No, don't use a polarized capacitor because there is no DC offset. The signal is feeding into a very high input impedance so you can use 0.1uF or even a little less. A value lower than that will not pass the lower frequencies but it would test if the potentiometer works correctly.Again, thank you all for your help and the generous explanation. Regarding the resistor and cradle, that is how I found the impedance above. I do not plan on using the headphone out for a variety of reasons. (If my math is correct, it is closer to 189 Ohms input impedance). And regarding the dc offset, I recently acquired an oscilloscope that registers dc coupling and there seems to be none showing up as the center of the 1 k signal sits at 0. I did find a 10 uf electrolytic cap in my tiny spare parts box. I read that one should use only polypropylene or metalized poly caps for input coupling…will the polarized cap be ok for test purposes?
That may be true but I don't know if it is the whole story.The problem of "louder towards the middle of the pot" was caused by connecting it directly to the grid input. At mid position, there was ,maximum resistance to ground. As you varied the pot either way, you reduced the resistance and upset the bias voltage on the grid, reducing the gain..
The entire audio source voltage is developed acros that low source resistance and so it would be present at the end of the control resistance. So while the impedance will be low, the signal voltage would be a maximum.That may be true but I don't know if it is the whole story.
The grid to cathode bias is set by the cathode to ground resistor. The voltage on the grid is at 0V through the 1MΩ resistor from the grid to ground.
I would think that it has more to do with the impedance to ground (a short to ground via the low output impedance of the source. (I would like to test this theory with my tube breadboard that I have.)
The cathode to ground resistor does cause a negative potential on the grid when current flows through it but that is not the whole story. With a very high value resistor connected between the grid and ground, the electron emissions from the cathode will result in a small grid current that flows through the resistor to give a volt or so of negative bias on the grid. It's called "grid current biasing". That will be shorted to ground if the resistor is made a much lower value, leaving only the cathode resistor to bias the grid.That may be true but I don't know if it is the whole story.
The grid to cathode bias is set by the cathode to ground resistor. The voltage on the grid is at 0V through the 1MΩ resistor from the grid to ground.
I would think that it has more to do with the impedance to ground (a short to ground via the low output impedance of the source. (I would like to test this theory with my tube breadboard that I have.)
by Jeff Child
by Jake Hertz
by Jake Hertz
by Aaron Carman