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Power Amp Capacitor

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Tommy_G, Feb 21, 2012.

  1. Tommy_G

    Tommy_G Member

    Nov 10, 2007
    Looking at the circuit between the PI and the power tubes of a typical EL34 power amp:

    On the anode of the "top" triode to the anode of the "bottom" triode on the schematic of my amp is a 250 pf capacitor.

    Although the rest of the circuit is based loosely on a JTM45 circuit, the JTM45 only has a 47 pf capacitor bridging across these same points.

    First, what is the technical name and function of this capacitor?
    Does the tone get affected by changing the value and how does the tone change depending on selection.

    My amp sounds like a "tuba" and JTM's I've heard sound like "trumpets" by comparison. I'm trying to get to the bottom of that issue. I need more forward mids and cut. Will changing this capacitor to JTM45 specs be worthwhile in this regard?
  2. Keyser Soze

    Keyser Soze Member

    Dec 14, 2008
    Johnson city, TN
    It is a form of localized negative feedback - the output signal coming of off each of those plates is 180 degrees out of phase, so whatever passes through that cap gets attenuated from output.

    Reducing the value will push the bottom of the attenuated signal higher - so progressively more mids and treble.
  3. muffin

    muffin Member

    Jun 13, 2011
    I believe that is a snubbing capacitor. Its primary function is to filter out RF oscillations, which implies that it won't make much difference to your guitar sound. It probably wont hurt your amp to change it, but i doubt it will solve the trumpet/tuba problem.

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