maximum 5 and minimum just under
unity. The op-amp unity gain
bandwidth is 3. 5 MHz so that is not a
concern and distortion will be nil as
long as the amplitude of the output
does not exceed VCC-two volts peak.
QIn the September ‘07 issue
(page 30, Figure 4), I am
interested in the fly zapper
schematic. I have a raccoon
tearing up our trash even though it is
heavily secure. The coon is smart, big,
and strong. Here’s my thought — I do
know a little about auto ignition and
for the coil to output a good strong
spark, it needs the correct ignition
cap. Cap needs to be around 0.06 µF,
but that depends on coil make. I have
two coils for old VW Beatles without
the caps and the local auto supply
store (Pep Boys) wants an arm and a
leg for the caps.
Also, for me to use your circuit I
would need it to fire about 2X per
second. I’ll call it RussKin’s Critter
Getter. Any ideas on that? I’ll bet that
arm and leg that many people have
the same problem with critters!
— Bob Jay (long-time subscriber)
AI can help you with the cap:
0.068 µF from Mouser, 72
cents, part number 5989-
1KV.068-F. You can connect
the cap across the coil primary and
eliminate R1 and D1. To make the
555 run at 2 Hz, change R2 to 1.2K
and R3 to 3. 3 meg. Use C2 = 0.1 µF,
Mouser part number 5989-250V.1-F.
■ FIGURE 3
QI have a circuit that I got
from a magazine (transistor),
but I wonder if I could
convert it to tubes. I know
tubes are old technology, but I like
them. My endeavor was the circuit in
Figure 4 — an OTL tube circuit. I had
to make some compromises but I
wonder if this scheme would work?
The first tube is a pentode and a triode
in one. The other tubes are power
pentodes. I’m not sure if the biasing is
right. I do know the transistor circuit
says that the 680K R2 is adjusted to
bring Q4, Q5 emitters to half the supply
voltage. Is this tube circuit practical?
One thing I know, the speaker will
have to be higher impedance.
— Craig Kendrick Sellen
ADC coupled tube circuits
are difficult. Tektronix used
to do it using NE-2 neon
bulbs for level shifting.
But let me list the problems with the
1) There is no tube equivalent for a
PNP transistor. The cascode circuit
will have low impedance pulling up
and high impedance pulling down.
That will not be a problem if the
speaker impedance is high enough;
but high impedance implies low
power, and I don’t know where you
would find such a speaker.
2) With the cathodes at different
potentials, you can’t connect the
3) With V5 cathode near ground and
V7 cathode at –V, V7 will be saturated. R8 should return to –V and V7
needs a resistor in series with the
cathode to limit the current.
4) The connection between V4 plate
and V5 grid is the place to put the
NE-2 neon to bring the grid down
close to –V.
Enough of the critique; vacuum