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.

In conclusion, it looks like the final stage operates best at zero grid
resistance:

- no efficiency loss
- minimum grid current for a given ouptut power.

In such conditions, the tube operates in class B (the fixed -33V bias causes an
idling plate current of about 10 mA), with a circulation angle of more than 180
degrees. Increasing the grid resistor causes a reduction of the circulation
angle, with no practical benefit and some drawbacks.

Where has the class-C efficiency advantage gone?

73

Tony I0JX