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First split cathode amps question ....

Discussion in 'Let's Talk Vintage' started by Joshabr1, Jun 11, 2021.

  1. Joshabr1

    Joshabr1 Well-Known Member

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    So when the lead amps started evolving .. when the split cathode amps came in to being was the .68uf cap on v2?? Or did it arrive later ??
     
  2. South Park

    South Park Well-Known Member

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    I looked at the schematic and did not see one on v2 . The 2204 has that cap on v1
     
  3. Joshabr1

    Joshabr1 Well-Known Member

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    The schematic from Marshall in 1970 had it there. I don’t know what schematic you are referring to. Every split cathode Marshall and master volume have the .68 on v1.
     
  4. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    From what I understand, in 1968 the split cathode on V1 appeared with 820 ohm/.68. The 4.7nF was added to vol 1. V2a got bypassed with the .68 and the tonestack changed to the 33k/560pF on super lead. The 50 watt lead model probably is the same.
     
  5. Joshabr1

    Joshabr1 Well-Known Member

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    So really the only time you don’t see it there on lead models is after the move to 100k in the neg feedback ??
     
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  6. stickyfinger

    stickyfinger Well-Known Member

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    I had this all figured out at one point but have forgotten the variations timeline but in a nut shell for lad spec...

    Some amps did not have a V2.68 in 71, some with 47k maybe a few with 100k feedback and some variations of the 4 or 8 tap or maybe even at the speaker jacks. 71 all over the place.

    72 was no .68uf v2, 100k 4ohm tap feedback.

    Mid 73 same as 72 with .68uf on presence and 4k7. (no scratchy pot)

    Later 70s (76?) Same as mid 73. Lower volts
     
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  7. Joshabr1

    Joshabr1 Well-Known Member

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    I’m more concerned with the 68-9 era amps
     
  8. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    68-69 should have the cap.
     
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  9. Ivan H

    Ivan H Member

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    The JMP (model 1987/1959) split cathode amps first appeared in late '67, with an 820r/.68uf on both V1b & V2a. This continued through '68, however sometime early in '69 the 100 watt model 1959 was changed to have the now common 2k7/.68uf on V1b (still 820r/.68uf V2a), the 50 watt model 1987 continued with the 820r/.68uf V1b, not exactly sure for how long. Hope this helps. Cheers
     
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  10. TAZIN

    TAZIN Well-Known Member

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    From what I can tell it looks like the 100w Super Lead received the split-cathode arrangement first starting around late 1967. These would be the SL/10xxx amps which had the gray colored RS 0.68uF bypass caps on V1B and V2A. These early examples don't appear to have had the volume pot bypass cap (5nF) but did have the 47K NFB resistor. The 50w lead amps (model 1987) seem to have received the split-cathode setup in February - March of 1968 and once again I don't believe they had the volume pot bypass cap. In April - May 1868 you see the addition of the 5nF volume pot cap for the 50w lead.
     
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  11. neikeel

    neikeel Well-Known Member

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    I echo what Tazin said, although I have never seen one without a cap on vol 1 pot at all. Even my early 67 45/100.(EL34 version) had a 100pF cap there.
     
    Last edited: Jun 13, 2021
  12. TAZIN

    TAZIN Well-Known Member

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    I forgot about your amp. I know the early 'Lead' amps had the 100pF but the examples I had on file either didn't have a cap or it was removed.
     
  13. stickyfinger

    stickyfinger Well-Known Member

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    Have you ever seen a 50 watt with the gray rs .68uf bypass caps? I have a pair for a build but I don't build 100 waters!
     
  14. TAZIN

    TAZIN Well-Known Member

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    Yes.
     
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  15. stickyfinger

    stickyfinger Well-Known Member

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    You wouldn't happen to have photo you could share?
     
  16. TAZIN

    TAZIN Well-Known Member

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    Not the greatest photo but it was the easiest to grab.
    RS caps.jpg
     
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