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Class-C stage grid resistor
On Mon, 1 Feb 2010 21:52:11 +0100, "Antonio Vernucci"
wrote: I have just completed building a class-C AM transmitter with an 829B (two tetrodes in parallel) driven by a T-368 VFO. The 829B bias is a fixed -33V (keeping the tube safe against lack of drive) plus the bias which develops across the grid resistor. It works fine, but I still have to adjust the grid resistor value for optimum performance In the 829B data sheet I read a value of about 7 kohm. In my Geloso AM transmitter (single 6146) they use 12 kohm. In the T-368 (single 4-400A) they use about 23 kohm. My practical experience is that lowering the grid resistor I always get more output power FOR THE SAME GRID CURRENT. In other words, every time I try a different resistor value I re-adjust the drive power so as to make the grid current equal to the allowable limit of 15 mA. At the moment I ended up using just 470 ohm but, despite that value gives me good output power, I suspect that it may cause some drawbacks that I cannot presently figure out. Does anyone have a CLEAR understanding of the trade-offs involved in selecting the grid bias resistor value? Or where I can find a practical and coincise discussion of the issue? Thanks & 73 Tony I0JX Rome, Italy I cannot give you actual values for your set up, but basically, if you have already provided optimum bias with your fixed bias voltage, then you will not need to develop any more bias across a grid resistor. peter |
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