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

  1. #36
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    Thank you.
    I will get the gain pot out to measure it with my multimeter.
    Probably it is a commercial log pot.
    I really appreciate the help everyone is giving with this thread, and all the link provided.

    =)

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  2. #37
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    This is the way I understood the instructions.


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  3. #38
    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by spunko View Post
    ...
    Approximately
    View Marshall Clean / Crunch circuit, and analyze the signal path of the part schematics V2a / V2b (switch SW5)

    https://drtube.com/schematics/marshall/6100-61-04%20iss9.pdf
    Clean/Crunch circuit

    https://drtube.com/library/schematics/69-marshall-schemas#AN30
    Marshall 6100

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  4. #39
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    Quote Originally Posted by vintagekiki View Post
    Approximately
    View Marshall Clean / Crunch circuit, and analyze the signal path of the part schematics V2a / V2b (switch SW5)

    https://drtube.com/schematics/marshall/6100-61-04%20iss9.pdf
    Clean/Crunch circuit

    https://drtube.com/library/schematics/69-marshall-schemas#AN30
    Marshall 6100
    I haven't tried this solution in the amp yet.
    I have looked at the marshall circuit, but I don't understand what I need to get this mod right.
    I have followed the signal path (I think) and it is basically the same as I have understood, my AC signal comes amplified from the plate of A (1st half) and gets inyected into greed of the second half to get amplified and goes out from the plate of the second half. That would be the normal path.
    With the mod, the singal bypasses the second half of the tube and goes to the second tube.
    The marshall switch circuit is more complex for my understanding, I don't get why it feeds the signal back to the grid of the second half, if the intention was to bypass that half.


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    Last edited by spunko; 07-16-2019 at 06:37 PM.

  5. #40
    Old Timer J M Fahey's Avatar
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    Have you at least tried the simple solution I suggested?

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    Juan Manuel Fahey

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    Quote Originally Posted by spunko View Post
    How can I mod it to clean the sound, or lower the gain?
    I'd try connecting a resistor in parallel with the gain pot VR6
    Gain reduction would be 15dB using 100k, 20dB using 56k.

    Both sides of the pot are at ground potential so there shouldn't be any pops when switching the resistor. Tone should be unchanged unlike when switching out the cathode bypass caps.

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    Last edited by Dave H; 07-16-2019 at 09:43 PM.

  7. #42
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    Quote Originally Posted by J M Fahey View Post
    Have you at least tried the simple solution I suggested?
    Too easy

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  8. #43
    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by spunko View Post
    ...
    The Clean / Crunch schematics is as an idea of how the signal path is going.
    Tube V2b gives gain for crunch.
    Schematics for Crunch is your personal choice, it is important is select choice Clean / Crunch. In Clean mode g1 V2b (crunch tube) via SW5a and R24 binds to ground so that V2b does not oscillate.

    Click image for larger version. 

Name:	Clean & Crunch.jpg 
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ID:	54354

    Path Signal for Clean (Red)
    Path Signal for Crunch (Green)
    Choice Clean/ Crunch (Blue)

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  9. #44
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    I'm sorry for the late reply, I was out of town.
    Thank you for the help. As I understood, it is necessary that 2b goes to ground via a resistor to avoid oscillation.

    is this ok?


    Also, I'm going to try Fahey solution.

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  10. #45
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    **Edit: I was asking for help in this post, because the amp started to oscillate, even at stock (no mods). Sorry for the double post

    It turns out I forgot to clean the flux from the PCB and it got conductive. Just after I asked for help, I cleaned it and the oscilation stoped.

    What do you think about the diagram above?

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    Last edited by spunko; 07-21-2019 at 05:55 PM.

  11. #46
    Senior Member vintagekiki's Avatar
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    Jet city JCA20H mod

    Quote Originally Posted by spunko View Post
    What do you think about the diagram above?
    Click image for larger version. 

Name:	JCA20H cc mod.jpg 
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ID:	54413

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  12. #47
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    Quote Originally Posted by vintagekiki View Post
    Click image for larger version. 

Name:	JCA20H cc mod.jpg 
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    Thank you very much, that is awesome!!

    One question, adding those resistors (1M and 2M2) wouldn't change the behavior or sound of the amp even in the crunch/normal mode??

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  13. #48
    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by spunko View Post
    One question, adding those resistors (1M and 2M2) wouldn't change the behavior or sound of the amp even in the crunch/normal mode??
    Resistors 1M and 2M2 do not affect the sound in any one mode.
    They generally serve to reduce the "click" that may possibly occur when switching SW1 from clean to crunch and vice versa.

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  14. #49
    Don't forget the joker g1's Avatar
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    Quote Originally Posted by vintagekiki View Post
    Resistors 1M and 2M2 do not affect the sound in any one mode.
    They generally serve to reduce the "click" that may possibly occur when switching SW1 from clean to crunch and vice versa.
    Why the change from 330K to 1M, is it so there is less attenuation on clean setting?

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    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  15. #50
    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by g1 View Post
    Why the change from 330K to 1M, is it so there is less attenuation on clean setting?
    The clean / crunch mode does not change the schematics JCA20H.
    Only added SW1 (clean / crunch switch) and "anticlick" resistors 1M and 2M2 .
    Resistors 1M and 2M2 do not participate in attenuation of the signal.
    1M provides polarization of g1 (V1a), and 2M2 allows "emptying" C24 when SW1b is in clean mode.

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  16. #51
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    Thank you very much for the help.
    I tried vintage kiki diagram and so far so good.
    I'll give you an update in a couple of days.

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  17. #52
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    Hello friends.

    An update to this thread. I tried J M Fahey in my first mod and it worked really well, but I ended up mixing it with another suggestion from another forum. (If you see my first diagrams) and the clean mod sounded really good, but when switched to gain/normal, and if you strum a chord, at the end of the trail the volume faded down and up again. This is the reason why I tried a different aproach with the suggestion I got from Dr. Olaf Krampe by email.
    Whe I switched back to normal, no mods at all, the amp stil behabed with the fade down/up problem, and it turned out to be flux residiu on the PCB that I didn't clean.
    Perhaps my first attemp would worked well if I had cleanned that flux residiu.

    Well, I tried the second mod suggested by Dr. Olaf, and improved by vintagekiki, and it is working great. It sounds really good.
    It drops the volumen quite a bit, but not that much to bother, and with the improved headroom, it sounds good and interesting. Like if its another amp.

    It would be awesome if we could still keep this thread alive, improving or changing the mod.

    *Edit:
    This is how I hooked the mod to the switch.
    Sorry vintage kiki for butchering your diagram.
    Click image for larger version. 

Name:	JCA20H cc mod.jpg 
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    Last edited by spunko; 07-28-2019 at 05:07 PM.

  18. #53
    Old Timer J M Fahey's Avatar
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    Oh, you can keep it alive , just test other options and post results.

    Glad you found a solution you like

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    Juan Manuel Fahey

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