The AA8V 6x2 Superheterodyne Receiver
by Greg Latta, AA8V
Parts and Construction Information
Though several parts suppliers are listed on this page,
their listing does not constitute an endorsement.
Introduction:
Building a receiver such as the 6x2 receiver is a formidable project and should
not be entered into lightly. Building a tube superheterodyne receiver or SSB
transmitter is perhaps the most difficult project a builder can ever undertake.
Just finding all of the parts can be difficult, and some of the parts,
particularly the IF and BFO coils, can be impossible to find and will most
likely need to be wound by the builder.
To guarantee success the following should be done:
1. Each circuit should be breadboarded and tested before it is
permanently wired into the chassis.
2. The actual final wiring of each circuit should be worked out on paper
before any soldering is done. Soldering components should be the last
step in assembling that circuit, since it is very difficult to add connections
later.
Critical Components:
Though most of the components in the 6x2 receiver are not critical, there are
some components in the local oscillator and BFO that must be of the highest
quality and/or must be made with the highest care. These include the following:
Any components in the local oscillator circuit V1B such as:
1. Main tuning capacitor C7
2. Local oscillator coil L4/L5
3. Band Set capacitor C5
4. All fixed capacitors in the local oscillator circuit except the 0.001uf
tickler bypass capacitor. These must all be silver mica capacitors.
Any components in the BFO circuit V2B such as:
1. BFO oscillator coil L8
2. BFO adjustment capacitor C8
3. All fixed capacitors in the BFO circuit except the 0.01uf plate decoupling
capacitor. These must all be silver mica capacitors.
Input Network Components
Diagram of Input Coil L1/L1A
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Input Coil L1a/L1
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| Input Network Coils L1a/L1 and
L3:
The input network coils L1a/L1 and L3 are made according to the specifications
in the diagram and picture above. The coils were originally specified to be
made from B&W 3012 Miniductor or Illumitronic 632 stock. However, these are
either no longer available or very expensive.. Instead, wind the coils using
#29 enameled magnet wire (or similar) on plastic tubing (PVC, CPVC, and
plexiglass) with a diameter of 3/4" and space the turns out 32 per inch. I
used 3/4" plexiglass tubing and cut grooves in the tubing 32 turns per
inch on a lathe. It was then very easy to let the wire just ride in the
grooves. Coil L3 is exactly the same, except it does not have the 3 turn
primary winding.
The inductance of L1 and L3 was measured with an MFJ antenna analyzer and found
to be 16.38uH when measured at 2.00MHz. When L1 or L3 is connected in
parallel with the band capacitor C1, the
combination should tune from below the 80m band to above the 40m band.
When mounting the coils, keep them apart and at right angles to each other
as shown in the photo at right to avoid any coupling between them.
|
Mount input coils L3 and L1/L1a at right angles
to each other to prevent unwanted coupling.
(By the way, small coil at the center of the photo is L2.)
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| Input Network Coil L2:
L2 consists of 18 turns of #22 enameled magnet wire wound on a 1/4"
diameter plastic form or old style 1W carbon resistor of 100k or more
resistance. The inductance of this coil was measured with an MFJ antenna
analyzer and was found to be 1.15uH at 25.0MHz. The inductance of this coil is
not critical.
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Input Coil L2
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| Band Capacitor C1:
Band capacitor C1 tunes the input network made up of L1, L2, and
L3 to the 80m or 40m bands. It is a dual section
capacitor with a maximum capacitance of 140pf per section.
My capacitor was made from a dual 365pf unit by removing several of the rotor
plates from each section. First, mesh the capacitor and use a capacitance meter
to measure the actual capacitance of one of the sections. To remove a plate,
mount a thin cutoff wheel in a Dremel rotary tool and cut the phenolic plastic
that holds the plate to the other plates. The plate is then carefully pried
out. Remeasure the capacitance with the meter. The difference will tell you how
much the capacitance will decrease with each plate removed. Continue removing
plates until you reach the desired capacitance. Don't remove too many
plates! It is better to have one plate too many than one plate too few.
Remove the same number of plates from the same positions in the other section.
The sections must have the same number of plates or they won't track properly.
When a section is places in parallel with L1 or L3, the combination should tune
from below the 80m band to above the 40m band.
My capacitor contains built in trimmer capacitors which are on the other side
of the capacitor and cannot be seen in the photo. If your capacitor doesn't
have built in trimmer capacitors, you will need to solder a small mica trimmer
capacitor (30pf, C1a) across the capacitor section that connects across L1.
This allows the
front end to be aligned so the two sections track with each other.
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Band Capacitor C1
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Local Oscillator Components
Diagram of L4 and L5
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Local Oscillator Coil L4/L5
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Local Oscillator Coil L4/L5:
Along with main tuning capacitor C7 and
band set capacitor C5 local oscillator coil
L4/L5 is the most critical component in the entire receiver. The local
oscillator coil is made according to the specifications in the diagram and
picture above. This coil was originally specified to be made from B&W 3012
Miniductor or Illumitronic 632 stock. However, these are either no longer
available or very expensive. Instead, wind L4 and L5 using #29 enameled magnet
wire (or similar) on plastic tubing (PVC, CPVC, plexiglass, acrylic, etc.) with
a diameter of 3/4" and space the turns out 32 per inch. I used 3/4"
plexiglass tubing and cut grooves in the tubing 32 turns per inch on a lathe
just deep enough to hold the wire in place. The wire must be wound tightly
so that there is no movement possible. If you do not use grooves then you
might consider gluing to turns in place after you have tested the coil. Keep
the connecting leads as short as possible. Any looseness in this coil or the
connecting leads can lead to receiver instability and microphonics.
The inductance of L5 was measured with an MFJ antenna analyzer and found to be
5.4uH at 5.21MHz.
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| Main Tuning Capacitor C7:
As the heart of the receiver, C7 must be of the highest quality. The
bearings must be very smooth and the capacitor should turn effortlessly without
any slop or backlash,
C7 has a maximum capacitance of 35pf. My capacitor is a multi-section capacitor
with four sections. I only use the front section with three rotor plates, which
happens to have exactly the right capacitance. A National 5 to 1 vernier drive
drives the capacitor through a flexible coupling to keep the tuning smooth.
With 35pf maximum capacitance, the receiver covers about 340kHz on each band.
The maximum capacitance can be different from 35pf. If it is more, the receiver
will cover a wider frequency range but the bandspread will be poorer. If the
the maximum capacitance is less, the receiver will cover a narrower range of
frequencies, but the band spread will be better.
If your capacitor has too much capacitance, one way to cut it down is to put a
high quality (silver mica) capacitor in series with the variable capacitor. The
formula for two capacitors in series is C=(C1 x C2)/(C1+C2). In the formula, C
is the maximum capacitance of the combination, C1 is the maximum capacitance of
your capacitor, and C2 is the capacitance of the fixed capacitor. For example,
if your variable capacitor has a maximum capacitance of 100pf, and you place a
50pf silver mica capacitor in series with it, the result is effectively a
variable capacitor with a maximum capacitance equal to (100pf x 50pf)/(100pf +
50pf)=33pf, which is just about right.
If the capacitor construction permits, you may also be able to remove rotor
plates to cut down the maximum capacitance. In many cases these can simply be
pulled or pried out. Since this is irreversible, think twice before you do it!
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Main Tuning Capacitor C7
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| Band Set Capacitor C5:
Band set capacitor C5 is a 100pf air variable that is used to set the lowest
frequency of the local oscillator to 5200kHz. Even though it is
only set once and
then left alone, it should operate smoothly with no jiggle, backlash, or
looseness.
It should be high quality, since any looseness or change with temperature will
directly affect the tuning of the receiver. The capacitor is easier to set if
it has a screwdriver slot on the end of the shaft. My capacitor already had a
screwdriver slot, but if yours is lacking one you can use a Dremel tool cutoff
wheel to grind a screwdriver slot into the end of the shaft. Be sure to wear
eye protection while using the Dremel tool!
If you can't find a 100pf capacitor, you can compensate by increasing or
decreasing the value of the
68pf
silver mica capacitor in parallel with L5. For example, if your variable
capacitor is only 50pf, increase the 68pf capacitor to 68pf + 50pf=118pf. If
your capacitor is 125pf, then decrease the 68pf capacitor to 68pf - 25pf=43pf.
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Band Set Capacitor C5
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IF and BFO Components
| BFO Capacitor C8:
BFO capacitor C8 is used to adjust the pitch of a CW signal or to fine tune a
SSB signal. It has a maximum capacitance of about 5 pf. If you can't find a
small enough variable capacitor, you can take a larger variable capacitor and
cut away unwanted plates.
I made my capacitor from a 50pf variable trimmer capacitor by using a Dremel
rotary tool and cut-off disk to cut away the rotor and the stator until only
one rotor plate and one pair of stator plates was left.
Another way to decrease the total capacitance is to put a high quality (silver
mica) capacitor in series with the variable capacitor. The formula for two
capacitors in series is C=(C1 x C2)/(C1+C2). In the formula, C is the maximum
capacitance of the combination, C1 is the maximum capacitance of your
capacitor, and C2 is the capacitance of the fixed capacitor. For example, if
your variable capacitor has a maximum capacitance of 15pf, and you place a
7.5pf silver mica capacitor in series with it, the result is effectively a
variable capacitor with a maximum capacitance equal to (15pf x 7.5pf)/(15pf +
7.5pf)=5pf, which is just right.
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BFO Capacitor C8
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| IF and BFO Coils L6, L7, and L8:
Coils L6, L7, and L8 were originally specified as inductors made by North
Hills, which has been out of business a long time. I was lucky enough to have a
collection of brand new Millen 69045 copper core coil forms, but it is very
unlikely you will be able to find those either. As a result, these coils
must be wound by experimentation. They must be wound on quality coil forms
of some type, and they must be adjustable unless you use variable capacitors
across them to tune them to resonance.
The BFO coil L8 in particular must be tightly wound with the greatest care, and
on the best coil form possible. The windings must not be allowed to move. After
winding and testing the coil you might consider gluing the turns in place.
If L8 is not made with the greatest care the BFO will drift or will exhibit
microphonics.
Since I didn't know the exact inductance needed, my coils were made with double
layered windings to give them a large adjustment range. I do NOT recommend
doing this on your coils unless absolutely necessary, especially on the BFO
coil. Instead, keep the windings spread out and use as much of the coil form
as possible.
After I tested my coils by breadboarding them in an actual circuit to make sure
they would tune to resonance, I removed them and measured their inductances. To
my surprise I found the inductance for all three to be about 74uH ,
which should give you something to go on. If the inductance is 74uH, the coil
should resonate at about 1.85MHz when a 100pf capacitor is placed in parallel
with it.
The Q of these coils is NOT critical, since the crystal filter gives the
receiver its selectivity. However, the coils must tune to resonance at 1700kHz
when wired into the circuit. This is best assured by breadboarding the
circuits to test their actual operation before wiring them permanently into the
chassis.
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IF Plate Coil L7
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Large Transformers and Chokes:
| Power Transformer T2:
The power transformer is a Chicago Stancor PC-8405. The transformer cost $10.30
as listed in a 1967 Allied parts catalog. You won't find one for that price
now!
The transformer ratings are as follows:
Primary: 117V at 60Hz
HV Secondary: 540V CT at 120mA
Filament Secondary #1: 6.3V at 3.5A
Filament Secondary #2: 5V at 3A
If you use diodes in place of the 5Y3 rectifier you will not need a 5V
secondary. Also, actual current draw in the 6x2 is about 70mA, so a HV
secondary with a lower current rating, say 90mA, would also do. The HV
secondary can also be a little higher or lower. The
750
ohm voltage dropping resistor in the power supply can be adjusted to
accomodate the difference.
I sometimes experience large line voltage fluctuations in my area so I run my
receiver off of an old constant voltage transformer. In that case the HV in the
receiver is only 205V, and it still runs fine. Thus, there is a good bit of
tolerance in the actual value of the HV.
Don't skimp on the 6.3V secondary. The tubes and dial lamps draw almost 3A, so
be sure the 6.3V rating is at least 3A. (Don't forget that you can use a
separate 6.3V filament transformer if necessary.)
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Power Transformer T2
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| Power Supply Choke L9:
Power supply choke L9 is a 158M choke manufactured by Hammond Manufacturing and
rated at 10H at 100mA with a resistance of 195 ohms. The resistance isn't
really important. Actual current draw in the 6x2 receiver is about 70mA, so a
choke with a smaller smaller current rating could be used.
As of February 2016 it was available at Antique Electronic Supply via the
following link: Amateur
Electronic Supply P-T158M.
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Power Supply Choke L9
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| Audio Output Transformer T1:
The audio output transformer used in the 6x2 receiver is a high quality S-14
universal output transformer made by United Transformer. It is rated at 10
watts and has primary taps for 2500 ohms, 4000 ohms, 7000 ohms, and 10,000
ohms,.
By experiment it was found that the 7000 ohm tap on the primary worked best
with the 6AQ5A audio output tube used in the 6x2 receiver.
Output taps are provided for 500 ohms, 15 ohms, 8 ohms, and 2 ohms . Since 8
ohm speakers are the most common, the 8 ohm output was used.
The chances of finding a transformer like this today are very small. What you
are looking for is an output transformer with a primary impedance between 4000
ohms and 7000 ohms and an output impedance between 3 ohms and 16 ohms. It
should be able to handle 2 to 5 watts. The transformer below was available in
February of 2016 from Antique Electronic Supply:
Hammond P-T1750C
It is the type of transformer that should work, though I haven't used it
myself. I list it only to give you an idea of what you are looking for.
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Audio Output Transformer T1 |
Miscellaneous Components:
| Terminal Strips:
Building vintage equipment is impossible without the use of phenolic lug type
terminal strips. They are used throughout the receiver to mount components and
to provide wiring tie points. They are made in a wide variety of styles.
Different numbers of lugs are available and the lugs can be grounded or
ungrounded. In the photo at right a 5-lug terminal strip is used to hold the
power supply filter capacitors. On this strip the middle lug is grounded and
the outside lugs are ungrounded. They can sometimes be found at hamfests, and,
as of February 2016, these tie strips were available from Surplus Sales of
Nebraska and Antique Electronic Supply at the following links:
Terminal
Strips at Surplus Sales of Nebraska
Terminal
Strips at Antique Electronic Supply
I suggest that you spend a bit of money and buy an assortment. You can look at
the interior photos of the 6x2
receiver to see how and where they are used.
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5-Lug Terminal Strip in the Power Supply
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| Dipped Silver Mica Capacitors:
Dipped silver mica capacitors must be used in the local oscillator and BFO
circuits. They have a zero temperature coefficient and using them will result
in minimal drift. The use of other types of capacitors, such as ceramic disk
capacitors, will result in drift and inferior performance. Do not try to cut
corners in the local oscillator or BFO!
As of February 2016, these capacitors were available from RF Parts and Surplus
Sales of Nebraska at the following links:
RF Parts -
Dipped Silver Mica Capacitors
Surplus Sales of
Nebraska - Dipped Silver Mica Capacitors
They are also available from DigiKey and
Mouser Electronics.
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Dipped Silver Mica Capacitors
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| Carbon Film Resistors - Through Hole:
All of the smaller resistors in the 6x2 receiver are carbon film through hole
resistors. They are more stable than the older carbon composition resistors and
are readily available from suppliers such as Mouser Electronics and
DigiKey. Most of the resistors in the 6x2
are 1/2 watt (500mW).
When searching for these at, for example, Mouser Electronics, be sure to specify that
you want carbon film resistors - through hole. Otherwise you will
probably get listings for the surface mount resistors.
The ones I used in the 6x2 receiver are the Xicon brand from Mouser
Electronics.
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Carbon Film Resistors - Through Hole
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| Ceramic Compression Trimmer Capacitors:
Ceramic compression trimmer capacitors are used in several places in the 6x2
receiver. These use a mica dielectric and pack a lot of capacitance in a small
package. One is used for the WWV circuit and several are used in the crystal
filter.
As of February 2016, these capacitors were available from RF Parts and Surplus
Sales of Nebraska at the following links:
RF
Parts - Ceramic Trimmer Capacitors
Surplus
Sales of Nebraska - Compression Trimmer Capacitors
|
Ceramic Compression Trimmer Capacitors
|
Electronic Parts Sources
Though I have dealt with all of these sources, listing them
does not constitute a legal endorsement of any kind.
Modern Electronic Parts:
Mouser Electronics, Digi-Key, and Arrow
Electronics
Between Mouser Electronics, Digi-Key, and Arrow you should be able to find just
about any modern electronics component you could need. They carry resistors,
capacitors, semiconductors, screws and hardware, fans, potentiometers, encoders
etc. You name it, they have it. On the off chance one doesn't have what you
want the other will. They are happy to deal with small buyers such as you and
me. Their web sites are:
Mouser Electronics
Digi-Key
Arrow Electronics
RF Parts
RF Parts is a great source for both modern and classic parts. They carry a full
line of transmitting tubes along with dipped silver mica capacitors, ceramic
trimmer capacitors, variable capacitors, transformers, chokes, coaxial
connectors, and much more.
RF Parts
Vintage and Surplus Electronic Parts:
Surplus Sales of Nebraska
As the name suggests, Surplus Sales of Nebraska deals in mostly surplus
equipment. They carry a good selection of ceramic trimmer capacitors, larger
variable capacitors, tubes, dipped silver mica capacitors, terminal strips,
transformers, and chokes. Transformers and chokes in particular can be
expensive, and can often be found surplus at attractive prices.
Surplus Sales of Nebraska
Antique Electronic Supply
Antique electronic supply now seems to cater heavily to the tube guitar
amplifier market. They carry a great selection of power transformers, chokes,
tubes, and other vacuum tube related parts, such as tube sockets, octal plugs,
terminal strips, and so on.
Antique Electronic Supply
Mechanical Parts Sources
Though I have dealt with all of these sources, listing them
does not constitute a legal endorsement of any kind.
Enco and MSC Direct:
Both Enco and MSC Direct are large industrial suppliers. They are both happy to
work with small individual buyers, and have monthly sales flyers that usually
contain great deals. They often have special sales where you can get free
shipping or 10% to 30% off. I recommend getting on their mailing list to
receive their monthly sales flyers. They not only carry tools, but also raw
materials, such as aluminum and brass. If you wait for one of their special
deals, you can get your aluminum or brass on sale and get it shipped shipped
free of charge too.
Enco
MSC Direct
OnLineMetals.com:
For any kind of metal or plastic you can't beat OnLineMetals.com. They carry
every conceivable metal and plastic in every form you can imagine.
OnLineMetals.com
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comments, you can send E-Mail to Dr. Greg Latta at
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