1st gen JTM60 and their clipping section: any comment?

Discussion in 'Marshall Amps' started by freefrog, Dec 31, 2019.

  1. freefrog

    freefrog New Member

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    Posting an answer about a JTM30 reminded me something that I had noticed a long time ago on another model of this series.

    In the last JTM60 and JCM600 schematics, if memory serves me, clipping diodes were implemented after the output of V2A, as a diode bridge to ground following a capacitor (itself replaced with a simple jumper in some of the most recent amps).

    In the first gen JTM60's, things were different: the signal going through the 4,7µ cap then through the diode bridge was sent back to the cathode of V1B.

    I was wondering why Marshall had designed this circuit like this then modified it...

    I've the sketch of an answer in my mind but I'd be glad to know how other Marshall users understand and would explain this.

    Comments, anyone?

    Thx in advance!

    PS: English is not my mother tongue. Thx for your understanding if ever my words or sentences are not correct. :)

    FF

    BTW, here is a schematic showing the convoluted path that I mention:

    http://www.prowessamplifiers.com/schematics/Marshall/Jtm_60-Poweramp.html
     
    Last edited: Dec 31, 2019
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  2. slide222

    slide222 Well-Known Member

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    is there a date on the JTM 60's i would like to know when mine was made
    my 60 has a diode on off switch and it is much better with the switch off
    and don't forget the JTM30 is a completely different amp to the JTM60 , not even close to being similar , not even close to the same , well that's what i was told ,lol
     
    Last edited: Dec 31, 2019
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  3. freefrog

    freefrog New Member

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    I don't remember if there's a date or not in JTM60's.

    This model appears to have been designed in 1995.

    The schematic with the "negative feedback clipping section" that I mentioned is dated 2 - 7 - 96.

    The schematic with the clipping diodes to ground is dated 4 - 11 - 96.

    Inspecting the signal path of the clipping section should reveal if the amp has been assembled before or after these dates, that's all I can say...

    I'm still all ears about how people explain these changes in the clipping section. :)

    EDIT- BTW, whatever are the obvious differences between JTM30 and JTM60, the 30 watters appear to have featured also a negative feedback clipping section, but located between the output and cathode of V1:

    https://www.thetubestore.com/lib/th...shall/Marshall-JTM30-30W-JTM310-Schematic.pdf
     
    Last edited: Dec 31, 2019
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  4. slide222

    slide222 Well-Known Member

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    I don't think the JTM 30 is much different to the JTM 60 , ignore that , that a reference to a very stupid comment somebody made and we had fun with him -that is the reason I got 2 likes
     
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  5. freefrog

    freefrog New Member

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    Yeah, slide222, I had smelt some private joke in your post. It has the advantage to bring a smile in this topic. Happy New Year!
     
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  6. Trelwheen

    Trelwheen Well-Known Member

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    Now you've got two more :agreed:
     
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  7. Trelwheen

    Trelwheen Well-Known Member

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    If English is your second language, you must be awesome at your native tongue. You're English is nearly perfect
     
  8. freefrog

    freefrog New Member

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    Thank you Sir.
    and Happy New Year! :cheers:
     
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  9. Trelwheen

    Trelwheen Well-Known Member

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    Happy New year!
     
  10. slide222

    slide222 Well-Known Member

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    I had a good 3 hours on the JTM60,2x12 tru a 2x12extension cab last night , and it always sounds good and playing tru 4 nicely worn in v30 's is superb , the amount of gain available on the red channel always surprizes me I have to have the gain low to get what I want , and a great clean channel .the JTM 60 has a raw 60's sound but I do turn the diodes off, which takes the fizz out
    the gain on these goes up to 20, i'm on 5
    if you can get one of these in good order jump on it
     
    Last edited: Jan 1, 2020
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  11. freefrog

    freefrog New Member

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    Glad to know that you rock your JTM. :)

    A few years ago, I've discussed by mails with Andy Brauer about the JTM line: he described the JTM30 and 60 as sleepers and keepers. I think I've still a scan of his article about that, somewhere in my archives.

    I've also gigged many times with a JTM60 1x12, generally coupled with 1 or several other combos (Vox AC15, Fender, Carlsbro, Jazz Chorus...). The JTM had and has still the merits to be cheap enough to avoid to be stolen, light enough to be dragged on stage easily, not too loud but with a useable MV and still able to cut the mix on request...
    Mine had crapped out 2 years after its purchase (its OT had to be changed and I had also added a cooling fan) but it has survived without further issues since this day.
    It remains the cheap amp that I've the least modified among those in my possession (compared for example to a Hot Rod in which I've done 17 mods). I've even left the JTM with its stock clipping diodes : I find the red channel playable as it is with the gain @ noon. But it has less gain and a less fizzy buzzy voicing than the JCM600 compared to it in the late 90's (when I was comtemplating the purchase of a second identical combo for my stereo rig).
    Granted, it's not the same than the stacks of my youth (I still recall my ears ringing because of a full stack JCM800) but it has been more than useful for me until now...

    To come back on topic: I've started to work on a LTspice simulation of the amp. I've also some testing gear waiting to be used again with the JTM when time will permit. I'll share later any experimental result related to my initial question. :)

    EDIT - In the meantime, I post below the frequency response of the mentioned JTM60 through a Marshall 1912 then through its own stock loudspeaker, both miked with a SM57 (monophonic signal). The other line shows the response of the amp captured from the OT. For comparison, same thing with two other amps on the right side of the pic (although the AC15 has been tested with a polyphonic signal, hence the more ragged curve)...

    https://servimg.com/view/15353905/219
     
    Last edited: Jan 2, 2020
  12. slide222

    slide222 Well-Known Member

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    mine had 3 mods , a fan , a switch for the diode and those small black and red inputs on the back panel for plugging in a multimeter to bias , all done before I got it-I sent it to marshall for bias and new power tubes , and then checked out the mods

    my first marshall was a early jcm 800, but I swopped it for a JMP mid 70's 50 watt head, and then later I got a 60/100 mk2 mesa boogie combo - I wish I kept that one
    ...yea plz keep me informed as you go forward with this , I have my collection of amps now and I intend to keep them all , hopefully I won't buy anymore , I might buy more speakers and more of the eminence empty palmer 2x12 cabinets tho , and I would like to see that article with andy , I love to know more about my amps and any history in design and development
     
    Last edited: Jan 2, 2020
  13. wmachine

    wmachine Well-Known Member

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  14. slide222

    slide222 Well-Known Member

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    thx for that , I saved it
     
  15. slide222

    slide222 Well-Known Member

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    the jtm60 2x12 is like a tank, only heavier
     
  16. slide222

    slide222 Well-Known Member

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    had a good play on the JTM60 pretty loud,with the diodes switched on and off , and its louder with them off and the wah wah sounds much better with the diodes off
    and its is quite fizzy with them on and deffinately more natural with the diodes off and much better in my mind, far better so they will stay off, more 60's jtm45 sounding on the green channel .and i'm only on 5 out of 20 on the red channel , unbelievable huge gain available...and the master volume was only on 3 , really loud amp , the power tubes, EL34's are nicely worn in-best sound I've had out of it today
     
    Last edited: Jan 4, 2020
  17. freefrog

    freefrog New Member

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    Yep, it's a goldmine. I had downloaded their JTM60 schematics years ago (I think it was in 2003) and am glad to see the site still alive. :)


    FWIW, one of the reasons why I've kept the diodes on the red channel is that I typically use the amp in "4CM", through a MultiFX sending back its preamp to the JTM but also to another amp. The overdriven signal becomes a bit too hot for this when it's not clipped by diodes... And the green channel is there to be cranked when I need "non clipping diodes", anyway.

    That said, when I use Lil' JTM alone, I like to push the green channel with a boost: although the tone stack is not typically Marshall on this side of the amp, it sounds really good when properly excited (by one of my home made Rangemaster clones made from NOS parts, for example). :)
     
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  18. slide222

    slide222 Well-Known Member

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    not all the links work , some do and some don't
     

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