How does a bright cap work?


I removed the bright cap on my 1974 Marshall 50 watt JMP. It makes the amp volume very usable all the way from 0 - 10 and sounds smoother. I understand that varying the value of the bright cap will change how much of that 'bark' is added to the sound. So, what exactly is going on? Is it bleeding low end out of the circuit while boosting high end? With the bright cap in, this amp would go into overdrive very quickly. May be cool to make the cap switchable but is there a way to know what value would sound good? Is it just trial and error? Also, would different types of caps change the tone when used as a bright cap?


a bright cap is a high pass filter.It simply allows more high frequencies to pass.Playing at home is fine but you may like it back playing in a band mix,or not.Depends.
A common value for a bright cap is 120pf.Higher values will be brighter up to a point and then the boost the volume and go the other way;that is,they pass less highs and add mids and allow wider lows to come through.However,with any bright cap as you turn the volume past about 6 or 7 the effect lessens to almost nothing.It only works with the volume up to that point and doesn't work any more.
Marshall also used a treble peaking circuit that consists of a cap and a resistor in series with the input.Some guys remove those too and use a 68k resistor in it's place.Experiment until you get what you want to hear.

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