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previous: tubegirl I'm just intrigued as to what the e... -- 10/4/1999 2:14 AM

Re: Excessive Henries.

10/4/1999 6:22 AM
Randall Aiken	Re: Excessive Henries. Most guitar amps use capacitor input filters as opposed to choke input filters, so the effect of a large inductance on dynamic response is not as great. If the plate supply were taken off after the choke instead of before, the dynamic response would be affected more. The preamp stage current draw is relatively small and relatively constant in comparison to the output stage current draw. Since a push-pull output stage has the benefit of inherent common-mode hum rejection, and the plate current in a pentode is not affected much by plate voltage variations, a well-filtered plate supply is not needed. The screen voltage, however, has a much greater effect on the plate current, and therefore must be well-filtered, so it is usually tapped off after the choke. This allows the manufacturer to use a much smaller choke than would be needed if the entire plate current of the amp had to flow through it. To answer your question, a larger inductance will result in a cleaner DC supply, but will also change the transient response of filter network. Usually, the reason for going to an inductor is to get the extra filtering without the DC voltage drop of a resistor. However, if, as in your example, you replace the resistor with a choke of the same DC resistance, it will have the same voltage drop, but the transient characteristics will change. The purely resistive filter will generally rise with no overshoot, but the large inductance may tend to make the supply overshoot or ring on transients, depending upon the size of the filter caps in relation to the inductance value (it will be underdamped, overdamped, or critically damped). However, as pointed out before, the preamp current draw is fairly constant, so there won't likely be any transients except at power up time. Because the preamp supply is relatively constant, the extra filtering won't make that much difference in touch response. This is why some amps get by with no choke, only a resistor for preamp supply filtering. If you were to change the 2.7K resistor to an inductor with much lower DC resistance, say 100 ohms, you would have better filtering and a higher preamp supply voltage, which would increase headroom, giving a slightly different feel. Randall Aiken

10/4/1999 6:22 AM

Randall Aiken

Re: Excessive Henries.

Most guitar amps use capacitor input filters as opposed to choke input filters, so the effect of a large inductance on dynamic response is not as great. If the plate supply were taken off after the choke instead of before, the dynamic response would be affected more. The preamp stage current draw is relatively small and relatively constant in comparison to the output stage current draw.

Since a push-pull output stage has the benefit of inherent common-mode hum rejection, and the plate current in a pentode is not affected much by plate voltage variations, a well-filtered plate supply is not needed. The screen voltage, however, has a much greater effect on the plate current, and therefore must be well-filtered, so it is usually tapped off after the choke. This allows the manufacturer to use a much smaller choke than would be needed if the entire plate current of the amp had to flow through it.

To answer your question, a larger inductance will result in a cleaner DC supply, but will also change the transient response of filter network. Usually, the reason for going to an inductor is to get the extra filtering without the DC voltage drop of a resistor. However, if, as in your example, you replace the resistor with a choke of the same DC resistance, it will have the same voltage drop, but the transient characteristics will change. The purely resistive filter will generally rise with no overshoot, but the large inductance may tend to make the supply overshoot or ring on transients, depending upon the size of the filter caps in relation to the inductance value (it will be underdamped, overdamped, or critically damped). However, as pointed out before, the preamp current draw is fairly constant, so there won't likely be any transients except at power up time.

Because the preamp supply is relatively constant, the extra filtering won't make that much difference in touch response. This is why some amps get by with no choke, only a resistor for preamp supply filtering. If you were to change the 2.7K resistor to an inductor with much lower DC resistance, say 100 ohms, you would have better filtering and a higher preamp supply voltage, which would increase headroom, giving a slightly different feel.

Randall Aiken