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Jet city JCA20H mod - can't find schematics or instructions

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  • #16
    Links to Jet City Schematic

    Originally posted by spunko View Post
    Maybe someone has kept the schematics or info about it, and could share it.
    http://starin.info/Product%20Info/-%20-%20Archive/Jet%20City/

    http://starin.info/Product%20Info/-%20-%20Archive/Jet%20City/Manuals/
    Jet City manuals & schematics

    https://music-electronics-forum.com/showthread.php?t=21418
    Jet City JCA20 Mods

    http://valvesphere.blogspot.com/2012/01/jet-city-333-jca20h-hsg-mod-high.html
    Jet City 333 JCA20H HSG Mod (high saturated gain) with schematic circuit diagram.

    https://valvesphere.blogspot.com/2012/01/triode-pentode-switch-in-jca20h.html
    JCA20H with EL84 Triode / Pentode Switch and schematic circuit diagram.

    https://valvesphere.blogspot.com/search?q=Jet+City
    Jet City JC20H output harshness

    https://docs.google.com/document/d/15nDuz_whyORWFFBaNSn74szUwBzMu8kpZ-InX6-Poag/edit#heading=h.gg08ch9zpnsk
    Biasing your Jet City Amp
    Last edited by vintagekiki; 07-09-2019, 10:13 PM.
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

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    • #17
      This is the way I think it might work. Haven't tried on the amp yet.

      What do you think?

      Last edited by spunko; 07-10-2019, 07:14 AM.

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      • #18
        Thank you very much!!

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        • #19
          Originally posted by spunko View Post
          This is the way I think it might work. Haven't tried on the amp yet.

          What do you think?

          Sorry but no.
          C27 , C15 and the attenuation resistor need a pair of switch contacts each.
          Juan Manuel Fahey

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          • #20
            Why it wouldn't work?

            I have connected all the parts/mod into the switch, and into the amp and tested with my fluke (without power on) and it apparently works, or at least it does what I intended with that connections.

            1) the upper strip just opens/closes grounds of C27, C15 and the 150K attenuator.

            2) the lower strip puts 220K in parallel with R28 (I noticed I put 100k in my diagram, typo)

            The two strips don't have continuity between each other (neither the lugs, just when engage them).
            When I flip the switch, the two strips do what intended with the diagram.

            I haven' tryed with the amp powered on yet, and I want to make sure it is ok before the real test.

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            • #21
              Originally posted by spunko View Post
              Why it wouldn't work?
              Because ...
              Originally posted by J M Fahey View Post
              C27 , C15 and the attenuation resistor need a pair of switch contacts each.
              After all, if you're so sure in your design, why you not try it how it works.
              Who does not know and knows that he does not know - teach him Confucius)
              Who knows and does not know that he knows - wake him Confucius)

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              • #22
                *Edit: Sorry I didn'y understand the post at first. Bad english.

                All the caps are going to ground. Why they need a single a terminal for each? Sorry, I want to understand.


                Originally posted by spunko View Post
                I haven' tryed with the amp powered on yet, and I want to make sure it is ok before the real test.
                I hope someone gives a hand here and point my mistakes in the diagram. I want to understand the way it should work.

                This could help a lot of people, not just me.

                Also, don't want to fry the amp
                Last edited by spunko; 07-10-2019, 04:23 PM.

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                • #23
                  All the caps are going to ground.
                  That's okay. What happens when the switch is in a different position, when the caps C27 , C15 and resistor 150k are not at ground?
                  Attached Files
                  Who does not know and knows that he does not know - teach him Confucius)
                  Who knows and does not know that he knows - wake him Confucius)

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                  • #24
                    Originally posted by vintagekiki View Post
                    All the caps are going to ground.
                    That's okay. What happens when the switch is in a different position, when the caps C27 , C15 and resistor 150k are not at ground?
                    When the caps are not grounded, they seem to lower the gain. Also your question make me notice something wrong with my diagram.
                    The switch is supposed to lift from ground C27, C15, and "connect" to ground the 150k resistor so it could be in parallel with R33, but it is doing the opposite with the 150k and the way I connected it.

                    Originally posted by J M Fahey View Post
                    Sorry but no.
                    C27 , C15 and the attenuation resistor need a pair of switch contacts each.
                    Maybe that is why the response of JM Fahey
                    Last edited by spunko; 07-10-2019, 05:06 PM.

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                    • #25
                      Originally posted by spunko View Post
                      Also your question make me notice something wrong with my diagram.
                      Schematics that you have attached, do not match the description you provided.



                      useful links

                      http://www.guitarstudio.tv/documents/Designing-V-T-Amplifiers.pdf
                      Designing-V-T-Amplifiers

                      http://guitar-gear.ru/forum/index.php?app=core&module=attach&section=attach&attach_i d=14475
                      The Electric Guitar Amplifier Handbook

                      https://www.4shared.com/get/jmzf8Vxb/Designing_Tube_Preamps_For_Gui.html
                      Designing Tube Preamps for Guitar and Bass
                      Who does not know and knows that he does not know - teach him Confucius)
                      Who knows and does not know that he knows - wake him Confucius)

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                      • #26
                        So I edited again the diagram. I think this should work as I intended to:

                        Position 1 (normal):
                        - upper strip Grounds C27, C15 and R33 (330K)
                        - lower strip does nothing

                        Position 2 (clean):
                        - upper strip lift C27 and C15 from ground, and grounds 100k instead of R33
                        - lower strip puts 220K in parallel with R28

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                        • #27
                          When the caps and R33 are not grounded by the switch, they are connected together. That is a problem. You are creating circuit paths that should not be there.
                          You need a 4PDT on-off-on so that the parts do not interfere with each other when not grounded.
                          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                          • #28
                            In a nutshell, you are connecting Audio at the third tube cathode straight to the first tube one ... what could go wrong?

                            Imagine sticking the singerīs microphone against its own PA speaker cone and you end up with about the same.

                            Thatīs why you need separate switches so each element has its own.
                            Juan Manuel Fahey

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                            • #29
                              Good intentional advice

                              In order to practically do mod it is assumed that the user is minimal knows how amplifier works, what is expected from mod, and only at the end drawing schematics and practical realization on the amplifier.
                              With How can, What do you think, Why it wouldn't work, do not repair amplifier, and the way to moding is is far away.
                              In this Thread there is an experience of members who want to help beginners.

                              A lot of effort has been invested to find the links that deal with the Jet City JCA20H mode problem, and we are always at the beginning.
                              Dear Colleague, I see you have a lot of enthusiasm to deal with the amplifiers, please begin by hearing some of the people who have passed the same path as you, from the beginning to get to the experience they now have.
                              Who does not know and knows that he does not know - teach him Confucius)
                              Who knows and does not know that he knows - wake him Confucius)

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                              • #30
                                Thank you very much for all the hel and info provided.
                                I have checked some of the URL posted in here and downloaded the pdf files to read them all, it will take a while. Unfortunately some URL have broken links, but there is a LOT of useful info.

                                Also I have a question related about those URL articles, like this one:
                                https://valvesphere.blogspot.com/201...harshness.html

                                About the harshness he says: "The solution was with the power amp section. A 330pf capacitor across the 47K negative feedback (NFB) resistor marked R12 in the service schematic"

                                But it doesn't tell what type of capacitor, and if it goes in parallel or series.



                                Also I tried my diagram, and it didn't work well as expected.

                                The clean part sound great IMO, also a friend came to hear the amp, and he likes it a lot. We compared it with his "unmoded" head, which is the exact same model and tried to clean it like the "moded one" and it couldn't get that clean.
                                Also we noticed that the unmoded amp when trying to get clean, it lacks of bass; while the moded amp in clean sounds with good tone (despite what a member of another forum told, that the clean will sacrifice tone)

                                The "normal" or crunch does not work well, as all of you already know. I think is oscilating, dunno how to describe it well. If I hit a chord, it will sustain but in the end oscilates the trail. Sorry for my bad english.

                                I have another news too.
                                I wrote an email to Dr. Olaf Krampe, who helped a lot with these mods in those threads of 2009 or so. He kindly repplyed to my email with these instructions of how to clean it:

                                "Cut out the preamp gain pot and get a new 1M log one.

                                From middle of pot (reconnect left/right pins to the board, flipped because the pins are now upside down) run into a switch (on/on).
                                One side back to the middle of the pins on the PCB (the original channel)
                                The other side to R36. This way you create a cleaner channel with just two gain stages"


                                Unfortunattely, with my bad english I'm a little lost.
                                Does he mean I have to install the pot backwards?
                                The left lug to the rigth pcb hole, and rigth lug to left pcb hole?
                                Last edited by spunko; 07-11-2019, 05:06 PM.

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