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