4001/Studio 15 losing Gain... or something

Discussion in 'Marshall Amps' started by jak, Apr 10, 2020.

  1. jak

    jak Member

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    I bought my 4001 in 1986. A couple of years ago, just because I never had the amp services—and despite it sounding like it was brand new—I took it to an authorized Marshall service center. The gentleman who worked on it told me all he had to do was clean the gain pot; that he didn’t have to change any tubes or do biasing or anything. Caps solid. He said everything was perfectly in spec‘s. When he was working on that amp I bought a spare. so when I picked the amp up from him, I only plugged it in once to make sure it was working OK and then I continued playing the new amp. Recently I began playing it again with the same exact pedals and settings I had in the past that gave me phenomenal sound. But over the last couple of months I’ve noticed something wrong, like every week or so I was losing more and more tone for lack of a better word. Then I noticed it seemed I was losing crunch. Long story short, finally today, after noticing that I had the gain on my OD pedal and the amp gain both on 10, I plugged my guitar directly into the amp. with the gain on 10 and the output level on five, and it sounded almost perfectly clean, no dirt. I get the same result whether the amp speaker is plugged into its 15W, 8ohm jack or in the attenuated headphone jack. Same thing.

    sooo... The power amp side master output level sounded plenty loud at five, so I’m assuming that side is OK. But safe to say somethings wrong on the pre amp side?

    any ideas?
    Thanks ahead of time.
     
    Last edited: Apr 10, 2020
    tce63 likes this.
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Plug the guitar straight into the amp no effects.
    Is it working right now?
    Are you sure this isn't caused by the OD pedal?
    Does the OD pedal use a battery?
    Does the guitar use a battery?
    Are the batteries new?
     
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  3. jak

    jak Member

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    Thanks. Yeah I mentioned that in my post. To be sure it wasn’t anything else I plugged directly into the amp, with the gain on 10, bass, treble & mid on 10, and output on five, and it’s so clean I’d say it’s all clean. With that set up in the past it would be very distorted. And no, no batteries in my setup, regardless. And I tried it with multiple guitars and cables and the result is the same. So it’s none of those variables. And my DSL5C sounds great so it’s not the mains.
     
    Last edited: Apr 10, 2020
  4. mickeydg5

    mickeydg5 Well-Known Member

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    Did you try known good 12AX7/ECC83 tubes in V1 and V2?
     
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  5. jak

    jak Member

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    No. I have no spares. I’ll see what I have in my DSL5C. Otherwise it’s mail order or a 2 hour drive.
    thanks.
     
  6. johan.b

    johan.b Well-Known Member

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    Swap preamp tubes between your two 4001 and see if problem follows the tubes or the amp. As long as it's only the two small bottles, no bias needed to be adjusted
    J
     
  7. jak

    jak Member

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    K. After swapping I don’t notice a difference sound wise. Not much distortion. But, turns out I have new tubes in there. Ruby ax7ac5. Maybe these are not as high gain as my originals? I have my originals using the euro ecc83 nomenclature. Can’t tell what they are as they’re 40yrs old and just have White powder coat stamp on the bottle ecc83A—the ‘A’ is ? cause it’s pretty faded.

    Tell me if the following is significant: on the ruby v2 one of the two filaments glows less intensely than the other. The shaft is slightly less intense but the typically bright core of the filament is markedly less bright. I swapped v1/2 and the faint filament follows the bottle to the the new location—v1, so it’s not the socket.

    so I took one of my old tubes and swapped it for the ruby with the suspect filament (currently in v1)!and now all four pins on v1/2 glow the same but I don’t notice any difference in sound.

    so, the dimmer filament would seem likely to be the culprit but even with it swapped out, nothing changes. Nice warm creamy sound but weak on the crunch.

    also the dim filament was originally in v2 which I’m thinking is less involved in the tone and distortion than v1.

    though the dim filament seems not to be a factor, for the record safe to say it indicates there’s a voltage drop across the plate?
     
  8. mickeydg5

    mickeydg5 Well-Known Member

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    First and formost:
    clean all jack contacts, cable plugs, tube pins and tub sockets with a good lubricating electronics contact cleaner.

    Then check all cables for proper resistances or swap out with known good cables.

    If it is not that then start looking at components and connections in the amplifier.
     
  9. jak

    jak Member

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    Thanks. Good idea. Will do. Is there a device specific for testing cables or would my multimeter work?

    any thoughts on the v2 dimmer glowing filament?
     
  10. mickeydg5

    mickeydg5 Well-Known Member

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    Yes, use a multimfeter to test cables. There should be next to zero measured at point to point in line like form TIP to TIP or SLEEVE to SLEEVE contact points and infinity between separated lines.

    Different tubes have different glow on heaters. Some can barely be seen and lights in the room have to be turned off to view. That is just how it is.
     
  11. jak

    jak Member

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    Ah. Thanks for telling me what to look for in the testing. That helps perfectly.

    as far as the heaters, I understand what you said. But different glow intensity between heaters of different tubes I would imagine. But did you mean it’s also normal for the two heaters in a single tube to glow at markedly different intensities? Just asking cause I’ve never seen that; I’ve always seen the heaters in a given tube glow symmetrically intense.

    just want to be clear.
    Thanks.
     
  12. mickeydg5

    mickeydg5 Well-Known Member

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    Oh, sorry. Those dual tubes have two filament halves that are connected in parallel for operation in this amplifier. One half may be more worn or disintegrating faster and malfunctioning.

    This is a 12AT7 but same thing going on. The center tap is at the connection of the two filaments.

    upload_2020-4-14_21-32-14.png
     
  13. jak

    jak Member

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    Ah. Excellent. Thanks for that. I looked up schematics for my AX tube, same as the AT in your pic, Save for the gain, but I couldn’t find descriptions for the parts. I see in yours pins 4 & 5 are the filament/heater connectors. got it. So you confirmed my assumption that the disparity in filament intensity is not normal. Interesting that whatever that symptom might represent, i can’t hear a difference even when swapping out the suspect tube for my good old one, but I’m sure it’s only a matter of time. Not the culprit this time is guess.

    I’ll have to get to testing the cables etc.

    Thanks again!
     
  14. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I wouldn’t worry about the filaments. If it were really a problem you would have noticed when you swapped the tubes.
     

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