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.