"Uwe" wrote in message
...
They have an alternative way to tune by inserting a mA meter into the key
lead but I do not get the little dips in current they are describing. For
me
it is more of a linear increase in current from the highest capacity
setting
(lowest current) to the setting where the cap is all open and the current
is
highest.

It seems as though your Plate Tuning capacitor has too little capacitance to
resonate at the frequency you are using.

The way things are the max output occurs more or less at the lowest anode
current of my power supply (about 35mA at 200V B+).

Why do I tune for minimum current??

The combination of the Plate Tuning capacitor and the inductor in the output
circuit comprise a parallel-resonant tuned circuit, which has its highest
impedance at resonance. Therefore, when the two are resonant, current is at
its lowest point. Tuning for maximum voltage on the antenna is not
guaranteed to get the plate circuit on frequency, at least not the frequency
you want. On many of those older tube transmitters, there was enough range
in the plate tuning capacitor that the circuit could be tuned to the second
harmonic of the desired frequency. Ie, if your desired frequency was 3500
kHz, the circuit could also be tuned to 7000 kHz. Measuring antenna voltage
(which many cheap rigs did, by using a "Relative Output" meter, just an rf
voltmeter connected across the antenna terminal) could lead you to adjust
for the wrong resonance frequency. (In the 1960s the 80-meter Novice subband
was 3.7-3.75 MHz, putting the second harmonic outside of any amateur band
and generating a lot of QSL cards from the FCC for unsuspecting owners of
transmitters like the Knight T-60, which had only a relative output meter.)

If you can find someone who has a "dip meter," you can determine whether
your plate circuit can be tuned to resonance, and, if not, how far off it
is. A dip meter is an oscillator with an exposed coil, and an analog meter
that dips when the oscillator is positioned near a resonant circuit. You
adjust the dip meter knob until its meter dips and read the frequency off
the dial. The dial calibration is not great, but if you have a
general-coverage receiver you can listen for the oscillator signal.

I would also investigate the coupling capacitor from the tube plate to the
ungrounded terminal of the plate tuning capacitor. It may have dried out and
shorted. Carefully measure for dc voltage from the ungrounded terminal of
the plate tuning capacitor to the chassis. There shouldn't be any. If there
is, replace the capacitor. As a rule of thumb, its voltage rating should be
4x the plate voltage, capacitance about 1000-1500 pF.

"PM"

In article , Uwe
writes:

in article , N2EY at
wrote on 3/13/04 15:42:

In article , Uwe

writes:

When I increase the plate voltage to 325V my 2. grid runs about 280V and
the
plate current increases to about 65 mA. I don't really dare to do that
without knowing the max values on that tube.
I guess I could lower the 2. grid voltage by increasing the value of the
resistor connectd to it.
I don't know if that would lower my plate current to what you get.

Uwe,

How are you tuning up the transmitter? Are you adjusting the plate
capacitor
for minimum current?

73 de Jim, N2EY

Jim,

Sofar I have connected a scope to the antenna and tuned for max waveform eg.
highest voltage. That is also the setting where my dummy load with bulbs is
brightest.
The original instructions for the AC1 explain how to tune but use
expressions like 'turn the capacitor clockwise' which is useless to me when
I am not using their exact part.

The tuneup procedure (adapted to your setup) is as follows:

1) Set both output capacitors to maximum capacitance (plates fully meshed)
2) Close the key and adjust the "plate" capacitor (the one nearest the tube in
the circuit diagram) for minimum plate current. Maximum output should occur at
the same time. Release the key.
3) Unmesh the "load" capacitor about 10 degrees or so
4) Repeat Step 2. The plate current minimum should be higher and there should
be more output.
5) Repeat steps 2, 3, and 4 in order until you get maximum rated plate current
at the minimum setting of the plate capacitor.

They have an alternative way to tune by inserting a mA meter into the key
lead but I do not get the little dips in current they are describing.

Then there is something wrong with your output circuit. What capacitors and
coil are you using? The original design had a 365 pf variable "plate" capacitor
and a similar capacitor for the "load" adjustment. The coil was different for
each band.

What components are you using in the output circuit? Plate tuning capacitor.
coil, load capaticor, plate RF choke, coupling capacitor? Any of them could be
the cause of your problem.

For me
it is more of a linear increase in current from the highest capacity setting
(lowest current) to the setting where the cap is all open and the current is
highest.

That says to me that your plate C is too low.

The way things are the max output occurs more or less at the lowest anode
current of my power supply (about 35mA at 200V B+).

Which should happen somewhere in the midrange of the plate capacitor
adjustment.

Why do I tune for minimum current??

Because that's when the impedance of the load circuit is closest to matching
the tube's output.

73 de Jim, N2EY

In article ,
"Paul_Morphy" writes:

The combination of the Plate Tuning capacitor and the inductor in the output
circuit comprise a parallel-resonant tuned circuit,

The AC-1 uses a pi network output circuit.

73 de Jim, N2EY

In article , Uwe
writes:

in article , N2EY at
wrote on 3/13/04 15:42:

In article , Uwe

writes:

When I increase the plate voltage to 325V my 2. grid runs about 280V and
the
plate current increases to about 65 mA. I don't really dare to do that
without knowing the max values on that tube.
I guess I could lower the 2. grid voltage by increasing the value of the
resistor connectd to it.
I don't know if that would lower my plate current to what you get.

Uwe,

How are you tuning up the transmitter? Are you adjusting the plate
capacitor
for minimum current?

73 de Jim, N2EY

Jim,

Sofar I have connected a scope to the antenna and tuned for max waveform eg.
highest voltage. That is also the setting where my dummy load with bulbs is
brightest.
The original instructions for the AC1 explain how to tune but use
expressions like 'turn the capacitor clockwise' which is useless to me when
I am not using their exact part.

The tuneup procedure (adapted to your setup) is as follows:

1) Set both output capacitors to maximum capacitance (plates fully meshed)
2) Close the key and adjust the "plate" capacitor (the one nearest the tube in
the circuit diagram) for minimum plate current. Maximum output should occur at
the same time. Release the key.
3) Unmesh the "load" capacitor about 10 degrees or so
4) Repeat Step 2. The plate current minimum should be higher and there should
be more output.
5) Repeat steps 2, 3, and 4 in order until you get maximum rated plate current
at the minimum setting of the plate capacitor.

They have an alternative way to tune by inserting a mA meter into the key
lead but I do not get the little dips in current they are describing.

Then there is something wrong with your output circuit. What capacitors and
coil are you using? The original design had a 365 pf variable "plate" capacitor
and a similar capacitor for the "load" adjustment. The coil was different for
each band.

What components are you using in the output circuit? Plate tuning capacitor.
coil, load capaticor, plate RF choke, coupling capacitor? Any of them could be
the cause of your problem.

For me
it is more of a linear increase in current from the highest capacity setting
(lowest current) to the setting where the cap is all open and the current is
highest.

That says to me that your plate C is too low.

The way things are the max output occurs more or less at the lowest anode
current of my power supply (about 35mA at 200V B+).

Which should happen somewhere in the midrange of the plate capacitor
adjustment.

Why do I tune for minimum current??

Because that's when the impedance of the load circuit is closest to matching
the tube's output.

73 de Jim, N2EY

In article ,
"Paul_Morphy" writes:

The combination of the Plate Tuning capacitor and the inductor in the output
circuit comprise a parallel-resonant tuned circuit,

The AC-1 uses a pi network output circuit.

73 de Jim, N2EY

"N2EY" wrote in message
...
In article ,
"Paul_Morphy" writes:

The combination of the Plate Tuning capacitor and the inductor in the
output
circuit comprise a parallel-resonant tuned circuit,

The AC-1 uses a pi network output circuit.

That's right, but the plate tuning cap and the inductor still operate as a
parallel-resonant circuit. That's why plate current dips at resonance. In
conjunction with the loading capacitor, the pi network also serves to match
the plate impedance to the load impedance.

73,

"PM"

"N2EY" wrote in message
...
In article ,
"Paul_Morphy" writes:

The combination of the Plate Tuning capacitor and the inductor in the
output
circuit comprise a parallel-resonant tuned circuit,

The AC-1 uses a pi network output circuit.

That's right, but the plate tuning cap and the inductor still operate as a
parallel-resonant circuit. That's why plate current dips at resonance. In
conjunction with the loading capacitor, the pi network also serves to match
the plate impedance to the load impedance.

73,

"PM"

"N2EY" wrote in message
...

2) Close the key and adjust the "plate" capacitor (the one nearest the
tube in
the circuit diagram) for minimum plate current. Maximum output should
occur at
the same time.

It is a characteristic of pi-network amplifiers that efficiency and power
output are slightly better when the plate is tuned slightly off resonance.
Tuning for a dip is good enough for most purposes.


Why do I tune for minimum current??

Because that's when the impedance of the load circuit is closest to
matching
the tube's output.

If plate voltage is 200 and plate current is 35 mA, plate impedance is
200/0.035, or 5714 ohms. The impedance of the load has nothing to do with
this, it is due to the reactance at resonance of the plate tuning capacitor
and the coil, which are equal but opposite in phase. You can have an open
circuit on the other side of the coil and still tune for a dip in plate
current. Assuming stable plate voltage it is possible to have the same plate
current over a wide range of load impedances. Because the pi network looks
like a lumped section of transmission line, and because the plate tuning
capacitor serves the dual functions of resonating the plate circuit _and_
matching the plate circuit to the input of the pi network, variations at the
load end require slight adjustments of the plate tuning capacitor. To get
maximum power transfer to the load, the pi network must transform the plate
impedance to the load impedance, while also resonating the plate circuit.
There's more going on than simple impedance matching.

73,

"PM"

"N2EY" wrote in message
...

2) Close the key and adjust the "plate" capacitor (the one nearest the
tube in
the circuit diagram) for minimum plate current. Maximum output should
occur at
the same time.

It is a characteristic of pi-network amplifiers that efficiency and power
output are slightly better when the plate is tuned slightly off resonance.
Tuning for a dip is good enough for most purposes.


Why do I tune for minimum current??

Because that's when the impedance of the load circuit is closest to
matching
the tube's output.

If plate voltage is 200 and plate current is 35 mA, plate impedance is
200/0.035, or 5714 ohms. The impedance of the load has nothing to do with
this, it is due to the reactance at resonance of the plate tuning capacitor
and the coil, which are equal but opposite in phase. You can have an open
circuit on the other side of the coil and still tune for a dip in plate
current. Assuming stable plate voltage it is possible to have the same plate
current over a wide range of load impedances. Because the pi network looks
like a lumped section of transmission line, and because the plate tuning
capacitor serves the dual functions of resonating the plate circuit _and_
matching the plate circuit to the input of the pi network, variations at the
load end require slight adjustments of the plate tuning capacitor. To get
maximum power transfer to the load, the pi network must transform the plate
impedance to the load impedance, while also resonating the plate circuit.
There's more going on than simple impedance matching.

73,

"PM"

Gary, you may be right regarding my power output measurement, but I thought
I knew how you calculate DC power and AC power of sinusoidial waveforms on
my scope, which, by multiplying with 0.707 you have to reduce first to the
aquivalent of a DC voltage.

What confuses me in your suggestion (printed below) is that even after you
rectify an AC signal and send it through a filter capacitor you STILL
suggest to multiply it times 0.707.
I would have thought that would give you the wrong result??

Where do I go wrong, if at all??

Uwe


in article , JGBOYLES at
wrote on 3/12/04 17:19:

So a 50 resistor serves as my dummy load and a meter connected to
the dummy load indicates around 10 volts RMS. If I did my homework that
would indicate around 2 watt output.

Uwe, The 10 Vrms that your meter indicates may not be accurate at 3.5 or 7
MHZ. It is probably OK at 60 hz. What you can derive from the 10 Vrms is
that
the AC-1 is producing power, just not sure how much.
One thing you might try is a method to remove the frequency dependency of
your measurements. Use a 10:1 voltage divider (10k and a 1.11k). Run this
thru a Germanium or Schottky detector diode and a .01 filter capacitor. You
now have a DC voltage that is proportional to power, and relatively frequency
independent.
To calculate the RMS voltage across the 50 ohm load: Read the DC volts out
of the detector-Vdc. Then Vrms=(Vdc*.707)*10.
Example: You read 2Vdc out of the detector. Vrms=14.14 volts. Power into
the
50 ohm load is then: 14.14^2/50=4 Watts. The diode drop in the dector will
introduce some error at QRP levels, hopefully not too much for what you are
trying to do.
73 Gary N4AST