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JSX mods on an Ultra

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  • JSX mods on an Ultra

    finally went ahead with some of the more common mods to my Ultra 60 212,
    namely:

    swapping out the C76 coupling cap after V1b from the (overly shrill) 1nf -> 2.2nf

    changing the parallel C75 470pf/470k before V2a to 2.2nf/470k

    upping the V1b C35 0.47uf cathode bypass cap to 1uf PP film

    (on the JSX these caps are C3, C8 and C21 respectively)

    for giggles I included a Ultra/JSX switch which puts a 75k resistor across the Ultra's 220k R7 which gives it the 56k value found at JSX R6

    as is my practice I bagged all the pulled parts labeled it and gorilla taped it to the inside of the head for total reversibility.

    I didn't add the ice pick 10nF cathode bypass cap the JSX has at V3b since a +10kHz boost seems a real bad idea...with many JSX mods removing this.

    end result:

    huge low end bloom with beautiful sustain and transition into controllable violin like feedback; the shrill "bee in a can" 5150 flavor is gone from the Ultra channel. Through my little ported cab this amp now makes a thick breathing rumble but with high end harmonics jumping out.

    The "JSX switch" does what is expected; dumping more signal to ground it sounds like one ball is lost; it instantly thins up (maybe why the JSX adds the 2.2nf/2.2M switched fat cap just upstream).

    We are talking the Ultra channel tone here...clean tone? WTFC as I never use it (or crunch).

    Didn't get pre mod clips so post mod seems pointless But subjectively I am VERY happy.

    now to add the 750-1k screen resistors and the JSX bias circuit so I can try kt77s!

    what a pain having to drill the rivets on the power tube board!!!!!!!...I 'll post when I have a good fastening strategy for it.
    Last edited by tedmich; 08-27-2010, 04:57 PM.

  • #2
    Oh come on, 8 rivets, 1/8" drill bit, done in 2 minutes.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Telling tales out of school...

      First time KB took that board out of a 5150, he left all the sockets riveted on the chassis and unsoldered all those long socket leads. Then had to figure out how to get them all back in the holes to resolder when he was done. 8 rivets all of a sudden was a blessing.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        drill and rivet every time? Well I guess the board needs to come out very infrequently...

        I'd drill out the tube socket rivet holes to 3/16" and add some PEM S-632-0 clinch nuts if I ruled the world,

        as i hate rivets

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