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10-31-2018, 11:42 AM
Hi Guys
As TUT refers to these resistors, they are for mixing and for isolation. Yes, contradictory functions
With a single signal path, the output of the level control could be fed more or less directly into the next gain stage. If we add a second input path and Level-2 control, the controls would interact with each other. Assuming we simply tie the wipers together, adjusting either pot effects both signals. To reduce this interaction, isolation resistors are added in series with each wiper.There is still interaction but it is greatly reduced.
With one level control set to zero, its resistor is grounded. The other pot can be set however we need and the signal sees a voltage divider comprised of the two resistors, which if they are equal values reduce the signal by half. If both pots are up, then both signals can pass to the next stage, both being attenuated by the divider-action of the two resistors.
If we add a cap across one resistor, that signal will have a treble emphasis. When the related pot is at zero, there will be a treble
de-emphasis for the other signal.
Why 470k versus 100k? The higher values load the pots less and allow the use of lower-value caps for frequency shaping.
In one of the versions of the Fender Bassman, the stage after the passive mixing had feedback applied, which made the mixing active as the overall circuit becomes a virtual-earth mixer. This minimizes the interaction between the pots and the frequency shaping works only on the signal path it is built into.
In neither form do the mixing/isolation resistors act as grid-stops in the true sense. There is usually quite a lot of wire between these Rs and the tube grid, so things can happen along the way. However, these Rs are large enough to work against the next stage's internal capacitance to impose a high-frequency roll-off.
Have fun
As TUT refers to these resistors, they are for mixing and for isolation. Yes, contradictory functions
With a single signal path, the output of the level control could be fed more or less directly into the next gain stage. If we add a second input path and Level-2 control, the controls would interact with each other. Assuming we simply tie the wipers together, adjusting either pot effects both signals. To reduce this interaction, isolation resistors are added in series with each wiper.There is still interaction but it is greatly reduced.
With one level control set to zero, its resistor is grounded. The other pot can be set however we need and the signal sees a voltage divider comprised of the two resistors, which if they are equal values reduce the signal by half. If both pots are up, then both signals can pass to the next stage, both being attenuated by the divider-action of the two resistors.
If we add a cap across one resistor, that signal will have a treble emphasis. When the related pot is at zero, there will be a treble
de-emphasis for the other signal.
Why 470k versus 100k? The higher values load the pots less and allow the use of lower-value caps for frequency shaping.
In one of the versions of the Fender Bassman, the stage after the passive mixing had feedback applied, which made the mixing active as the overall circuit becomes a virtual-earth mixer. This minimizes the interaction between the pots and the frequency shaping works only on the signal path it is built into.
In neither form do the mixing/isolation resistors act as grid-stops in the true sense. There is usually quite a lot of wire between these Rs and the tube grid, so things can happen along the way. However, these Rs are large enough to work against the next stage's internal capacitance to impose a high-frequency roll-off.
Have fun
