"Presence" pot in J5

  • In J5Mk1, switch SW5 "harshness filter" looks like a on/off "presence" control. I usually prefer to have this switch off, but sometimes I think it would be fine if it was adjustable with a pot, as it was a kind of presence control. So, do you think that replacing that switch with a pot 1 MOhm log will work that way? Or perhaps it has to be connected otherwise?


    Best regards
    Alvaro

  • Re: "Presence" pot in J5


    Hi Alvaro,
    first you should replace C14 by 2x 4,7nF in series and place the variable resistor in between the two caps (otherwise you will have DC at the variable resistor). I would try a 100K, 1 Meg is too much.
    Regards,
    Martin

  • Re: "Presence" pot in J5


    Quote from MAD

    first you should replace C14 by 2x 4,7nF in series


    Thank you Martin for that advice.


    Replacing C14 (10nF) for 2x4,7nF in series, wouldn't reduce the effective value to one half? Shouldn't I use 2x22nF instead? So the resulting value of both in series would be similar to the original C14, or am I missing something?


    By the way, the sound of this amp is totally addictive!


    Best regards
    Alvaro

  • Re: "Presence" pot in J5


    I just tested this mod with a 47K log pot I have at hand. In order to better hear any tone differences I injected white noise from a CD into the input of the amp. Also I inserted a switch that allowed me to compare the original switch-off sound with the one from the pot at 47K. I would expect that both sounds where nearly identical, but I found that there is a subtle but definitive loss of high frequencies even when the pot is at maximum 47K. And those high frequency harmonics are responsible of that chime tone I like in clean and slightly crunchy sounds: I do not want to lose them ;D


    So that I have two options: to use a 250K pot, for example, with the hope of not losing high frecuencies or to return to the original configuration with the switch. The problem of the 250K pot is that surely It will be too unresponsive for the most of its turning movement and then perhaps it will be not worth at all that mod.


    Best regards
    Alvaro

  • Re: "Presence" pot in J5


    Hi Alvaro,
    I think that's a misunderstanding, the 100K pot wouldn't allow you to disable the function, it's only for changing the sound. If you want to completely disable the sound change, you need to turn it off with a switch :(
    Regards,
    Martin

  • Re: "Presence" pot in J5


    Finally I found a simple way to maintain both on/off and a potentiometer control of high frequencies roll-off :).
    I use a 250K Fender "no load" pot, which is used for tone control of guitars, between these two 22nF condensers early mentioned. It is a 250K log which disconnect the circuitry when it is fully clock wise at "ten". So having it at "ten" is just like the switch off, and one it is rotated you can adjust the tone between cero to ten.
    So, although it is not really a "presence", it is similar regarding the tone shaping. Thanks!

  • Re: "Presence" pot in J5



    I usually set this switch at "ten" position (off) because this usually produce the best clean sound for my guitar. But sometimes, depending of the position of the amp, room type, etc, I would like to adjust it with a bit of less bright tone. With the original switch it sometimes eliminated too much top-end tone, so neither position was good for me. This is the purpose of this mod.


    I agree that the effect is not dramatic, but noticeable if your speaker has a good top end reproduction and you turn quickly the pot. If you want even more amount of top-end reduction with this pot, it also possible to reduce the vales of R18-19.


    Best regards
    Alvaro

  • Re: "Presence" pot in J5



    Right, its R17/R18! My mistake.
    Yes MAD, you exactly described what I did and the resistance values are just these. So I have one potentiometer more in the front panel.


    Best regards


    ALvaro

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