AMATEUR VACUUM TUBE MAKING
BUILDING A HAND HELD GETTER FLASHER cont.
Here are some photos and details of the construction. Follow these plans closely if you want the best chance of success because there
are many parts of the design that are critical. This is especially the case of the shielded oscillator box and the construction of
the output tuned circuit.
The oscillator box is made to enclose the circuit board completely. It is not necessary to solder a top onto the box. The bottom can
either be soldered to the sides or small screws can be positioned in the corners to hold it so it can be easily removed later if necessary.
Make the box size such that it will not touch the outer case. The box and power supply are mounted to a piece of 6mm perspex
or delrin plastic sheet to insulate it from the outer case.
The potentiometer shaft must also be carefully insulated from the outer
case. This unit uses a small swtiching supply for the 12 volt supply instead of the transformer. The switching supply must be modified
slightly to get the signal for the zero crossing trigger.
The heatsink is mounted such that holes can be drilled for air flow through the outer case. This one the holes are drilled through
the perspex mounting plate and the outer case. Matching holes are drilled in the top cover. This version shows the 120 volt design
with the bridge rectifier mounted to the heatsink with the power FETs. Wire the FETs with short direct heavy wire. The protection
zeners should be soldered directly to the FET pins. The trigger switch is mounted on a small bracket. Be sure the body of the switch
does not touch live wiring. The perspex mounting plate does not have ot extend past the heat sink. The tuned circuit components will
be mounted directly to the case.
The tuned circuit terminals are made of sheet copper. Use a dremmel tool to cut slots in the end piece for the electrodes. Make the
electrodes the length of the set of capacitors you use. Fold the copper sheet around the plastic as shown. This uses 1/2 mm thick
copper sheet obtained from an art supply store. You can get copper sheet by cutting lengthwise a section of 15mm dia. copper refrigeration
tubing. This is then flattened out giving a surprisingly large sheet of copper. Mount the plastic to the bottom tray with small screws.
Make matching mounting holes in the cover so the plate will be held firmly.
The coil is wound with #10 or #12 AWG solid wire. (4mm or 3mm dia) Square wire is easiest to wind but much more difficult to find.
Clamp the end of the wire to a form, which can be a piece of wood or metal with the inside diameter you want. Tightly wind the wire
into two side by side turns. Then, carefully wind the third turn over the second, and the 4th turn over the first. This is tricky
to do but you can do it if you work at it. Be careful not to damage the insulation. If you only have bare wire, use kapton tape to
insulate the wire first. Bend the ends of the coil wires to match the width between the terminals on the end plate.
Solder some copper screw lugs to the ends of the coil wires. These screw onto the end plate terminals. By using screw lugs, you can
change coils if you later want to make a special size. If you are not planning to change coils, you can solder the coil wires directly
to the end plate terminals.
Use short direct wiring between the driver box and the output FETs. Note the heavy common wire between the shield box and the output
FETs. This wire should also connect to the heatsink fins to eliminate RF radiation from it.
The output coupling capacitors are wired
with as short of leads as practical. All of this fits nicely in the housing. The tuning lamp is mounted such that a small hole can
be drilled in the cover to see it. Note the very short leads on the .033 mfd resonating capacitors. Use copper sheet to wire from
the capacitor wires to the output terminals. This can add 25% more power to the output than just bending the capacitor wires to the
The first scope photo shows the trigger waveforms. One pulse on the collector of the transistor for each time the rectified AC goes
to zero. It is essential that this wave be stable. Adjust the 2k resistor from the base to common if necessary to get the waveform
to be solid.
The second photo shows the overlap of the driver outputs. The two waveforms are positioned using the vertical position
controls to just overlap. There must be no overlap above the 3 volt level which is where the FETs turn on. It is best to have a slight
dead space between the two waves, but the falling wave with no dead space is acceptable so long as the voltages of the two waves are
below 3 volts when they cross. Adjust the 1000 ohm resistors to increase the dead time. Do not make the dead time too much, because
then you will have to run more current through the FETs to get the full output power.
The first photo is of the output of the oscillator gates. Note the obvious dead time between the rise and fall of the two waves.
second photo is of the gates of the power FETs showing the dead time. The photo is the same as the one above but closeup so you can
see the details better. The scale is 5 volts per division so you can see that there is no overlap above the 3 volt turn on threshold
of the FETs.
If there is excessive ringing or overshoot your wiring is too long. Use the shortest most direct wiring you can between
the sections. The common connection must be heavy wire.
If you follow these plans and get the waveforms shown you should have
no trouble at all in making a good solid unit that will work perfectly to flash getters. Remember that this unit is for short duration
activation only. Keep below 10 seconds at a time and let the unit cool off for a few minutes. This is far more time than it takes
to flash a getter so you should have no trouble at all.
If you have questions you can email me at ronsoy2 (at symbol) att.net.
The email address has to be done this way to prevent spam robots from causing trouble.