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Thread: ripple question

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    ripple question

    Hello, can someone explain what means the ripple modulation in my second pic,please? That meant ripple voltage over first anode filter cap, looks like sort of modulation when draw a certain amount of current. Thanks.
    iddle
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    400mA
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    sorry for pic quality
    This pics was done at 400cps . Of course it looks worse at low freq. and very smooth going up. It is a sign for bad cap ? Looks like it not decouple enough at low freq.? should be worried about, please ? Thanks

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    Last edited by catalin gramada; 09-28-2017 at 08:24 PM.
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    Is that with an input signal? It looks like the normal modulation you'd get with a fixed frequency input signal of 400Hz. See LTSpice sim below.

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    Hi. Yes 400hz input signal was applied. The "modulation" start to be visible from 300mA up. I just wonder if is a sign for a bad cap or not enough decoupling capacitance...? or it just looks fine enough ..? Thanks

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    It looks OK to me. The sim was with a 'perfect' cap.

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    Supporting Member loudthud's Avatar
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    The lower the frequency that the amp is outputting, the bigger the modulation you see on B+. More capacitance will reduce the modulation, but also interacts with how bass notes sound and feel. A bass amp needs more capacitance than a guitar amp or it will sound too mushy on low notes. It's worth experimenting with different caps to see what you like.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Hi. The cap is large enough to keep topped up the voltage even at max power but I.ll keep that, to try to suprime as much considering how it works in tone effect. Thanks

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    Last edited by catalin gramada; 09-29-2017 at 05:20 AM.
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    Supporting Member tubeswell's Avatar
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    Quote Originally Posted by loudthud View Post
    The lower the frequency that the amp is outputting, the bigger the modulation you see on B+. More capacitance will reduce the modulation, but also interacts with how bass notes sound and feel. A bass amp needs more capacitance than a guitar amp or it will sound too mushy on low notes. It's worth experimenting with different caps to see what you like.
    +1.

    A decent amount of capacitance on the smoothing cap supply node in Pii filters where the smoothing cap directly feeds the screen supply, will help prevent screen current feedback of LF even order harmonics when the amp is dimed. Getting the rolloff under 10Hz is beneficial, getting under 5Hz is awesome. This is why some amps have higher value smoothing caps.

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    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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    Senior Member Old Tele man's Avatar
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    Yep, see these old (02-23-2009) postings on this subject: Using large value screen resistors w/ bypass caps

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    what kind of amp?

    look at one of the output tube plates of a push pull amp at the onset of clipping and you will see the waveform dance all over the place at 120 Hz. this gets cancelled in the OPT so you do not need a huge filter to swamp this out.

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