PLASTIC JACKS

Discussion in 'Marshall Amps' started by dragonvalve, Nov 16, 2020.

  1. Gene Ballzz

    Gene Ballzz Well-Known Member

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    @dragonvalve

    First and foremost, given that these are on the speaker output of a JTM30 it is absolutely critical to understand that:

    #1 - All connections (tip, sleeve and chassis) of any replacement jacks must end up being absolutely identical to the original.
    #2 - All switching functions operate exactly the same as the original, or else you will likely be looking for an output transformer or such!
    #3 - Few users understand how the goofy (and very short lived) configuration of the output jacks actually operate.
    A - The "Internal" jack is connected to the 16 ohm output of the transformer only when it is the only jack used. It gets connected (through switching in the jacks) to the 8 ohm transformer tap once you plug an additional (hopefully) 16 ohm cabinet/speaker into the "External" jack, for a total impedance of 8 ohms.
    B - Now here is the REALLY goofy part. The "External" was intended and can be used as a single 8 ohm output, but will only operate on its own if a "dummy, connected to nothing" jack is plugged also into the "Internal" jack!
    C - Most overheating and/or failure problems reported with this amp model can be easily traced back to this misunderstanding, along with the assumption that other power tubes can be used! It really wants to have the rather unusual "5881/6L6WGC, SOVTEK" power tubes that end being not really either a 5881 or 6L6, but just a tad different. The power section and non-adjustable, fixed bias was designed around this particular tube and the output transformer is rather "fragile" and part of the magic of these amps, as well as it's health/longevity being dependent upon these particular power tubes. This is also compounded by the power section being non-adjustable fixed bias.​
    #4 - Of course, a smart guy could follow and trace what is going on and replace this goofy/convoluted/confusing switching output jack setup with an actual switch (requires a small hole to be drilled) to choose 8 ohm/16 ohm, or have only one output jack with the switch in the other jack's hole, to avoid drilling!
    #5 - I own and have properly/mercilessly pounded the poop out of a 1x12 JTM since new in '92 and it has been one of my favorite amps of my +50 year "Screechin' & Squawkin" career! It has never failed me and even still has the original tubes!
    Given that the original style jacks may not be currently available, I'd likely opt for #4, with genuine Cliff UK jacks, drill a hole for the switch and be done with it forever! Here's a link to the schematic, if you're hip to understanding such:

    https://drtube.com/schematics/marshall/jtm3-61-02.pdf

    And to those who allude to cliff jacks being faulty: The moronic act of stepping on the cable connected to the input of your amp does nor equate to poor design of the component, but rather to opertor error! I've done it too, but am smart enough to right it off to my own stupidity! Cables and their connections are one of the most important and fragile parts of our great guitar rigs and need to be treated as such!

    Just My $.02,
    Gene
     
    Last edited: Nov 18, 2020
  2. coolidge56

    coolidge56 Well-Known Member Double Platinum Supporting Member

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  3. coolidge56

    coolidge56 Well-Known Member Double Platinum Supporting Member

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    According to the schematic a single 8 ohm output will operate fine on its own. When a plug is inserted it breaks the connection between the internal jack and the 16 ohm OT tap and instead connects the 16 ohm external jack to the 8 ohm OT tap e.g. both jacks are then connected to the 8 ohm tap. So you could use a single 8 ohm speaker or two 16 ohm speakers in parallel, one plugged into the internal jack and the other plugged into the external jack.

    The odd duck is this 3rd switch in the schematic between the two jacks which controls the standby switch path to OT common when an 8 ohm external speaker is connected, but nothing is plugged into the 16 ohm internal jack.
     
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  4. dragonvalve

    dragonvalve Well-Known Member

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    Thanks much for that important clarification.

    I've got new 5881's (Sovtek) as spares but what's in there now is old Peavey STR's 6L6's. They are a little longer than the Sovteks so I had to back the tube guard off the chassis a little using some rubber spacers.

    I'm not a fan of using other type tubes in amps. Whatever they were designed for that's what I go with other than brand selections.

    Since I'm using the JTM 30 for recording there are many things that can be done in the DAW to rely on slick tube arrangements to get a tone.

    I get that players like to define things with "their" tone but sometimes the tone doesn't fit the song in the mix.

    I have not used the amp with other speakers other than the 2x10's that came from the factory. The switch mod would not be something I would be needing. I've always liked 2x10's specially for recording and that the combo is open backed. No bass-woof to deal with.

    Only change I made to the amp was the baffle board. I swapped out the factory (MDF? or particle board?) baffle and made a multi-ply Birch baffle from some scrap at a lumber yard.

    I don't think the amp was over heating. The studio is air-con'ed when the amp is used.

    Thanks for the schematic coolidge56!

    [​IMG]
     
  5. dragonvalve

    dragonvalve Well-Known Member

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    Update:

    I got a reply from Joanna Green of BLETCHLEY Marshall CUSTOMER SERVICES MANAGER.

    She said these are the model numbers for the jacks for this amp.


    "the internal socket is SKTP-00005 5 pin 6.35mm and the external is SKTP-00004 9 pin 6.35mm."


    Who makes them was not revealed.
     
  6. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Here are couple links to goggle results of who actually has those jacks in stock. They appear to have been made by "JALCO" that I've not heard of before, but may be out of a large group in Australia.
    SKTP-00005
    SKTP-00004


    And @coolidge56 , nice catch on that "odd duck" with the standby! There have been rumors that some small details got changed/evolved/revised over the course of different production runs. Mine is, I believe, a fairly early one and the speaker jacks have always operated as I described above and so have a couple other units I've used over the years. I'm not sure what was intended with that switching connection to the stanby or the logic behind it.
    Please Keep Us Posted?
    Gene
     
    Last edited: Nov 18, 2020
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  7. dragonvalve

    dragonvalve Well-Known Member

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    Thanks for the links.

    I think from photos of open chassis on the JTM30 I've seen but not well enough one photo shows the back of the two jacks having a white portion of plastic on the rear.

    jalco 9 pin - Copy.png Jalco 9 pin

    https://reverb.com/item/27596019-marshall-9-pin-jack-dsl-tsl-jcm-2000-effects-loop-return

    jtm30 chassis - Copy.jpg

    Will have to open up the JTM when time allows.

    Due to the specific switching circuitry it might be prudent to replace the jacks with the original type parts.

    [​IMG]

    When it comes to switches my inner-retard bubbles up.
    [​IMG]

    [​IMG]


    [​IMG]


    Update:

    Amplifed Parts has them:
    https://www.amplifiedparts.com/products/jack-marshall-pc-mount-9-pin-effects-loop-return

    https://www.amplifiedparts.com/products/jack-marshall-pc-mount-5-pin-effects-loop-send

    [​IMG]
     
    Last edited: Nov 24, 2020

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