A THREE TUBE HYBRID AUTODYNE by gary // wd4nka

A THREE TUBE HYBRID AUTODYNE

( Click here for the complete article I wrote called
" A Three Tube Regenerodyne Receiver "  in 1998 )

WHAT IS IT?

The accompanying .bmp file shows a circuit that can best
be described as a very simple garden variety regenerative 
detector and one stage of audio with a frequency converter 
in front of it.   This is a departure from the types of Super 
Gainers normally described, however certainly not a new 
idea: even in more modern venues, an 80 meter receiver 
has been made multiband by adding a conversion stage 
before it, causing the 80 meter portion to assume the role 
of the IF strip, tuning in a 'window' of intermediate 
frequencies corresponding to the desired band of operation.  
A tunable detector, as it were.  This is commonly done with 
Direct Conversion Projects.

This project accomplishes the same thing, only using tubes 
and what in my opinion is a far more sensitive and 
misunderstood detector.  Because we are using a different 
method of tuning and combining the features of Superheterodyne 
and Autodyne operation, I will refer to this hybrid approach as 
a 'Regenerodyne ' scheme.

ADVANTAGES:

There are some advantages to this scheme of operation:

Selectivity-    The detector is allowed to operate at a
        considerably lower frequency where this type
        performs best, exhibiting greater selectivity
        by nature.

Stability-      Any time you isolate a regen detector
        from varying input reactance, you tend to
        stabilize it.  Also, RF radiation is greatly 
        attenuated ( and, on a frequency other than
        what you are listening to. )

Tuning-     Since the detector tunes a fixed range for 
        all bands, dial markings will remain accurate
        for all bands, provided the same decimal is
        used, in other words, if you use a 10.0, a 13.0,
        and a 17.0 MHz LO xtal, the dial decimal 
        markings will also remain constant.  

Sensitivity-    Although a regen detector is sensitive as 
        it is, there is conversion gain in the mixer 
        which can benefit the detector in that weak     
        signals may be heard with the regen setting 
        backed off a little, improving stability.

Construction-   Owing to the simplicity, parts placement is 
        not critical, beyond observing normal precautions
        and good practice.

Price-      My first prototype was built in the mid
        1990s, and prices have changed. However since 
        parts are few, and all chokes are hand wound,
        plus contributions from your junque box 
        and use of relatively cheaper tubes, this 
        project is not much more expensive or 
        difficult to build than an average 2 or
        3 tube 'genny.

A BRIEF CIRCUIT DESCRIPTION:

The incoming signal is coupled to the mixer through L1 
and is tuned through L3/C2.  This circuit can become a 
double tuned filter by the addition of another tuned circuit 
matching L3/C2, separated by either a coil link, or a 5 or 
10 pf cap.

The Local Oscillator (LO) signal is introduced to the 
front end by applying it to the cathode of the 6J7 
( or 6K7 ) through a .01 uF cap.  This is but one 
of several ways to accomplish mixing : look at the 
older handbooks for other ideas.

The triode used in the LO was one I had handy 
from an old National SW-3 project ( 6C5 ).  Use 
what you have handy, using appropriate voltages, 
biasing and such.  No tight tolerances here, gang.  
Note that this is an untuned pierce oscillator.  
I would suggest using an HC-6 xtal.  I have used 
the good ol' FT-243, but they can be slow starters 
in this circuit.  Go ahead, use that surplus 10 meg 
computer xtal !

The output of the mixer is NOT TUNED.  The IF 
is tuned by the detector alone.  There are several 
reasons for this.

The main reason is that we are not dealing with 
one IF frequency here, but a 'window' or range of 
them.  While it is true that one can track two 
variable caps ganged together to tune this window, 
another thing we are trying to do is allow the 
detector to operate stably.  Extra loading of the 
detector defeats that goal, which is simplicity and 
stability.  L3 is wound directly over L4, matching 
the number of turns in L4.  L5 is wound 1/2 inch 
above L4.   This is a rough manner of impedance 
matching between the mixer and the detector, a 
method used in the 1929 Bearcat model 3B in the 
August 1929 QST for matching the RF amp to the 
detector.

Images are kept at a minimum by a combination 
of the high Q inherent to the input LC of the detector, 
and the IF that is fairly low, yet not so low as to 
create images so close to operating frequency 
that extra filters and such are needed to prevent them.  
The front end LC is sufficient to prevent all but the 
loudest images, even at 15 MHz.  I have had some 
birdies from NMR, but an extra LC at the front end 
took care of that.

Of course , the detector is nothing more than a 
standard 6SN7 regenerative receiver, therefore 
no BFO is needed.  I settled upon using this tube 
because I had a lot of them from other regen projects.  
Use whatever you prefer.

I use an outboard 100v power supply to power this 
thing, but again, use what you desire.  My project 
should be viewed as a pattern to build around.  
If you want to use substitutions, different components 
or circuitry, do so.

CONSTRUCTION

There is no critical layout considerations beyond that
which I would consider normal practice.  Remember that
when you are making anything that generates RF that
is tunable, you are in essence making a VFO, and it
should be built like one.  This means considering the
components liable to be affected by surrounding environ
ment.  I remember having to operate a Drake with
the VFO shielding removed . . . . a total disaster !!
Put that shield on and it was a solid as the rock of 
Gibraltar.  So keep all coils and frequency determining
components as far from heat generating components as
possible.  Keep RF wiring distant from chassis bottom
as possible, and the filament wiring as close to chassis
bottom as you can.  Wire the grid leak (R3/C8) away from
filament wiring, as well.  Keep leads as short as you can.
Just out of habit I always solder in the bypass caps first.
Then the biasing resistors ( all the grounded resistors.)

When you wind the coils, which will not be very time
consuming because we are not dealing with REALLY low
frequencies, in fact, 3 MHz is actually fairly high for an IF,
but it is low enough to get good stable performance from
a well built regen.  Heathkit and Swan used to like 3 megs
for IF use as well.

In winding, I used those black 1-1/4 inch diameter 35 mm 
film canisters.  Wind L4 first.  Go clockwise or counter 
clockwise with your turns, but which ever way you go, do
the same direction for L5 and L3.   Any other suitable
coil form will do.  Pill vials, Quarter containers from Coin
shops are OK too.  Wind L5 about 1/2 inch above L4.
Next, wind L3, the primary.  I used a wide piece of two-way
carpet tape to cover L4 and wound L3 on it.  The tape helped
keep the windings in place.  Or, you can use stiff paper to
cover L4, and wind L3 on that, glueing the windings in place.

On one model, I found a coil form that slid over the 1-1/4 inch
form, leaving a space between the two forms which just allowed
for the #28 magnet wire.  This was great, because I had the
ability to vary the coupling some, though I never needed to.

L2 is wound on the same size form using the same size
wire.  

The windings as described, when coupled with about 150 - 
200 pF of variable capacitance should put you right about
2.8 to 3.3 MHz.

Just another comment here: you do not have to sweat over
doing exactly as I did.  If you feel more comfortable using
Toroids,  air-dux, whatever, and you feel confident you can
make a similar transformer with these, go for it !

If you want to try toroids, you might want to wind L3 and L4
on separate forms, ( the blue mix is good.) and wind L5 over
L4.  Then, tape the two toroids together.  Or mount them on
a spindle of sorts to vary the distance and vary coupling.
I never tried this approach, but it might work.

For the RFCs, you have freedom here too.  I sometimes 
recycle my defunct audio transformers by using the primaries
as RF chokes.  Especially as Filter chokes in small current
power supplies !

If you want to use high impedance headsets, you can wire
them in series with the B+ right off of pin 2 of the 6SN7.  Just
watch out for higher voltage.  You might want to use a .001uf
isolation cap, on second thought.

I wanted to use readily available hi-fi headphones, so I took 
the opportunity to place a .22 uF cap across the primary
of the plate - to - voice coil transformer, which does a good
job filtering off the high audio frequencies, effectively adding
some shape to the audio output.

I usually wire the power supply separately and away from
the actual receiver, just to keep the hum potential at a minimum,
but ground the chassis of the power supply to the chassis
of the receiver if you do this, do not rely on the ( - ) negative
voltage lead.

Even though you are tuning a lower frequency circuit,
I still recommend the use of Vernier Dials if you can find
them.  If not, use an insulated shaft to the tuning cap.
In ANY receiver it is a good idea to keep all varying reactance
as far from the VFO as possible.  I am fortunate to have
several PW reduction 50 : 1 HRO gears and a couple HRO
dials.  If you can locate one ( they go pretty cheap on eBay-
many collectors don't know what they are yet. )

If you see good verniers at the Hamfests, grab them !
They are not only functional, but great looking on the
ol' home-brew as well.


OPERATION and FIRE-UP

Once you have given a thorough wiring check, 
connect the voltages and turn it on.  Put a few feet of 
antenna on as well.  Test the detector first to make 
sure it's oscillating.  If it isn't, check the coil polarity, 
making sure all turns are going the same direction 
between L4 and L5.

Once you are satisfied the detector is working, plug 
in a LO xtal and tune the antenna preselect slowly.  
At two points you will notice a rise in noise level.  
At one point you may find a dead spot.  The dead 
spot is the preselect actually tuning the LO.  The 
two peaks are the two images.  Yup, for every one 
xtal you get two bands 6 MHz apart from each other.  
This reminds me of the Drake 2 series receivers.  
This can be used to your advantage: if you choose 
the right LO frequency, you can get 2 useable 
bands for the price of one!  The antenna preselect 
should provide enough selectivity to attenuate one for 
the other.  Using the 17.0 MHz xtal for 14.0 MHz, I can
also bring up 20.0 MHz and listen to the teletype.
A loud signal from one band has never caused me
grief in the other, except in one instance whereas I added
another LC to the antenna circuit and it was gone.

( A little later on I was listening to 20 meters and sheesh,
there was that image again on my commercial rig.  I didn't
feel so bad after that.  I added an extra LC to it as well. )

If you hear nothing, remove and replace xtal while listening.  
If there is no change, the LO may not be oscillating.  
Re check wiring: there's not a lot to go wrong.  Make 
sure the xtal is working.

One thing I have done is use a general coverage receiver, 
with the regenerodyne's antenna input connected to the input 
of  it.  With the detector regeneration set high, you can 
hear the actual heterodyne of where the receiver is tuned.  
It is weak, but it is there.  The mixer is working in reverse!

I would use a short antenna with these units, although I 
do use mine from time to time on the vertical on the roof.  
But after a rain, and the power lines go nuts, I use a fifteen 
foot piece of wire in a configuration one would use in an 
indoor attic random length antenna. I still hear the signals 
well enough to work them.  A separate receive antenna 
makes antenna switching less complicated as well.

ENDING COMMENTS

This has been a very brief version of an article written by
myself in the mid 1990s ( which goes into far more detail ).
However this text along with the schematic should suffice
to get you started.  I don't think I left out anything important.

Please let me know if you require added info, or want a photo
copy of the original piece.  It was intended for publication, but
never was sent do to relocation and subsequent distractions.

Available also is a modified mixer, using the same basic
scheme, but in a feedback arrangement for use as a Q
multiplier.  I call the project a " Q-dyne ", and can be used
as a stand-alone unit for other radios.  Incorporated
the same crystal pierce used in the r-dyne. (Regenerodyne)

vy 73
gary // wd4nka
" . . . who forgets the past forfeits the future . . ."