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