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previous: T.B. Hi John, Is it bett... -- 7/17/2000 12:44 PM

Re: Signal grid resitors values & output tube distortion???

7/17/2000 8:20 PM
Aaron V.	Re: Signal grid resitors values & output tube distortion??? Hi TB, Under idle conditions the grid of the output tube is sitting at a negative voltage - the bias voltage - and lets suppose it is -30V. When the grid is hit with a large signal, say a 40V peak signal, the voltage on the grid will be positive for a period of time. When this happens, some of the negatively charged electrons coming from the cathode will be attracted to the positve charge on the grid. In terms of conventional current, there will then be current flowing from the grid to the cathode, much like the current flowing from the plate or screen. One of the problems here is that the capacitive coupling from the plate of the previous stage cannot feed the grid the current it wants to draw. That is what the cathode follower is for. Try to think of it as still being voltage driven, but we just have to find a way to give the grid the current it wants. IOW, the current isn't doing any driving, it is just a side-effect of hitting the grid with such a big voltage. Look on GEO under the Mosfet Follies section for a more detailed description of the negative things that happen when driving the grids positive via capacitive coupling. I think RA might have a description on his site too. HTH, Aaron

7/17/2000 8:20 PM

Aaron V.

Re: Signal grid resitors values & output tube distortion???

Hi TB,

Under idle conditions the grid of the output tube is sitting at a negative voltage - the bias voltage - and lets suppose it is -30V. When the grid is hit with a large signal, say a 40V peak signal, the voltage on the grid will be positive for a period of time. When this happens, some of the negatively charged electrons coming from the cathode will be attracted to the positve charge on the grid. In terms of conventional current, there will then be current flowing from the grid to the cathode, much like the current flowing from the plate or screen. One of the problems here is that the capacitive coupling from the plate of the previous stage cannot feed the grid the current it wants to draw. That is what the cathode follower is for.

Try to think of it as still being voltage driven, but we just have to find a way to give the grid the current it wants. IOW, the current isn't doing any driving, it is just a side-effect of hitting the grid with such a big voltage.

Look on GEO under the Mosfet Follies section for a more detailed description of the negative things that happen when driving the grids positive via capacitive coupling. I think RA might have a description on his site too.

HTH,

Aaron