On Mon, 01 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

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