Many old 1920s TRF sets have a tone control. It's a rheostat that adjusts the filament voltage to the grid-leak detector, typically an '01A tube.

With the voltage turned up, there is more bass. Lower, it has more treble.

What's the theory behind this?

:Many old 1920s TRF sets have a tone control. It's a rheostat that adjusts the filament voltage to the grid-leak detector, typically an '01A tube.
:
:With the voltage turned up, there is more bass. Lower, it has more treble.
:
:What's the theory behind this?

One wild guess is that changing the filament voltage has the effect of changing the equivalent resistances of the tube, similar to using a potentiometer with a capacitor to get the same effect.

Doug,
I would think the action is similar to a cathode bypass cap, where the low end frequency response can be altered with different values, but set once a fixed value is installed. The bypass cap effectively lowers the instantaneous AC impedance in the cathode circuit. We had this discussion several months ago about the same subject, whether a fixed voltage on the cathode or altering the instantaneous AC cathode impedance with a cap effected changes. I think we came to a draw in opinions, but seems you have now suggested at least a partial conclusion to that discussion.

marv

:Many old 1920s TRF sets have a tone control. It's a rheostat that adjusts the filament voltage to the grid-leak detector, typically an '01A tube.
:
:With the voltage turned up, there is more bass. Lower, it has more treble.
:
:What's the theory behind this?

:Many old 1920s TRF sets have a tone control. It's a rheostat that adjusts the filament voltage to the grid-leak detector, typically an '01A tube.
:
:With the voltage turned up, there is more bass. Lower, it has more treble.
:
:What's the theory behind this?

I am not sure about your circuit, but according to a 1924 radio book, what I see is a rheostat to adjust filament volatge AND a "resistor of some 300 ohms connected across the A battery and having a sliding contact in the grid circuit; and it is used to prevent the amplifiers from becoming unstable or generating oscillations."

The same resitor is used across all tube's (including detector) filaments, having the wiper go to ground and both extremeties of the pot going to each filament's connection.

Z-

Z,
I've seen the same 300 ohm across the filaments of a couple TRF's I've done. It just provides a ground reference for the tube to operate, but without a huge load on the filament circuit. Tubes with indirectly heated cathodes don't need it.

marv

::Many old 1920s TRF sets have a tone control. It's a rheostat that adjusts the filament voltage to the grid-leak detector, typically an '01A tube.
::
::With the voltage turned up, there is more bass. Lower, it has more treble.
::
::What's the theory behind this?
:
:I am not sure about your circuit, but according to a 1924 radio book, what I see is a rheostat to adjust filament volatge AND a "resistor of some 300 ohms connected across the A battery and having a sliding contact in the grid circuit; and it is used to prevent the amplifiers from becoming unstable or generating oscillations."
:
:The same resitor is used across all tube's (including detector) filaments, having the wiper go to ground and both extremeties of the pot going to each filament's connection.
:
:Z-

The potentiometer accross the filament circuit was occasionally labeled the tone control. It allowed the operator to vary the point of the grid return relative to the A battery voltage, giving the operator come control of the grid bias of the amplifier tubes. the potentiometer in this application fades quickly from the scene after the widespread adoption of the C battery in the 1925 season.
:Z,
:I've seen the same 300 ohm across the filaments of a couple TRF's I've done. It just provides a ground reference for the tube to operate, but without a huge load on the filament circuit. Tubes with indirectly heated cathodes don't need it.
:
:marv
:
:::Many old 1920s TRF sets have a tone control. It's a rheostat that adjusts the filament voltage to the grid-leak detector, typically an '01A tube.
:::
:::With the voltage turned up, there is more bass. Lower, it has more treble.
:::
:::What's the theory behind this?
::
::I am not sure about your circuit, but according to a 1924 radio book, what I see is a rheostat to adjust filament volatge AND a "resistor of some 300 ohms connected across the A battery and having a sliding contact in the grid circuit; and it is used to prevent the amplifiers from becoming unstable or generating oscillations."
::
::The same resitor is used across all tube's (including detector) filaments, having the wiper go to ground and both extremeties of the pot going to each filament's connection.
::
::Z-

OK, but why does in change the tone?

:The potentiometer accross the filament circuit was occasionally labeled the tone control. It allowed the operator to vary the point of the grid return relative to the A battery voltage, giving the operator come control of the grid bias of the amplifier tubes. the potentiometer in this application fades quickly from the scene after the widespread adoption of the C battery in the 1925 season.
::Z,
::I've seen the same 300 ohm across the filaments of a couple TRF's I've done. It just provides a ground reference for the tube to operate, but without a huge load on the filament circuit. Tubes with indirectly heated cathodes don't need it.
::
::marv
::
::::Many old 1920s TRF sets have a tone control. It's a rheostat that adjusts the filament voltage to the grid-leak detector, typically an '01A tube.
::::
::::With the voltage turned up, there is more bass. Lower, it has more treble.
::::
::::What's the theory behind this?
:::
:::I am not sure about your circuit, but according to a 1924 radio book, what I see is a rheostat to adjust filament volatge AND a "resistor of some 300 ohms connected across the A battery and having a sliding contact in the grid circuit; and it is used to prevent the amplifiers from becoming unstable or generating oscillations."
:::
:::The same resitor is used across all tube's (including detector) filaments, having the wiper go to ground and both extremeties of the pot going to each filament's connection.
:::
:::Z-

The few I've encountered were either fixed center tapped ww (10W), and hardly a tone control, or a ww pot (10W) with wiper tied to ground. In both cases, buried in the chassis without a knob or labeled "Tone Control"

marv

:OK, but why does in change the tone?
:
:
::The potentiometer accross the filament circuit was occasionally labeled the tone control. It allowed the operator to vary the point of the grid return relative to the A battery voltage, giving the operator come control of the grid bias of the amplifier tubes. the potentiometer in this application fades quickly from the scene after the widespread adoption of the C battery in the 1925 season.
:::Z,
:::I've seen the same 300 ohm across the filaments of a couple TRF's I've done. It just provides a ground reference for the tube to operate, but without a huge load on the filament circuit. Tubes with indirectly heated cathodes don't need it.
:::
:::marv
:::
:::::Many old 1920s TRF sets have a tone control. It's a rheostat that adjusts the filament voltage to the grid-leak detector, typically an '01A tube.
:::::
:::::With the voltage turned up, there is more bass. Lower, it has more treble.
:::::
:::::What's the theory behind this?
::::
::::I am not sure about your circuit, but according to a 1924 radio book, what I see is a rheostat to adjust filament volatge AND a "resistor of some 300 ohms connected across the A battery and having a sliding contact in the grid circuit; and it is used to prevent the amplifiers from becoming unstable or generating oscillations."
::::
::::The same resitor is used across all tube's (including detector) filaments, having the wiper go to ground and both extremeties of the pot going to each filament's connection.
::::
::::Z-