It has become apparent through
email correspondence that some would be constructors of the receivers I've
described are unsure as to some aspects of their construction.
So, I'm attempting to summarise
on this page some important points.
First, the receivers in
question:
http://www.users.bigpond.com/cool386/6c4/6c4.htm
http://www.users.bigpond.com/cool386/12at7/12at7.html
http://www.users.bigpond.com/cool386/new12at7/new12at7.html
http://www.users.bigpond.com/cool386/tradio/tradio.htm
http://www.users.bigpond.com/cool386/amfm/amfm.htm
The main tuned circuit also
applies to the pulse counting receiver:
http://www.users.bigpond.com/cool386/pcrx/pcrx.html
The tuned circuit:
For the usual 88-108Mc/s
coverage, this consists of 4 turns of 18 B&S tinned copper wire air
cored, with a diameter of 10 mm. A drill bit or suitable felt pen makes
a good former to wind the coil.
The associated tuning condenser
needs to be 15pF for full band coverage. You can use other values of tuning
condenser as I have done with some of the receivers; e.g. a 33pF in series
with a 60pF variable will give correct tuning range, albeit with some extension
of low frequency coverage.
Varicap diodes can be used
as per the Model T Ford car radio; in which case you'll need to find suitable
diodes, and provide the tuning supply from a well regulated source. Ideally
this is based around a temperature compensated regulator IC specially made
for the purpose. Typical types are TAA550 and ZTK33. Despite the way they
are used, these devices are not zener diodes. Zener diodes on their own
will cause tuning drift.
The best tuning device is
an air spaced variable condenser. Failing that, the modern kind with plastic
dielectric will work, although the losses are higher. Least to be preferred
are varicap diodes with their low Q.
If you can get silver plated
wire for the coil, it's worth using, but don't go out of your way to get
it. I haven't tried the more common enamel or just bare copper wire in
these circuits, but from experiments long ago these kinds of wire should
work.
Those 15uH RF chokes:
A very critical part of
the design. Get these wrong and the receiver will not oscillate across
the whole band. The chokes I used were once sold by Dick Smith Electronics.
They are no longer available from them. Don't ask for a part number because
I had no idea who manufactured them. However, to describe, they are wound
on a ferrite former and encased in blue heatshrink tubing. Both leads emerge
at the one end, so they stand on end when mounted on a PCB.
There is a more common kind
of choke which looks just like a 1/4W resistor. These do not work. I haven't
measured their characteristics to determine why, but I suspect lack of
Q.
The choke I now use and
recommend is home made, not just because the original is hard to get, but
because the performance is far superior. The chokes consist of 75
cm of 25 B&S enamelled copper wire wound on a 6.3mm plastic form.
Heater wiring:
The 12AT7 can have its two
heaters connected in series to allow 12.6V operation, or in parallel for
6.3V. However, in these circuits this appears not to be a good thing to
do.
I would prefer would be
constructors to wire the 12AT7 for 6.3V operation. Pin 9 is to be earthed,
and pins 4 and 5 connected together and fed from 6.3V. A 1000pF (.001uF)
ceramic condenser is also connected from pins 4 and 5 to earth. Why is
this all so critical when the 12AT7 has indirectly heated cathodes? The
reason is due to the heater to cathode capacitance being very significant
at VHF. So, the heaters are actually live at RF and therefore need to be
bypassed to earth. Not doing so means the heater wiring will affect the
operation, particularly in terms of the detector oscillating.
As for not recommending
the heaters being wired in series, it would appear that despite bypassing
pin 9, some RF is coupled from the detector back to the RF amplifier internally
via the heaters. If you must persist with wiring the heater for 12.6V operation,
bypass pin 9 to miminise interaction between the two heaters and earth
the detector side of the heaters. Of course bypass the 12.6V supply to
the heater. Again, I don't recommend this and it could be responsible for
improper operation. For series heater circuits, the 12AT7 should be the
last in the chain so that its heater is actually earthed.
Mechanical construction:
One constructor who was
having problems had assembled the receiver simply by connecting the parts
together in mid air. There was no chassis. Not only must lead length be
kept as short as possible, the receiver must be built on a metal chassis.
The tuning condenser must be mounted rigidly; not a trimmer capacitor suspended
in mid air by wires.
Voltages:
Keep the B+ at 140-150V.
Consumption is around 4ma including the RF amplifier. Heater requirements
are 6.3V at 150 ma for 6C4 and 6.3V at 300 ma for the 12AT7 or 12AU7. Audio
output from the 12AT7 detector plate (via the filtering components) is
around 300mV. It is meant to be fed into a high impedance load; e.g.. 500K
or 1M. Not solid state amplifiers with their low input impedance! If you
must use a low impedance input, then use a FET input stage at the amplifier
input or a cathode follower on the 12AT7 output.
Output is suitable for a
two stage amplifier. Typical examples would be valves like 6BM8 or 6GW8,
or where separate valves are used, 12AX7 and 6AQ5, 6J7 and 6V6 etc.
High impedance headphones
can be driven straight from the filtered audio output at a level similar
to a crystal set.
Valves:
With the commercially made
15uH chokes, 12AT7 must be used. With the home made chokes, performance
is improved to the point where the lower gain valves 12AU7 and 6C4 (equivalent
to half a 12AU7) will work. Interestingly, despite their lower gain, the
detected audio output appears to be somewhat higher than the 12AT7 provides.
Obviously if using 6C4's instead of the 12AT7, two are needed (assuming
the RF amplifier stage is being retained).
Note that in varicap (and
possibly plastic dielectric condenser) tuned receivers, the Q is reduced
to the point where 12AT7 must be used; the 12AU7 won't work even with home
made RF chokes.
As a final note, please read all the articles and look at all pics and circuits before thinking of constructing.