Plate current measurement
 

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

Plate current measurement
 

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

Plate current measurement
 

Plate current measurement
 

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

Plate current measurement
 

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."

Plate current measurement
 

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.

Plate current measurement
 

On Jan 11, 12:11*am, Kenneth Scharf wrote:
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.



Just get a clip on ammeter or hall effect sensor and mesure the
plate direct ?

G ..

Plate current measurement
 

On Jan 11, 12:11*am, Kenneth Scharf wrote:
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.



My skanti-trp5000 tunes the valve stage by measuring the rf
voltage on the plates ....the 'level' is set at voltage
such that the tubes are matched to the correct load .. and maximum
power out occures ..all you need to do is select medium power and
tune (2 tone test) to the pre defined voltage level .. thats 150
watts carrier power out ,

G .

Plate current measurement
 

Another idea would be to connect an rf voltmeter to the antenna connection and
just tune for max rf voltage.

I found THE solution to measure current flowing though a wire at high potential
without risks!

Solution is to use an Hall-effect transducer of the same type used in the newer
clamp meters (those able to measure both AC and DC currents). One can order
transducers with full-scale currents ranging from as low as 10mA to tens of
Amps. The high-potential wire passes through a hole having a diameter of two
centrimeners, so it is fully insulated from the measurement circuitry,

The transducer can directly drive a meter having a full-scale current of 20 mA
or less. See http://www.chenyang-ism.com/

73

Tony I0JX

Plate current measurement
 

Antonio Vernucci wrote:
Another idea would be to connect an rf voltmeter to the antenna
connection and just tune for max rf voltage.

I found THE solution to measure current flowing though a wire at high
potential without risks!

Solution is to use an Hall-effect transducer of the same type used in
the newer clamp meters (those able to measure both AC and DC currents).
One can order transducers with full-scale currents ranging from as low
as 10mA to tens of Amps. The high-potential wire passes through a hole
having a diameter of two centrimeners, so it is fully insulated from the
measurement circuitry,

The transducer can directly drive a meter having a full-scale current of
20 mA or less. See http://www.chenyang-ism.com/

73

Tony I0JX

If the plate supply has it's own power supply with a separate power
transformer or winding the meter can be placed in the negative lead of
the power supply. Be sure to place the meter outside of any bleeder
resistors so you don't measure the bleeder current.