amount of audio can pass, the filament voltage may not be entirely shut off but too low to make the filament glow.

    Take out the tube,
    Measure DC voltage between pins 5 and 9. Maybe 1-2 volts is still happening.
     
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  19. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    :welcome: To the forum :cheers:
     
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  20. Kanakanaka77

    Kanakanaka77 New Member

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    Thanks again for all the replies! Great information in here!
    @ampmadscientist - I wish I was in Oahu! Any suggestions on where I could buy the suggested parts?
     
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