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Thread: Do leaky tone caps effect tone?

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    Supporting Member Randall's Avatar
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    Do leaky tone caps effect tone?

    I have had two amps in recently with one leaky tone stack cap, causing dc on some pots, and scratchiness. Replacing them resolved the issue, but I am wondering if and how this may effect tone? One of the amps went home last week, and I got a text saying that it sounded fantastic, so I wonder if replacing the cap had something to do with that. Seems like having 3 - 4 volts sitting on the preamp signal can't help.

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    It's weird, because it WAS working fine.....

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    Supporting Member The Dude's Avatar
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    Of course leaky caps effect tone. A leaky cap is no longer a cap. It's a resistor. You can't expect the circuit to work correctly.

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    Bent Member Chuck H's Avatar
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    Consider that following any voltage division in the tone stack whatever DC remains is on the following stages grid. Since preamp tubes typically bias under 2V it doesn't take much to throw it off.

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    Senior Member mhuss's Avatar
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    In any case, it's definitely not working as the designer intended!

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Chuck H View Post
    Consider that following any voltage division in the tone stack whatever DC remains is on the following stages grid. Since preamp tubes typically bias under 2V it doesn't take much to throw it off.
    True 'nuff & let's also consider stages where there is no tone stack - output tube grid drives, input to splitter/inverter, reverb mix, etc. NONE of them like to be misbiased. So - stop the leakers & enjoy good tone, the way it was meant to be. Also, leaky caps have a tendency to not leak at a steady rate IOW they create noises we don't want to hear. Often, but not always.

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    Yeah, if you wanna look at in terms of percentages, takeca typical triode stage that wants to be biased at -1to2 volts... Add a leaky coupling cap at +1to2 volts, thatsa pretty hefty misbias. Not only affects tone, but can blow up some stuff, probably a minute or so right after it sounded better than ever! At least, that's the way Jack Darr explained it in 1972.

    Kinda the effect of biasing your power tubes at 150-200% dissipation...

    Justin

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Justin Thomas View Post
    Yeah, if you wanna look at in terms of percentages, takeca typical triode stage that wants to be biased at -1to2 volts... Add a leaky coupling cap at +1to2 volts, thatsa pretty hefty misbias. Not only affects tone, but can blow up some stuff, probably a minute or so right after it sounded better than ever! At least, that's the way Jack Darr explained it in 1972.

    Kinda the effect of biasing your power tubes at 150-200% dissipation...

    Justin
    With a large value resistor standing in the way, in the plate circuit, probably won't wreck most preamp tubes. Nonetheless, they're best operated where they expect to be.

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    Supporting Member Randall's Avatar
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    I suppose I already figured it would effect tone somehow, but I was really wondering if it is something that could be identified, like the scratchy pot clue. Or in other words, does a leaky preamp cap have an identifiable characteristic sound?

    Or should I just shut up and go lick doorknobs?

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    It's a bad time to be licking doorknobs!

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    does a leaky preamp cap have an identifiable characteristic sound?

    Or should I just shut up and go lick doorknobs?
    Geeze, I wouldn't suggest that, even without filthy dog germs present everywhere!

    Bad caps can emulate all the noises you'd think a bad tube can make - squeeks, pops, outer-space noises - just about everything but hum. And the effects on tone can vary quite a bit, from sounding thin & weedy to dull & lackluster. Also splodgy, a sort of annoying indistinctness.

    Now let the dog lick doorknobs. Or kiss Lucy...

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