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

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.

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

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 ..

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

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

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

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.

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