| Home |
| Search |
| Today's Posts |
|
#10
February 3rd 10, 11:46 PM
posted to rec.radio.amateur.boatanchors
|
|||
|
|||
Class-C stage grid resistor
On 3 Feb, 17:54, Kenneth Scharf wrote:
Antonio Vernucci wrote: The higher the grid resistor, the higher the bias voltage that must be overcome by the drive. *Hence, higher drive, more power lost in the grid resistor, and lower conduction angle. So, too high a grid resistor and you'll need to beef up your drive stage. *Plus (as mentioned), your conduction angle decreases, and your final-stage efficiency may suffer. Get the grid resistor too low, your conduction angle will increase, and your final-stage efficiency may suffer. Note that I say "may" -- there's an optimum conduction angle. *There's handbook values for it (which I can't remember!) but I'll bet that no one amplifier works best right at the handbook value. If you _really_ want to be scientific about it then for each grid resistance value monitor your final stage input power, the amplifier output power, and calculate the grid resistance dissipation. *If nothing else, that'll help you make an informed choice. Otherwise, if it's given, calculate the grid resistor value to get you both the desired current and the RF p-p voltage, or the rated bias voltage, whichever is listed for your tube in that service. -- www.wescottdesign.com Thanks for your comments. I agree that there should be an optimum grid resistance value (even if rather dull), but in my case the optimum occurs at zero grid resistance. Let me report you some tests I have made, increasing the grid resistance in steps (starting from R=0) and then re-adjusting the drive power each time (and also re-optimizing the Pi-network controls): - increasing the grid resistance and then adjusting the drive power so as to keep the GRID current constant, the plate current - and hence the output power - decreases. Therefore, to obtain maximum output power, the grid resistance must be zero - conversely, increasing the grid resistance and then adjusting the drive power so as to keep the PLATE current constant, the output power remains about the same for a quite wide range of grid resistance values (except when resistance becomes very high). It should be noted that, increasing the grid resistance at constant plate current, the grid current increases significantly, to the extent that, for fairly high grid resistance values, the grid current gets beyond the allowable limit. |
| Thread Tools | Search this Thread |
| Display Modes | |
|
|
Similar Threads
|
||||
| Thread | Forum | |||
| Class envy, class warfare, begins with Democrats creating it? | Shortwave | |||
| 811A's, Dual Grid and Class B triodes | Homebrew | |||
| DSP in IF Stage? | Equipment | |||
| DSP in IF Stage? | Equipment | |||
| Q-Multiplier at RF stage? | Homebrew | |||
Threaded Mode
