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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 |
#2
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Class-C stage grid resistor
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 My fixed bias is set for an idling plate current of 10 mA. The only criterion behind that bias setting was to keep the final tube safe. So, it may not be optimum with regard to the final stage efficiency. On the contrary, it is surely not optimum because, without extra bias, the tube operates class B. More bias is needed (produced by the grid resistor) to have the tube operating class C. 73 Tony I0JX |
#3
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Class-C stage grid resistor
On Feb 2, 6:29*pm, "Antonio Vernucci" wrote:
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 My fixed bias is set for an idling plate current of 10 mA. The only criterion behind that bias setting was to keep the final tube safe. So, it may not be optimum with regard to the final stage efficiency. On the contrary, it is surely not optimum because, without extra bias, the tube operates class B. More bias is needed (produced by the grid resistor) to have the tube operating class C. 73 Tony I0JX With a fixed bias supply , do you really need a grid resistor as such ? how is the -Ve bias reaching the grid ? ....with the cathode earthed .. the fixed bias is 'required' .. I would think for a given drive level ... increasing the -Ve grid bias would reduce the conduction angle and slide the stage from class b to c as the peak drive level would need to overcome the bias to allow the grid to conduct .. thus the conduction angle is reduced ? G .. |
#4
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Class-C stage grid resistor
On Feb 2, 7:05*pm, Graham wrote:
On Feb 2, 6:29*pm, "Antonio Vernucci" wrote: 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 My fixed bias is set for an idling plate current of 10 mA. The only criterion behind that bias setting was to keep the final tube safe. So, it may not be optimum with regard to the final stage efficiency. On the contrary, it is surely not optimum because, without extra bias, the tube operates class B. More bias is needed (produced by the grid resistor) to have the tube operating class C. 73 Tony I0JX With a fixed bias supply *, do *you *really *need a *grid *resistor as such ? how is the *-Ve *bias reaching the *grid ? ....with the cathode earthed .. the fixed bias is *'required' *.. I would think for a *given drive *level ... increasing the *-Ve *grid bias would *reduce the *conduction angle and slide the *stage *from class b to *c *as the *peak drive level *would need *to *overcome the *bias to allow the *grid to *conduct .. thus the *conduction angle is reduced ? G ..- Hide quoted text - - Show quoted text - The original data has guide lines on the vlaves use in class C telephony ..... http://www.r-type.org/pdfs/829b.pdf G .. |
#5
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Class-C stage grid resistor
With a fixed bias supply , do you really need a grid resistor as
such ? Yes, because the fixed bias I have chosen causes the tube to operate in class B, whilst I wish it to operate in class C. how is the -Ve bias reaching the grid ? ....with the cathode earthed .. the fixed bias is 'required' .. I would think for a given drive level ... increasing the -Ve grid bias would reduce the conduction angle and slide the stage from class b to c as the peak drive level would need to overcome the bias to allow the grid to conduct .. thus the conduction angle is reduced ? G ..- Hide quoted text - - Show quoted text - I agree with your statements but they do not help me much with regard to my original doubts The original data has guide lines on the vlaves use in class C telephony ..... http://www.r-type.org/pdfs/829b.pdf I have several data sheets for the 828B of various manufacturers but with the grid resistor value they specify I obtain less output power than with a lower resistance value 73 Tony I0JX G .. |
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