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Thread: 50 watt Marshall build w/ Depth control problem

  1. #1
    Woodgrinder/Pickupwinder copperheadroads's Avatar
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    50 watt Marshall build w/ Depth control problem

    I have recently slowly finished my 50 watt jcm 800 build all is good & everything is working as it should i guess But I've added a Depth/Resonance control which is basically a 1 meg pot with a 4.7nf cap from the 100k negative feedback resistor then out to speaker out . but all that happens when i turn the depth up is the volume drops & may get a little more gain ,so i use it with the depth control turned off ,
    Maybe be related to this problem & maybe not , but for a 50 watt amp this not extremely loud & I wouldn't say it's any louder than my 20 watters .Now it can still make you ears ring ..
    I've done this mod on a couple other amps & i worked as it suppose to .. I wonder whats up with this
    "UP here in the Canada we shoot things we don't understand"

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    Quote Originally Posted by copperheadroads View Post
    I have recently slowly finished my 50 watt jcm 800 build all is good & everything is working as it should i guess But I've added a Depth/Resonance control which is basically a 1 meg pot with a 4.7nf cap from the 100k negative feedback resistor then out to speaker out . but all that happens when i turn the depth up is the volume drops & may get a little more gain ,so i use it with the depth control turned off ,
    Maybe be related to this problem & maybe not , but for a 50 watt amp this not extremely loud & I wouldn't say it's any louder than my 20 watters .Now it can still make you ears ring ..
    I've done this mod on a couple other amps & i worked as it suppose to .. I wonder whats up with this
    So put it on a load and input a signal and see what wattage your actually getting. Could be a problem or could be inefficient speakers.

    nosaj
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    Quote Originally Posted by copperheadroads View Post
    .. I've added a Depth/Resonance control which is basically a 1 meg pot with a 4.7nf cap from the 100k negative feedback resistor then out to speaker out . but all that happens when i turn the depth up is the volume drops...
    Maybe your feedback loop is positive rather than negative?
    Check whether the power amp gain is higher or lower with the loop open.

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    Woodgrinder/Pickupwinder copperheadroads's Avatar
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    Quote Originally Posted by pdf64 View Post
    Maybe your feedback loop is positive rather than negative?
    Check whether the power amp gain is higher or lower with the loop open.
    I had to reverse the primary's on the output transformer to stop the feedback prior to this problem .
    & loop open ? disconnect the wire from the impedance selector ?
    "UP here in the Canada we shoot things we don't understand"

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    Quote Originally Posted by copperheadroads View Post
    I had to reverse the primary's on the output transformer to stop the feedback prior to this problem .
    & loop open ? disconnect the wire from the impedance selector ?
    Check how much output you have, then you'll know where your at, since you stated it sounded less than 50 which is subjective.
    If you car was going slower than anticipated would you start changing stuff or would you look at the speedometer? Same theory.

    nosaj

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    Senior Member Enzo's Avatar
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    If your 50 watt were only putting out 25 watts, it would only be a 3db loss.
    Education is what you're left with after you have forgotten what you have learned.

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    Woodgrinder/Pickupwinder copperheadroads's Avatar
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    I'm not entirely sure how to check the output of the amp .I haven't really had time to dig in .how to check how much output
    ..I tried a little research on what method & found out it is also a subjective topic .
    So please , what should i do ?
    "UP here in the Canada we shoot things we don't understand"

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    Quote Originally Posted by copperheadroads View Post
    I'm not entirely sure how to check the output of the amp .I haven't really had time to dig in .how to check how much output
    ..I tried a little research on what method & found out it is also a subjective topic .
    So please , what should i do ?
    There's nothing really subjective to calculating the real power, the subjective parts is from the marketing guys.
    I use a signal generator HP200cd input 1k signal at .5vac( the .5 vac I use cause it's roughly equivalent to a guitar input signal)into one of the amp inputs. On the speaker jack I have a 8ohm load( big resistor array). I use a scope on the input and one on the output so that I can see the amp passing a clean signal on the output as I turn it up before clipping. I have a meter on the output jack also measuring VAC.

    Once I get VAC into a 8ohm resistor I use ohms law for example if I get 25volts into 8hms would give 78 watts.
    25 squared divided by 8ohms = 78.1watts

    If you want just rough estimates you can skip the scope if you don't have one and just max the vol to check full output with clipping.

    nosaj

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    Woodgrinder/Pickupwinder copperheadroads's Avatar
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    Ok so basically crank the amp & play a bit with the looper ,with the meter on the 8 ohm jack across the tip & sleave measuring VAC ..After that I will have to ?
    do the math
    Voltage Squared (VAC x itself ) divide by 8 ohm ? = amp wattage
    "UP here in the Canada we shoot things we don't understand"

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    Quote Originally Posted by copperheadroads View Post
    Ok so basically crank the amp & play a bit with the looper ,with the meter on the 8 ohm jack across the tip & sleave measuring VAC ..After that I will have to ?
    do the math
    Voltage Squared (VAC x itself ) divide by 8 ohm ? = amp wattage
    If you have a cell phone that can do apps you could download a signal generator. You want a steady continuous sine wave 1khz at .5vac. The 8 ohm jack needs to have an 8 ohm dummy load attached.
    Then you can measure the output.

    nosaj
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    Woodgrinder/Pickupwinder copperheadroads's Avatar
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    I know it needs a load ,I have 100W 8 ohm resistors on the way (in the mail) BUT I will do the speaker test .As I'm not afraid of my Neighbors . or consequences or
    repercussions
    "UP here in the Canada we shoot things we don't understand"

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    Supporting Member Jazz P Bass's Avatar
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    [QUOTE=nosaj;453208]If you have a cell phone that can do apps you could download a signal generator. You want a steady continuous sine wave 1khz at .5vac. The 8 ohm jack needs to have an 8 ohm dummy load attached.
    Then you can measure the output/QUOTE]

    Personally, for testing purposes, I would not go above .100Vac (100millivolts) for the input signal.

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    [QUOTE=Jazz P Bass;453212]
    Quote Originally Posted by nosaj View Post
    If you have a cell phone that can do apps you could download a signal generator. You want a steady continuous sine wave 1khz at .5vac. The 8 ohm jack needs to have an 8 ohm dummy load attached.
    Then you can measure the output/QUOTE]

    Personally, for testing purposes, I would not go above .100Vac (100millivolts) for the input signal.
    I think I had chosen .5 because that was an average for humbuckers. (I think) Is .1 vac equivalent to single coils?

    Thanks,
    nosaj

    A 1kz tone in a big amp( I think 50 watts is big). Is pretty annoying even with a champ amp.

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    Reading your 1st post it sounds to me that you may have a simple wiring error.
    Have you got the 1M pot + 4n7 cap connected to the right end of the 100K feedback resistor. Check that with the pot at zero ohms end you still have 100K in the feedback path.
    The schematic trace I did for the YETI50 may help:
    Here:
    YETI50 Set to Work
    Note that the feedback resistor is reduced to 39K to give the depth/resonance control more effect.
    Cheers,
    Ian

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    Woodgrinder/Pickupwinder copperheadroads's Avatar
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    Thanks for all the Suggestions ,I will get back at it in a little bit when i find some time
    "UP here in the Canada we shoot things we don't understand"

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    Woodgrinder/Pickupwinder copperheadroads's Avatar
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    Thanks for all the Suggestions ,I will get back at it in a little bit when i find some time
    "UP here in the Canada we shoot things we don't understand"

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