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boosting highs/ adding presence control?

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  • boosting highs/ adding presence control?

    I repaired this amp, got it knowing it needed some attention. Its a Guyatone GA-2000 MKII. The speaker is an Altec 417-8h Series II.

    I'm not sure if the amp is working totally as it should in terms of tone. I mean the amp is quiet - I eliminated some ground/hum issues and replaced all the Electro caps - both on the Preamp board and the power board - except for the three 333uf caps seen in the schematic.

    My questions are: There is a 1M resistor that is used in conjunction with the Treble pot via a toggle switch. When the toggle is flipped, the treble seems to be boosted. Would adding a larger value resistor boost the highs even more.

    Second question. Any other tips to help add Presence? Could I add presence control? If so, how would I go about doing it. Perhaps the amp is just generally a dark sounding amp, but I'd like to know if I can improve the tone in any ways. Attached is the schematic, the resistor for the treble is outlined in yellow.

    Also, would increasing the resistance of the input resistors help in boosting the signal?


  • #2
    I recall a presence mod for a tremolux I did that was in the feedback loop and involved decressing the value of resister and putting a 2.5k to 5k varistor in the loop. It acted like a loudness control and worked really nice for low volumes.

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    • #3
      [QUOTE=beedoola;292444]

      My questions are: There is a 1M resistor that is used in conjunction with the Treble pot via a toggle switch. When the toggle is flipped, the treble seems to be boosted. Would adding a larger value resistor boost the highs even more.QUOTE]

      It is not the 1 meg resistor that is giving you the treble boost.
      It is the 240pf caps.
      With the switch closed the two caps are in parallel, so they add up there values.
      The 1 meg is most probably there to eliminate a pop when the switch is toggled.

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      • #4
        [QUOTE=Jazz P Bass;299603]
        Originally posted by beedoola View Post

        My questions are: There is a 1M resistor that is used in conjunction with the Treble pot via a toggle switch. When the toggle is flipped, the treble seems to be boosted. Would adding a larger value resistor boost the highs even more.QUOTE]

        It is not the 1 meg resistor that is giving you the treble boost.
        It is the 240pf caps.
        With the switch closed the two caps are in parallel, so they add up there values.
        The 1 meg is most probably there to eliminate a pop when the switch is toggled.
        Its a voltage isolating resistor and I agree. Caps are for high frequency pass and inductors are for low frequency pass.

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