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01-02-2019, 02:49 PM
Hi jmcd
One thing I forgot to mention above is that in the TUT Fig.9-90 example, attenuation has been added so that the signals at the mute points would be within the capability of the BJTs. A BJT can only handle an AC signal of about 6V peak before it clips the signal. The signal level needs to be quite low where the the channels come together prior to the final tube stage, which makes it convenient as a switching point and allows proper balancing of the channel loudness.
In the LP-PRE kit, based on the London Power Standard preamp, switching is more likely to be by jfet or relay and the signal levels are still quite low at the various muting points.
The plate voltage to the preamp will influence the signal sizes at the various points throughout the circuit. Obviously, with a high B+ the plate signal of a second or later stage triode in a high-gain preamp can easily be 120V peak or more - often asymmetric, say +80V and -120V. Needless to say, there is no place in the circuit other than driving the output tube grid or the OT itself where we NEED such a high signal, which is why there are interstage attenuators everywhere. Even at the power amp input we only need 2V maximum to clip the output, so there is a lot of opportunity within the cascaded stage design to add muting and switching elements.
For a clean path, a single muting point is sufficient to kill the clean sound, where a high-gain channel should have at least two mutes.
One thing I forgot to mention above is that in the TUT Fig.9-90 example, attenuation has been added so that the signals at the mute points would be within the capability of the BJTs. A BJT can only handle an AC signal of about 6V peak before it clips the signal. The signal level needs to be quite low where the the channels come together prior to the final tube stage, which makes it convenient as a switching point and allows proper balancing of the channel loudness.
In the LP-PRE kit, based on the London Power Standard preamp, switching is more likely to be by jfet or relay and the signal levels are still quite low at the various muting points.
The plate voltage to the preamp will influence the signal sizes at the various points throughout the circuit. Obviously, with a high B+ the plate signal of a second or later stage triode in a high-gain preamp can easily be 120V peak or more - often asymmetric, say +80V and -120V. Needless to say, there is no place in the circuit other than driving the output tube grid or the OT itself where we NEED such a high signal, which is why there are interstage attenuators everywhere. Even at the power amp input we only need 2V maximum to clip the output, so there is a lot of opportunity within the cascaded stage design to add muting and switching elements.
For a clean path, a single muting point is sufficient to kill the clean sound, where a high-gain channel should have at least two mutes.
