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Thread: Input Capacitor??

  1. #1
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    Input Capacitor??

    This is the input of a Jet City JCA50. There's a 47uf cap in series with the guitar input to the 1st grid. What is the effect of this cap? I've never seen this before.

    38629430806_d106ea3b6c_c.jpg

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  3. #3
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    Thanks. I should add that the schematic isn't completely accurate as the input jack is a Cliff style, so the tip is grounded without a guitar cord inserted. What would happen if I removed the cap and kept the Cliff Jack?

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    Regardless of that cap, shorting jack always grounds the tip; it prevents HF from entering the grid. So, without the cap it still does it's job by grounding R48/47 junction.

  5. #5
    Senior Member Enzo's Avatar
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    The cap blocks any DC that might be on the input signal.
    Education is what you're left with after you have forgotten what you have learned.

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    Doesn't this cap have an effect on tone or feel? Nobody uses this.

  7. #7
    Old Timer J M Fahey's Avatar
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    47uF in series with 1M is nonsensical high.
    Way more reasonable would be something between .0047 and .047uF .
    And definitely not an electrolytic; besides being too large, any possible DC leaking from, say, a pedal or guitar preamp would be positive relative to ground ... so that capacitor would be reverse biased, lossy, and definitely not doing what it would be supposed to do.
    MAYBE it´s a typo and original design required 45nF there.
    Dave H, g1, eschertron and 1 others like this.
    Juan Manuel Fahey

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    Thanks and no, they are certainly 47uf caps. There's one on the effects return as well.

    I compared this schematic to a Soldano Hotrod 50 which is what it's based from and they are nowhere to be found in that amp. I went ahead and plucked them, and jumpered the traces. The result is a more forward detailed, less bloated tone. I wonder if they did this to make the amp sound worse than the Soldano??

    Thanks fellas.


    http://schems.com/bmampscom/soldano/...100_hotrod.pdf
    Attached Thumbnails Attached Thumbnails ouegf.jpg  

  9. #9
    Senior Member Enzo's Avatar
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    No they did it to block DC. MANY amps have input caps.
    Education is what you're left with after you have forgotten what you have learned.

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    In this thread ^^ Enzo says "The input cap is there to keep DC off the tube grid. Imagine if you have some effect pedal that has a few volts of DC offset at the output. Now how will your amp react if the input stage now has +5v there? or -5v there? A largish value cap that passes signal to the grid but blocks DC, solves that problem."

    Then Merlin indirectly says "I find that hard to believe, considering all pedals have an output capacitor already! An extra cap at the input of the amp would simply reduce the effective coupling capacitance a tiny bit (assuming it has a large value)."

    Anyway, why would a pedal have a DC offset or how common is that? I never thought of a pedal passing anything to an amp other than an AC signal.

    the reason I ask is because if there is a DC offset it seems like that would add hum to the noisefloor as well as affect bias.

    So if some pedals cause hum in noisefloor to increase I guess an input cap would be good for that. But is that how it would actually work?

    I find pedals to typically increase hiss in noise floor rather than hum, but just wondering

  11. #11
    Senior Member Enzo's Avatar
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    I wouldn't say it is common, but they are out there. Cheap insurance. I don't expect 5 volts of DC, but half a volt? A lot more likely. And going right onto a grid it would certainly change the tube operating point.

    Is it necessary? No, Fender never used one for the last what 70 years? I was just explaining why someone WOULD put one there. I see the reason, whether or not someone thinks it is worth the effort is a separate question.
    Education is what you're left with after you have forgotten what you have learned.

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    but would it increase noise floor or just affect bias? or both?

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    Supporting Member The Dude's Avatar
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    Theoretically, it shouldn't do anything except block DC.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    would the DC offset from a pedal or whatnot, if not blocked by the input coupling cap, cause hum

  15. #15
    Old Timer J M Fahey's Avatar
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    No, but would cause bias shift.
    Considering typical first triode bias is between 1.2V and 2V something, anything above, say, 50 or 100mV will be quite noticeable.
    Full 2V or more will straight cause tube cutoff, either polarity, because plate will either be at ground or full rail voltage.

    Most pedals have an output cap for the very good reason that they are fed 9V single supply, so in principle output should idle around 4.5V , say between 2V and 6 or 7V , all way beyond what´s needed to cutoff or block next stage is left free to reach it.
    The Dude and nsubulysses like this.
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  16. #16
    Supporting Member loudthud's Avatar
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    Quote Originally Posted by Enzo View Post
    ... Is it necessary? No, Fender never used one for the last what 70 years?
    In the early 50's, Fender used grid leak bias which requires a cap.

    Example: http://el34world.com/charts/Schemati...DELUXE_5C3.pdf
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

  17. #17
    Senior Member Enzo's Avatar
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    You are right. When I wrote that I had in mind the conventional cathode biased triode stage common in Fender amps.
    Education is what you're left with after you have forgotten what you have learned.

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