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Hacking a Jet City Jca2112rc

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  • Hacking a Jet City Jca2112rc

    Hey,
    So i have a Jet City Jca2112rc and recently started to mod it. I did some mods but then i fcked up somewhere and saw smoke coming from the PT, so i turn it off. Well i took it to a tech and he fixed it, luckily neither of the transformers/tubes were blown. So when i took it back i noticed 2 strange things.
    The master volume became overly responsive, to get at the same level as 2,5 on the MV pre-mods i have to turn it only to like 0,04-0,05 now, and it increases just like that afterwards(i've only tried it to ~3 on the "new" MV, didnt go higher, cause i guess i might overstress the tubes or the transformer or something).
    And the presence control is at maximum(9/9) right now, and it doesnt sound shrill at all here(9/9 on presence pre mods was hell). In fact if i turn it counter-clockwise it gets shrill, then after like 1/15 turn it unleashes this high pitched squeal at maximum master volume(much like a feedback, but it happens even if the guitar isnt plugged in).It stops if i turn it to maximum again.
    And all the mods i dont were in the preamp and i havent touched the master volume and the presence control, just got more gain out of it and changed the voicing.
    Any idea what the problem can be?

  • #2
    Maybe the negative feedback loop on the power amp has opened up (due to resistor burning out). Got a schematic?
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      What exactly did you mod in the amp? Almost sounds like the problem when the OT leads are reversed causing the NFB to become positive.
      "In theory, there is no difference between theory and practice. In practice there is."
      - Yogi Berra

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      • #4
        Click image for larger version

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        JCA20H service schematic.pdf
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        Well my amp is a jet city jca20h with a reverb, but the jca schematic isnt as close as the astroverb. And JCA20H_mod_1,1 is the basis for my mods.

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        • #5
          well i dont know if the tech reversed the transformer leads, but i didnt do it certainly.
          Here are some schematics, and JCA20H_mod_1,1 is the basis for my mods.
          Click image for larger version

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ID:	820063JCA20H service schematic.pdfClick image for larger version

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          • #6
            "it unleashes this high pitched squeal at maximum master volume" Sounds like an oscillation! Best to go back to the original design and get your feel on firm soil.

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            • #7
              It sure sounds like the output transformer primary leads where reversed.
              Hint #1: Presense control is reversed.
              Hint #2: It squeals.
              The best test for this condition is to run the amp into a dummy load at a set output amplitude.
              Then the feedback resistor is disconnected.
              If the amplitude rises, then you have a negative feedback condition, which is correct.
              If the amplitude goes lower, then that is positve feedback, which is incorrect.

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              • #8
                Thanks guys, but i somehow fixed it. If i remember correctly then one of the speaker jacks connected with the ground via chassis(cause i cant take out the pcb without unsoldering the transformer leads, so it just hangs there). But i've gone far from that, now made a kind of jcm800 clone out of the amp, sounds good!

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                • #9
                  Not enough b+

                  Hey, so i have been modding my amp(jet city jca2112rc) for like 5 months(and devoting a lot of time into it). I have modded the preamp for gain, then made it a jcm800 clone and modded a lot, then a bogner xtc red channel clone, and recently a carvin legacy clone. The problem i encountered was that per carvin schematic it had 385 b+ on the preamp tubes. The amp is el84 based so the max i can get is 330v, and with 330v preamp tube b+ and the schematic values i get 100,124v plate voltage on v1a and v1b when they should be 165-180v. As a result(at least i think so) i have insane amount of gain and the amp is oscillating on higher gain settings. So i though about lowering the anode resistor from 220k to 100k on both stages and got ~177-184 plate voltage and the amp got a bit brittler sounding and sounded more midsy and pushy(in a good way), BUT the oscillations were huge. I cant even play the lower gain settings now.
                  I really have no idea how to cure this. Any advice would be helpful i guess.

                  Edit:And my guitar volume pot started sounding really scratchy.
                  Last edited by shreditup; 04-12-2011, 04:29 PM.

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                  • #10
                    you won't be able to get 385 b+ unless you switch out the transformer.

                    The most under load is going to be about 360 on the plates which is what I run mine at.

                    In order to raise the b+ on the preamp tubes you'll have to change the dropping resistors.

                    The JCA stock uses something like 10k for each stage. I get about 345/350 on the first preamp tube using 840 ohm power resistors.

                    I've got 4 preamp tubes in mine though so you'll probably want to use a bit more, around 1k or 1.5k.

                    Use 2 watt or more resistors for those.

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                    • #11
                      Well to get those 330v i took out the dropping resistors completely(and put a wire in their place). And do you have a jca20 or a jca2112rc because i even measured the place right after the diode bridge(before any kind of dropping resistors, if there are any left lol) and got 332, not sure how you got 345-350 there tbh.

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                      • #12
                        You need dropping resistors for filtering or else your amp will oscillate.

                        I've got a jca20 head.

                        Measured right after the rectifier with no load at 380vdc. Rectified with 4 soft recovery diodes.

                        Your transformer should have a sticker on it saying 270-0-270 as secondary output. Thats 380 no load after rectification

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                        • #13
                          fuse- 100v-120v fuse 120v
                          output: 266v 150ma
                          27vac 100ma
                          6.3v 2000ma

                          thats the pt. Anything of interest?
                          And what about those dropping resistors? I still have those oscillations at med-high gain, so i'll consider any variants. Where and what resistance do i need to put them?

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                          • #14
                            The pt should do 370-380 unloaded.

                            The dropping resistors should be r31 - 10k, r49, r45, and r3

                            you can reduce those to whatever you need.

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                            • #15
                              so i have been modding my amp(jet city jca2112rc) for like 5 months(and devoting a lot of time into it). I have modded the preamp for gain, then made it a jcm800 clone and modded a lot, then a bogner xtc red channel clone, and recently a carvin legacy clone.
                              i have insane amount of gain and the amp is oscillating on higher gain settings.
                              BUT the oscillations were huge. I cant even play the lower gain settings now.
                              I really have no idea how to cure this. Any advice would be helpful i guess.
                              I guess your amp suffers a *severe* case of mod-itis.
                              "low B+" is the least of your problems.
                              The illness is so advanced I can't even offer a solution, besides scrapping it and buying a "real" Bogner or Carvin or whichever amp catches your fancy.
                              Following faithfully a schematic and getting the "right" parts is only half the journey; the other half lies in layout, grounding, etc. , which might not be physically possible inside the Jet City chassis.
                              Sorry.
                              Juan Manuel Fahey

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