JMP 50W help to identify mods and replate issue

Discussion in 'Let's Talk Vintage' started by Malibu2006, Sep 9, 2019.

  1. neikeel

    neikeel Well-Known Member

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    The bias supply on these is derived from one half of the ac HT (that white wire from the rectifier diode block).
    The bias circuit needs to see this voltage as soon as standby is on (before standby is powered) but if you have moved the standby switch so that should be ok.
    If the B+ is higher than usual (400-420v is usual on these) ie if the PT is a later 1202-324 from a JCM era then the stop resistor (47k) is the one to change to get into the correct range for the tubes you have installed.
    Suggest for now you tidy up lead dress and then piggy back with croc clips across the 47k to get closer to -40 to -45v range on pin 5. Then you can decide if you are going to mod the amp to fit these old tubes or get a burned in fresh pair and set up for those once you are happy, my 2cents.
     
  2. john hammond

    john hammond Well-Known Member

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    fuck we're good.
     
  3. john hammond

    john hammond Well-Known Member

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    a further 2 cents, high-ish plate voltage and those spragues in a marshall is not my idea of tone mecca. this thing is gonna be very bright, piercing.
     
  4. john hammond

    john hammond Well-Known Member

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    this is a diagram with a later jcm800 50 watt power transformer, these resistor values will get you a higher bias voltage. note rb2 which is 120k, yours has 220k there. You can match these bias sub circuit resistors to what you have there, theres only a couple of them.
     

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  5. Dblgun

    Dblgun Member

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    This knowledge and a willingness to share and assist is in large part why I frequent the forum. While I don't know Malibu2006 I benefited from the assistance you gave.
     
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  6. john hammond

    john hammond Well-Known Member

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    now, i like that, thankyou.
     
  7. Malibu2006

    Malibu2006 New Member

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    Thanks for the prompt responses and recommendations.
    The bias feed is coming from one leg of the secondary. You see it as the white wire soldered to one side of the AC input to the bridge rectifier.
    I will try changing the value of the 47K and/or 220K resistor to see if I can boost the bias into upper -40's.
    Meanwhile I measure 204VAC at the junction of the 220K resistor and diode.
    I measure -47.7VDC on the other side of the diode so that is too low going into the rest of the bias circuit.

    Is my plate voltage too high for this amp?
    Should I switch PT's to lower secondary voltage(the one in there measures 663VAC no load) to get B+ to around 400VDC?
    I am Canadian so favor Hammond transformers since they are in Canada.(I work in the electronics industry)
    I will also investigate that the correct PT primary leads are soldered to the voltage input switch(I am thinking that whomever wired it swapped the 110V tap for the 120V tap which would boost the secondary output voltage)
    Do any of you have a color code chart for the leads on the 1202-164 and 1202-324 xfmrs?

    What should I replace those Sprague orange drops with to get a better, less bright/piercing tone?

    Appreciate the help.
    Rgds
    Dave
     
  8. neikeel

    neikeel Well-Known Member

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    Oh, you were quoting 450vdc in your first post. As before 400 to 420 is normal. The -164 and -324 have same colour codes as I recall.
    My personal preference in these are Phillips mustards, lots of hype, lots of rubbish spouted but nothing else is as smooth and defined (I have tried all other commonly available axial caps. The Wima Tropfoyl are pretty close but 0.022 @ 400v are even harder to find. Old Sozos an Mallory caps do the job but lack something. The newer ones might be better?
    As I said piggy back a resistor or two on the bias stop end 47k to get into correct range first.
     
  9. john hammond

    john hammond Well-Known Member

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    - yes, ( double yes )

    unfortunately, signal caps are vital.
    theyre not vital to getting a great amp sound, theyre vital to getting the precise sound of that particular year/type amp.
    they, together with the correct o/t get you 95 percent there, glass and plate voltage will finish the job.
    for example, if you threw those little philips green .022 from 1977, it would sound more like an ac/dc crunch ( my canadian 77 has these).
    the issue is going to be finding mustard signal caps, they are for sale, but 99 percent of them are counterfeit.
    TAD sell genuine .1 uf.
    toneloco123 ebay seller sells genuine .1 and .022 also, that guy is a hoot to deal with, you have no idea, he is unique, a funny, funny dude. ( hes in canada )

    btw the worst signal caps that he could have put in there, he did.
    those particular orange drops are really wide range, lots of bass, really high tops ( keep them and put them in a 5e3 clone, theyre amazing in that application)
     
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  10. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    @Malibu2006 stated that the voltages given were of an unloaded amplifier. That means the voltages will be higher.

    An amplifiier with lower B+ will have a much hotter(lower) negative bias voltage.

    Has a good set of power tubes been tried yet?
     
    Last edited: Sep 12, 2019
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    When the tubes are installed, and the bias is adjusted, the plate voltage will drop.
    More load on the power supply = less voltage.

    2. There is nothing wrong with Orange Drop capacitors.
    No, they do not cause "bright piercing tone."
    The brightness of a Marshall is caused by the design of the amp itself....not Sprague capacitors.

    Now the short explanation:
    The amp is "bright" on purpose. It's designed that way deliberately.
    The design causes the highs to break up and sound crunchy, the lows are clean.
    This is the way that British amps are made to sound.
    The sound is only effective when the amp is cranked way up.
    If you are playing the amp on: "2," then you have the wrong amp.
     
  12. john hammond

    john hammond Well-Known Member

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    yeah, i remember you telling us recently that output transformer brand changes wont see a change in tone either, i get your message ampmad..i get it.
    wood choice has no bearing on guitar tone..etc..etc..etc.
     
  13. john hammond

    john hammond Well-Known Member

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    to ampmad and anyone else that cant hear tone changes after changing components..

    - just because you cant hear any changes, don't mean there aint changes.
    ( there HAS to be changes, it's called ' physics'.)

    I hear these differences like the difference between seeing night, and seeing day, and it's what defines my limits as a musician.
     
  14. neikeel

    neikeel Well-Known Member

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    Re the coupling caps afraid I have to disagree with AMS on this, there is a difference even I can hear!!
    Build it as a bass spec 1986 +/- a switchable split cathode and/or bright cap and you will have a decent amp.
     
  15. john hammond

    john hammond Well-Known Member

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    NEWS FLASH!

    george metros 68 exposed as ceriatone clone !, same circuit design right? - should have known all along.


     
  16. john hammond

    john hammond Well-Known Member

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    georges clones just never sound right to me, (with the outstanding exception of the kt66 earlier plexis 45/100)...somethings too bright.
     
  17. CoyotesGator

    CoyotesGator Well-Known Member

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    Dude, accidentally post this in the wrong thread?
     
  18. South Park

    South Park Member

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    Get the stand by switch in the right place. The clasic tone transformers are a real good choice. The bias circuit is good. If want tone caps I use Russian paper in oil
     
  19. john hammond

    john hammond Well-Known Member

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    nah, its good where it is.
     
  20. Malibu2006

    Malibu2006 New Member

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    Hello All. Happy to report I got the amp working nicely with your combined help. Turned out it was an intermittent contact on the impedance selector, one bad EL34 and a lowish bias range. I bumped up the bias range by putting a 200K resistor where the 220K one was and got -29 to -41V swing.
    I put in a new pair of GT EL34's and biased them up as equally as I could. After 30 minutes I have 34.7mA on one and 36.3mA on the other.
    So far I like the tone of the amp, it is bright and clear, not shrill. I am just starting to play so as my skills improve I can tweak things to suit.
    I am happy owning a piece of working Marshall history.
    Thanks again for the help, comments and suggestions.
    Dave
     
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