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.

Plate current measurement
 

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.

Thanks for your suggestion, but in my initial message I had mentioned that my
power supply is structured in such a way not to permit measuring plate current
on the negative lead as it would otherwise be normal to do.

As a matter of fact the low-voltage supply is part of the high-voltage supply;
they are not separate. Therefore the current flowing in the negative lead is the
sum of the currents of the two supplies, and it is then not possible to isolate
the plate current of the final tube.

Unfortunately no alternative to measuring it on the high-voltage lead.

73

Tony I0JX

Plate current measurement
 

Antonio Vernucci wrote:
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.

Thanks for your suggestion, but in my initial message I had mentioned
that my power supply is structured in such a way not to permit measuring
plate current on the negative lead as it would otherwise be normal to do.

As a matter of fact the low-voltage supply is part of the high-voltage
supply; they are not separate. Therefore the current flowing in the
negative lead is the sum of the currents of the two supplies, and it is
then not possible to isolate the plate current of the final tube.

Unfortunately no alternative to measuring it on the high-voltage lead.

73

Tony I0JX

Most good panel meters (Triplet, etc) 2" or larger in diameter in thick
Bakelite cases having glass windows are probably insulated well enough
to safely be used in the high voltage side up to at least 1kv (or more).
I wouldn't try it with a cheap imported meter in a thin plastic case
and a plastic window. Back in the days when REAL transmitters were
housed in relay rack cabinets it was the norm to put an ammeter in the
HV lead. Note that trying to switch a single meter between grid,
screen, plate, and HV readings (meter is used as a voltmeter and reads
voltage across shunts for current) gets tricky, a WELL insulated switch
is required with good isolation between selections! Using every other
position of a rotary switch and yanking out the unused contacts might
have been common.

Plate current measurement
 

Most good panel meters (Triplet, etc) 2" or larger in diameter in thick
Bakelite cases having glass windows are probably insulated well enough to
safely be used in the high voltage side up to at least 1kv (or more). I
wouldn't try it with a cheap imported meter in a thin plastic case and a
plastic window. Back in the days when REAL transmitters were housed in relay
rack cabinets it was the norm to put an ammeter in the HV lead. Note that
trying to switch a single meter between grid, screen, plate, and HV readings
(meter is used as a voltmeter and reads voltage across shunts for current)
gets tricky, a WELL insulated switch is required with good isolation between
selections! Using every other position of a rotary switch and yanking out the
unused contacts might have been common.

My Geloso transmitter just adopts the arrangement you mention. But I would
prefer to avoid it, mainly because of the HV present on the meter switch.

The Hall-effect transducer seems to be perfect for measuring current forgetting
about HV. When I will get it I will report whether it really works fine for our
applications or not.

73

Tony I0JX

Plate current measurement
 

On 15 Jan, 21:43, "Antonio Vernucci" wrote:
Most good panel meters (Triplet, etc) 2" or larger in diameter in thick
Bakelite cases having glass windows are probably insulated well enough to
safely be used in the high voltage side up to at least 1kv (or more). I
wouldn't try it with a cheap imported meter in a thin plastic case and a
plastic window. *Back in the days when REAL transmitters were housed in relay
rack cabinets it was the norm to put an ammeter in the HV lead. *Note that
trying to switch a single meter between grid, screen, plate, and HV readings
(meter is used as a voltmeter and reads voltage across shunts for current)
gets tricky, a WELL insulated switch is required with good isolation between
selections! *Using every other position of a rotary switch and yanking out the
unused contacts might have been common.

My Geloso transmitter just adopts the arrangement you mention. But I would
prefer to avoid it, mainly because of the HV present on the meter switch.

The Hall-effect transducer seems to be perfect for measuring current forgetting
about HV. When I will get it I will report whether it really works fine for our
applications or not.

73

Tony I0JX

Tony , I should think it will be ok , may be will need a little rf
screening ... as a Idea try using a coax as the plate
supply cable , with the braid earthed ... the copper 'should'
not affect the magnetic field ... try it out with a low voltage
psu and resistor as a load ?

G ..

Plate current measurement
 

Tony , I should think it will be ok , may be will need a little rf
screening ... as a Idea try using a coax as the plate
supply cable , with the braid earthed ... the copper 'should'
not affect the magnetic field ... try it out with a low voltage
psu and resistor as a load ?

G ..

I will try but I would not expect RF problems because the Hall transducer will
be located inside the modulator screened enclosure and inserted before the
modulation transformer where there is plenty of bypass capacitance to ground.

73

Tony I0JX

Plate current measurement
 

There are a number of rigs that measure cathode current rather than plate
current. Depending upon the tube, screen current is only a small percentage
of plate current, so even a 10% change in screen current may result in only
in a 1 or 2% in cathode current.

I have one other bizarre thought - which I admit has not been given much
thought - suppose you fed the screen with a constant current source, so that
screen current would not change at resonance?

Also, off this subject and back to screen modulation techniques, the old
Radio Handbook mentions that many beam power tubes will self modulate, with
no additional screen audio supplied. Apparently, it can vary from tube to
tube. Despite that stateement, I know of many am transmitters using beam
plate final tubes that do have audio applied to the screen, so take the
statement with a grain of salt.

73, Colin K7FM

Plate current measurement
 

There are a number of rigs that measure cathode current rather than plate
current. Depending upon the tube, screen current is only a small percentage
of plate current, so even a 10% change in screen current may result in only in
a 1 or 2% in cathode current.
I have one other bizarre thought - which I admit has not been given much
thought - suppose you fed the screen with a constant current source, so that
screen current would not change at resonance?

That was the very initial question of my post. My concern was 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). I am not feeding the screen through a
resistor from the HV because I do not wish to add a clamp tube to avoid screen
voltage rising too much in absence of drive. I am feeding it from a very stable
low-voltage supply, so screen current will vary.

Also, off this subject and back to screen modulation techniques, the old Radio
Handbook mentions that many beam power tubes will self modulate, with no
additional screen audio supplied. Apparently, it can vary from tube to tube.
Despite that stateement, I know of many am transmitters using beam plate final
tubes that do have audio applied to the screen, so take the statement with a
grain of salt.

The self-modulation method is just the one I am using. I have a 20H choke on the
screen path, and that will (I hope so!) self-modulate the screen. I plan to
finish work in a month from now. At that time I will be able to report to which
extent screen self-modulation works.

73

Tony I0JX
Rome, Italy

Plate current measurement
 

Antonio Vernucci wrote:
There are a number of rigs that measure cathode current rather than
plate current. Depending upon the tube, screen current is only a
small percentage of plate current, so even a 10% change in screen
current may result in only in a 1 or 2% in cathode current.
I have one other bizarre thought - which I admit has not been given much
thought - suppose you fed the screen with a constant current source,
so that screen current would not change at resonance?

That was the very initial question of my post. My concern was 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). I am not
feeding the screen through a resistor from the HV because I do not wish
to add a clamp tube to avoid screen voltage rising too much in absence
of drive. I am feeding it from a very stable low-voltage supply, so
screen current will vary.

Also, off this subject and back to screen modulation techniques, the
old Radio Handbook mentions that many beam power tubes will self
modulate, with no additional screen audio supplied. Apparently, it
can vary from tube to tube. Despite that stateement, I know of many am
transmitters using beam plate final tubes that do have audio applied
to the screen, so take the statement with a grain of salt.

The self-modulation method is just the one I am using. I have a 20H
choke on the screen path, and that will (I hope so!) self-modulate the
screen. I plan to finish work in a month from now. At that time I will
be able to report to which extent screen self-modulation works.

73

Tony I0JX
Rome, Italy

Depending on the choke inductance value and the final tube you may or
may not get full 100% modulation. But it will be close enough for
government work and will sound just fine.

Plate current measurement
 

Depending on the choke inductance value and the final tube you may or may not
get full 100% modulation. But it will be close enough for government work and
will sound just fine.

Let'see what will happen. By the way, also the T-368 adopts the self-modulation
method. As a matter of fact the screen resistor is connected to the cold side of
the modulation transformer.

73

Tony I0JX