I remember posts where Zaphod Phil explained that increasing the value of the power tube's bypass cap from 125R to 1000R would tighten the bass. But I don't understand why.
ZP: could you please explain the technical reason behind this ?
thanks
power stage bypass cap
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tinisuta
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zaphod_phil
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A 125R bypass cap?? 
Anyway, the reason you can get boomy bass with a cathode-biased amp is that the bias voltage at the cathode gets modulated by large bass notes. However, using a very large cathode cap (1000uF to 2200uF) forces the cathode voltage to hold itself much more constant and only change slowly with the amplified signal. Therefore the bias voltage doesn't get modulated by bass notes, and follows the overall envelope of the signal instead, which is what it really should be doing. It also helps get rid of transient spikes in the bias voltage which can make the distortion tone sound rough or splattery. The amp will feel a little stiffer than before, and a little more like a fixed-bias amp, but without losing the character of a cathode-biased amp.
I also generally recommend a 180 ohm cathode resistor when using the very large cathode caps, as the power tubes will tend to otherwise run a little warmer on average.
HTH
Anyway, the reason you can get boomy bass with a cathode-biased amp is that the bias voltage at the cathode gets modulated by large bass notes. However, using a very large cathode cap (1000uF to 2200uF) forces the cathode voltage to hold itself much more constant and only change slowly with the amplified signal. Therefore the bias voltage doesn't get modulated by bass notes, and follows the overall envelope of the signal instead, which is what it really should be doing. It also helps get rid of transient spikes in the bias voltage which can make the distortion tone sound rough or splattery. The amp will feel a little stiffer than before, and a little more like a fixed-bias amp, but without losing the character of a cathode-biased amp.
I also generally recommend a 180 ohm cathode resistor when using the very large cathode caps, as the power tubes will tend to otherwise run a little warmer on average.
HTH
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tinisuta
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silly me, I was refering to the bypass cap in // with the biasing resistor of 125R 
if understood:
- with no bypass cap, the grid voltage modulates the anode current, which flows into the cathode resistor. Therefore the cathode voltage is also modulated, which creates cathode-current feedback
- with a bypass cap, the cathode voltage gets 'stabilized', in the sense that this forms a sort of low-pass filter. Input signals above the cut-off frequ do not modulate the biasing voltage anymore. The larger the cap value, the lower the cut-off frequency
Now, the bypass cap, in many designs, is around 100µF. Moving to 1000 or 2200µF is a huge step. Is there another reason than the above to axplain why using such a huge value ?
rgds,
if understood:
- with no bypass cap, the grid voltage modulates the anode current, which flows into the cathode resistor. Therefore the cathode voltage is also modulated, which creates cathode-current feedback
- with a bypass cap, the cathode voltage gets 'stabilized', in the sense that this forms a sort of low-pass filter. Input signals above the cut-off frequ do not modulate the biasing voltage anymore. The larger the cap value, the lower the cut-off frequency
Now, the bypass cap, in many designs, is around 100µF. Moving to 1000 or 2200µF is a huge step. Is there another reason than the above to axplain why using such a huge value ?
rgds,
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trobbins
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tinisuta
Don't forget to add to your list of influences that the voltage on the capacitor doesn't just act as a filter to smooth the influence of signals, but also acts in a DC magnitude sense. As the average current in a large cap increases, the DC voltage increases, and that then moves the PP operating locus (eg. the idle point changes to 'hotter' or colder biasing).
Don't forget to add to your list of influences that the voltage on the capacitor doesn't just act as a filter to smooth the influence of signals, but also acts in a DC magnitude sense. As the average current in a large cap increases, the DC voltage increases, and that then moves the PP operating locus (eg. the idle point changes to 'hotter' or colder biasing).
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