In the AM transmitter I am building, the power supply is structured in such a
way that it is not possible to measure the final tube plate current at a
low-potential point.

So I have decided to connect the tube cathode to ground via a 1-ohm resistor and
measure the voltage that develops across that resistor.

I know that, doing so, I will be measuring the sum of plate current (around 200
mA), screen current (around 20 mA) and grid current (around 7 mA). But tuning
the transmitter Pi network for minimum total current should still work fine. Any
comment?

73

Tony I0JX
Rome, Italy

On Jan 9, 8:41*am, "Antonio Vernucci" wrote:
In the AM transmitter I am building, the power supply is structured in such a
way that it is not possible to measure the final tube plate current at a
low-potential point.

So I have decided to connect the tube cathode to ground via a 1-ohm resistor and
measure the voltage that develops across that resistor.

I know that, doing so, I will be measuring the sum of plate current (around 200
mA), screen current (around 20 mA) and grid current (around 7 mA). But tuning
the transmitter Pi network for minimum total current should still work fine. Any
comment?

73

Tony I0JX
Rome, Italy

You are exactly right..Be sure to bypass the the cathode resistor with
something like a point 01 cap . Good luck W4PQW

wrote:
On Jan 9, 8:41 am, "Antonio Vernucci" wrote:
In the AM transmitter I am building, the power supply is structured in such a
way that it is not possible to measure the final tube plate current at a
low-potential point.

So I have decided to connect the tube cathode to ground via a 1-ohm resistor and
measure the voltage that develops across that resistor.

I know that, doing so, I will be measuring the sum of plate current (around 200
mA), screen current (around 20 mA) and grid current (around 7 mA). But tuning
the transmitter Pi network for minimum total current should still work fine. Any
comment?

73

Tony I0JX
Rome, Italy

You are exactly right..Be sure to bypass the the cathode resistor with
something like a point 01 cap . Good luck W4PQW
The only thing you won't be able to do is to measure the plate input
power of the transmitter, which for your application isn't a requirement.

The only thing you won't be able to do is to measure the plate input power of
the transmitter, which for your application isn't a requirement.

My only concern is that tuning for the minimum cathode current may not be as
sharp as tuning for the minimum plate current, because when plate current
decreases the screen current increases (though only about 1/10 in magnitude).

73

Tony I0JX

Antonio Vernucci wrote:
The only thing you won't be able to do is to measure the plate input power of
the transmitter, which for your application isn't a requirement.

My only concern is that tuning for the minimum cathode current may not be as
sharp as tuning for the minimum plate current, because when plate current
decreases the screen current increases (though only about 1/10 in magnitude).

Is there any way that the tube can fail in such a way that the cathode current
is good but the plate current is not? Screen damage?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey wrote:
Antonio Vernucci wrote:
The only thing you won't be able to do is to measure the plate input power of
the transmitter, which for your application isn't a requirement.
My only concern is that tuning for the minimum cathode current may not be as
sharp as tuning for the minimum plate current, because when plate current
decreases the screen current increases (though only about 1/10 in magnitude).

Is there any way that the tube can fail in such a way that the cathode current
is good but the plate current is not? Screen damage?
--scott
Another idea would be to connect an rf voltmeter to the antenna
connection and just tune for max rf voltage. This is the best way to
tune a screen grid power tube anyway. Most ham rigs have a meter
position to tune for max power output, and this usually is just a diode
connected through a small capacitor to the antenna output of the rig.
The diode is connected through a resistor to the meter, and the meter is
bypassed by another capacitor. Sometimes a resistive voltage divider is
connected between the diode and the coupling capacitor to the antenna
terminal (for high power rigs). Should be some circuit ideas in older
ARRL HB's.