On Fri, 21 Nov 2003 21:29:19 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote:

I've built several GDO's over the years. My favorite circuit is the old
standby using a split stator variable capacitor and a shunt fed coplets
oscillator. This capacitor can be 100-250 pf per section. My favorite
tube is the 6CW4 nuvistor, though a 6DV4 might
be a better choice at VHF. I put a TO-5 transistor heat sink over the
tube fearing it might run hot! I use a cathode resistor of about
150-330 ohms (un-bypassed) and connect a bnc connector to the cathode
via a 1000pf capacitor to steal a little signal to drive my frequency
counter. With the counter set for low level input I get good drive up
to about 100mhz or so. My coil socket is an RCA (phono) connector and
the coils use a matching plug. A better idea might be to use a 3 pin
DIN plug and socket, this allows for a ground connection to go to a coil
center tap. If you add the center tap the circuit will work below
1.5mhz with proper coils, otherwise it will not oscillate below this
frequency! (Having coils that go to 455khz would be nice to test if
stages). Power supply used two 6.3v 300ma filament transformers back to
back with a bridge rectifier. You can find transformers small enough to
fit the supply into a box small enough to house the instrument, yet not
too big to be hand hold-able. The coils can be wound on 1/4" or 3/8"
plastic water line intended for ice maker use. These will fit into
small places. Use of peaking chokes in the plate and filament lines can
help eliminate "drop outs" in frequency coverage. Sensitivity can be
adjusted with a plate pot, or put the pot in the meter circuit. I have
used sub-mini meters salvaged from old jap tape recorders (vu-meters).
These are typically around 500ua sensitivity. See my web site at
www.qsl.net/wa2mze.

Thanks. Do you reckon there's any benefit in using toobz over FETs,
then?

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

On Sat, 22 Nov 2003 13:50:38 GMT, "Michael A. Terrell"
wrote:


You can use a capacitor in series with the variable capacitor to reduce
the tuning range, but you would need a switch to select the high or low
capacitance range, and to move the coil connection to the active
components. That, or use a dual capacitor, build two circuits, and
select one oscillator at a time.

in practice - a not so good idea!
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On Sat, 22 Nov 2003 13:50:38 GMT, "Michael A. Terrell"
wrote:


You can use a capacitor in series with the variable capacitor to reduce
the tuning range, but you would need a switch to select the high or low
capacitance range, and to move the coil connection to the active
components. That, or use a dual capacitor, build two circuits, and
select one oscillator at a time.

in practice - a not so good idea!
--
remove ,xnd to reply (Spam precaution!)

Paul Burridge wrote:
On Fri, 21 Nov 2003 21:29:19 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote:


I've built several GDO's over the years. My favorite circuit is the old
standby using a split stator variable capacitor and a shunt fed coplets
oscillator. This capacitor can be 100-250 pf per section. My favorite
tube is the 6CW4 nuvistor, though a 6DV4 might
be a better choice at VHF. I put a TO-5 transistor heat sink over the
tube fearing it might run hot! I use a cathode resistor of about
150-330 ohms (un-bypassed) and connect a bnc connector to the cathode
via a 1000pf capacitor to steal a little signal to drive my frequency
counter. With the counter set for low level input I get good drive up
to about 100mhz or so. My coil socket is an RCA (phono) connector and
the coils use a matching plug. A better idea might be to use a 3 pin
DIN plug and socket, this allows for a ground connection to go to a coil
center tap. If you add the center tap the circuit will work below
1.5mhz with proper coils, otherwise it will not oscillate below this
frequency! (Having coils that go to 455khz would be nice to test if
stages). Power supply used two 6.3v 300ma filament transformers back to
back with a bridge rectifier. You can find transformers small enough to
fit the supply into a box small enough to house the instrument, yet not
too big to be hand hold-able. The coils can be wound on 1/4" or 3/8"
plastic water line intended for ice maker use. These will fit into
small places. Use of peaking chokes in the plate and filament lines can
help eliminate "drop outs" in frequency coverage. Sensitivity can be
adjusted with a plate pot, or put the pot in the meter circuit. I have
used sub-mini meters salvaged from old jap tape recorders (vu-meters).
These are typically around 500ua sensitivity. See my web site at
www.qsl.net/wa2mze.


Thanks. Do you reckon there's any benefit in using toobz over FETs,
then?

Probably not, but there isn't any disadvantage to using a tube, except
for having to use an AC operated supply. If you need a portable grid
dipper (to check antennas up on the tower for example) then you want a
solid state battery operated unit (which sure beats 100 feet of
extension cord dragging behind you when you climb the tower!).

Not all fets function well into the vhf range. If you want to build a
solid state dipper using fets make sure you get vhf units, not ones
intended for audio use. There are circuits using fets that simulate
negative resistance (like tunnel diodes) which make for an interesting
oscillator since no coil taps are needed and a single gang cap can be used.

Paul Burridge wrote:
On Fri, 21 Nov 2003 21:29:19 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote:


I've built several GDO's over the years. My favorite circuit is the old
standby using a split stator variable capacitor and a shunt fed coplets
oscillator. This capacitor can be 100-250 pf per section. My favorite
tube is the 6CW4 nuvistor, though a 6DV4 might
be a better choice at VHF. I put a TO-5 transistor heat sink over the
tube fearing it might run hot! I use a cathode resistor of about
150-330 ohms (un-bypassed) and connect a bnc connector to the cathode
via a 1000pf capacitor to steal a little signal to drive my frequency
counter. With the counter set for low level input I get good drive up
to about 100mhz or so. My coil socket is an RCA (phono) connector and
the coils use a matching plug. A better idea might be to use a 3 pin
DIN plug and socket, this allows for a ground connection to go to a coil
center tap. If you add the center tap the circuit will work below
1.5mhz with proper coils, otherwise it will not oscillate below this
frequency! (Having coils that go to 455khz would be nice to test if
stages). Power supply used two 6.3v 300ma filament transformers back to
back with a bridge rectifier. You can find transformers small enough to
fit the supply into a box small enough to house the instrument, yet not
too big to be hand hold-able. The coils can be wound on 1/4" or 3/8"
plastic water line intended for ice maker use. These will fit into
small places. Use of peaking chokes in the plate and filament lines can
help eliminate "drop outs" in frequency coverage. Sensitivity can be
adjusted with a plate pot, or put the pot in the meter circuit. I have
used sub-mini meters salvaged from old jap tape recorders (vu-meters).
These are typically around 500ua sensitivity. See my web site at
www.qsl.net/wa2mze.


Thanks. Do you reckon there's any benefit in using toobz over FETs,
then?

Probably not, but there isn't any disadvantage to using a tube, except
for having to use an AC operated supply. If you need a portable grid
dipper (to check antennas up on the tower for example) then you want a
solid state battery operated unit (which sure beats 100 feet of
extension cord dragging behind you when you climb the tower!).

Not all fets function well into the vhf range. If you want to build a
solid state dipper using fets make sure you get vhf units, not ones
intended for audio use. There are circuits using fets that simulate
negative resistance (like tunnel diodes) which make for an interesting
oscillator since no coil taps are needed and a single gang cap can be used.

On Sat, 22 Nov 2003 18:20:46 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote:



Not all fets function well into the vhf range. If you want to build a
solid state dipper using fets make sure you get vhf units, not ones
intended for audio use. There are circuits using fets that simulate
negative resistance (like tunnel diodes) which make for an interesting
oscillator since no coil taps are needed and a single gang cap can be used.

Right. I don't like fets, execpt as switches, RX RF amplifier and in
PA. We had a discussion in a radio club among designers 30 years ago
if it was any reason for using fets in oscillators instead of
bipolars, but couldn't really find any good reason, except they were
fancy.
It is also a circuit using a jfet and pnp-combination which works in a
negative impedance mode, but usually found on lower frequencies. Used
it for some time to check resonnant frequency of unknown tuned
circuits with a counter, but the problem is to control oscillation
level. It worked fine for 50% of the applications while the counter
indicated some higher (or lower) frequency for some circuits, so it
was abandomed. Believe I have seen it mentioned for griddipmeters,
too.
It seems to be a reason why PNP UHF transistors are used in most UHF
tuners found here, usually BF479, BF679, BF979. I've copied some of
the text for DL7QY's dip meter covering up to 1400MHz on
http://home.online.no/~la8ak/m3.htm

-JM


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On Sat, 22 Nov 2003 18:20:46 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote:



Not all fets function well into the vhf range. If you want to build a
solid state dipper using fets make sure you get vhf units, not ones
intended for audio use. There are circuits using fets that simulate
negative resistance (like tunnel diodes) which make for an interesting
oscillator since no coil taps are needed and a single gang cap can be used.

Right. I don't like fets, execpt as switches, RX RF amplifier and in
PA. We had a discussion in a radio club among designers 30 years ago
if it was any reason for using fets in oscillators instead of
bipolars, but couldn't really find any good reason, except they were
fancy.
It is also a circuit using a jfet and pnp-combination which works in a
negative impedance mode, but usually found on lower frequencies. Used
it for some time to check resonnant frequency of unknown tuned
circuits with a counter, but the problem is to control oscillation
level. It worked fine for 50% of the applications while the counter
indicated some higher (or lower) frequency for some circuits, so it
was abandomed. Believe I have seen it mentioned for griddipmeters,
too.
It seems to be a reason why PNP UHF transistors are used in most UHF
tuners found here, usually BF479, BF679, BF979. I've copied some of
the text for DL7QY's dip meter covering up to 1400MHz on
http://home.online.no/~la8ak/m3.htm

-JM


--
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J M Noeding wrote:
On Sat, 22 Nov 2003 18:20:46 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote:



Not all fets function well into the vhf range. If you want to build a
solid state dipper using fets make sure you get vhf units, not ones
intended for audio use. There are circuits using fets that simulate
negative resistance (like tunnel diodes) which make for an interesting
oscillator since no coil taps are needed and a single gang cap can be used.


Right. I don't like fets, execpt as switches, RX RF amplifier and in
PA. We had a discussion in a radio club among designers 30 years ago
if it was any reason for using fets in oscillators instead of
bipolars, but couldn't really find any good reason, except they were
fancy.
It is also a circuit using a jfet and pnp-combination which works in a
negative impedance mode, but usually found on lower frequencies. Used
it for some time to check resonnant frequency of unknown tuned
circuits with a counter, but the problem is to control oscillation
level. It worked fine for 50% of the applications while the counter
indicated some higher (or lower) frequency for some circuits, so it
was abandomed. Believe I have seen it mentioned for griddipmeters,
too.
It seems to be a reason why PNP UHF transistors are used in most UHF
tuners found here, usually BF479, BF679, BF979. I've copied some of
the text for DL7QY's dip meter covering up to 1400MHz on
http://home.online.no/~la8ak/m3.htm

-JM


--
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The negative resistance fet circuit I mentioned used a pair of jfets,
one P channel and one N channel with the sources tied together and the
gates of each transistor tied to the drain of the other. In this way
each transistor acts as the self bias resistor of the other. The
resulting voltage/current curve of the combo exhibits a region where the
current decreases as the voltage increases, looking much like the 'kink'
in the plate circuit curves of an old time tetrode tube (such as the
24A). The major disadvantage of this circuit for a gdo is that it is
such a strong oscillator that you don't get much of a dip (you need to
sample the output and feed it to a rectifier and meter) unless you load
the coil down with a shunt resistor....and you need a different value
for each range. Also P channel fets with good vhf performance are as
rare as hen's teeth.

J M Noeding wrote:
On Sat, 22 Nov 2003 18:20:46 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote:



Not all fets function well into the vhf range. If you want to build a
solid state dipper using fets make sure you get vhf units, not ones
intended for audio use. There are circuits using fets that simulate
negative resistance (like tunnel diodes) which make for an interesting
oscillator since no coil taps are needed and a single gang cap can be used.


Right. I don't like fets, execpt as switches, RX RF amplifier and in
PA. We had a discussion in a radio club among designers 30 years ago
if it was any reason for using fets in oscillators instead of
bipolars, but couldn't really find any good reason, except they were
fancy.
It is also a circuit using a jfet and pnp-combination which works in a
negative impedance mode, but usually found on lower frequencies. Used
it for some time to check resonnant frequency of unknown tuned
circuits with a counter, but the problem is to control oscillation
level. It worked fine for 50% of the applications while the counter
indicated some higher (or lower) frequency for some circuits, so it
was abandomed. Believe I have seen it mentioned for griddipmeters,
too.
It seems to be a reason why PNP UHF transistors are used in most UHF
tuners found here, usually BF479, BF679, BF979. I've copied some of
the text for DL7QY's dip meter covering up to 1400MHz on
http://home.online.no/~la8ak/m3.htm

-JM


--
remove ,xnd to reply (Spam precaution!)
The negative resistance fet circuit I mentioned used a pair of jfets,
one P channel and one N channel with the sources tied together and the
gates of each transistor tied to the drain of the other. In this way
each transistor acts as the self bias resistor of the other. The
resulting voltage/current curve of the combo exhibits a region where the
current decreases as the voltage increases, looking much like the 'kink'
in the plate circuit curves of an old time tetrode tube (such as the
24A). The major disadvantage of this circuit for a gdo is that it is
such a strong oscillator that you don't get much of a dip (you need to
sample the output and feed it to a rectifier and meter) unless you load
the coil down with a shunt resistor....and you need a different value
for each range. Also P channel fets with good vhf performance are as
rare as hen's teeth.

Uncle Peter wrote:

It is kind of hard to get the proper coupling on PCB style coils.

Bingo.

Those were intended for larger tube circuit designs, not PCB
based gear.

Daily Double!

Dana