How does this reflex receiver work?

[THINKER_KAM_MAN]

just to say

in the circuit only the AF is feed back if it went around again it
wold echo or oscillate at audio frequencies.
the reason it won't go round a gain is because at audio
frequency the 100 k resistor is the load and the choke is
a low value resistor, so no audio voltage is developed across it.
i can not see where the RF is fed back, but if you want to feed
the RF back wind another coil round the ferrite rod and use a
resistor or capacitor to control the level of +Ve feed back from anode to grid. (call this then would be a re-gen reflex receiver)

well now thinking about it i can now see where
the rf is feed back, i think its via the ratio of
the diode capacitance to the RF decoupling capacitor connected to GND
controlled by the feed back resistor and the cap value.

its a simple circuit and it seems to work first time
ha now that the circuit has been improved by the committee design team, can you now put up the improved
circuit for further comments.

 

[betwixt]

Incorrect - there is no RF feedback in the original schematic. The feedback is AF, in other words V1A works as an RF amplifier once and then again as an AF amplifier. The diode is there to rectify the RF, producing a negative voltage proportional to the amplitude of the signal (its an AM receiver!) which is then used to bias the grid. C3 is there to prevent RF feedback, it's value is chosen to 'short out' any RF but not isn't large enough to seriously attenuate the higher AF frequencies.

Brian.

 

[THINKER_KAM_MAN]

Incorrect - there is no RF feedback in the original schematic. The feedback is AF, in other words V1A works as an RF amplifier once and then again as an AF amplifier. The diode is there to rectify the RF, producing a negative voltage proportional to the amplitude of the signal (its an AM receiver!) which is then used to bias the grid. C3 is there to prevent RF feedback, it's value is chosen to 'short out' any RF but not isn't large enough to seriously attenuate the higher AF frequencies.

Brian.

well i so-pose i will have to go think again
but resistor/cap phase change = 90 deg,
coil to base = > 90 deg. one doe's not need
lots of RF feed back, just a bit to boost Q and
sensitivity, the capacitor together with the
connecting wires will have inductance so it will
not completely remove the RF.
also too much RF fed back will cause the
receiver to lock onto strong signals and
weak signals will not be detected.
but if the design is to be used at FM
then the lock in function would be useful.
ho and yes the grid is biased by the DC voltage
across the CAP. but one can have RF riding on DC
if one was to adjust the value of C3 RF as well as
AF feed back could be had.

 

[neazoi]

No, the relatively high value of C3 is to effectively ground the bottom of the tuned circuit to RF. It isn't real ground and it actually has the audio across it but to higher frequencies it looks like a short-circuit. Ignoring the very tiny effects of the capacitance inside the triode, the only tuning components are C2 & L2.

Your next challenge is to make it more sensitive and selective by using controlled RF feedback. To do this, create a new coil on the ferrite rod from a few turns of wire. Its difficult to predict how many would be needed but I would start with 5 turns. Connect one end of the coil to the anode of V1A and the other end to ground through a variable capacitor of say 100pF. Start with the capacitor set to minimum capacitance and tune into a station, it should give similar performance to what you see already. Now increase the value of the new variable capacitor and see what happens. There are two possibilities, one is that the signal will stay the same or even get weaker, (this indicates the new coil is wired the wrong way around) the other is that the signal gets stronger and maybe it starts to oscillate. If the coil is reversed, swap it's wires over, you want it to boost the signal with positive fedback rather than reduce it with negative feedback.

The tuning and feedback conrols will interact slightly so expect to go from one to the other to get best results but you should be able to achieve results comparable with top of the range receivers. If you tune over the amateur bands you can resolve CW and SSB transmissions by adjusting the feedback to the point where it has just started to oscillate. They can be very versatile receivers, the only reason they are not popular is the need to adjust two controls instead of one.

Brian.

All clearly understood Brian, thanks!
I will definitely try this modification with the initial values you refer, as my ultimate goal is to build a single tube low +b RX with relatively good performance for the amount of components used. This will be a reflexed-regen receiver, similar to the transistorized version of reflexodyne.

As it can be seen from the other posts (and form internet reading I have done), it is confusing to many what is the difference between the reflex and the regen topologies. It is clear to me now that the original circuit is a reflex design that amplifies RF only once and audio only once, at the same first triode. The use of a regeneration will add positive RF feedback, so RF will be "amplified" more than once.

To completely remove any semiconductor devices, I looked for a tube that has an embedded diode, yet operating in low +B by design.
The only tube I have found is the 12DW8, which I have tested on the circuit with good results. This is a dual non similar triode with embedded diode inside. In fact, each triode anode is split into two plates. One triode has these two plates connected, but the other has only one of them connected to +B. The other plate stands unconnected and the grid between this plate and the cathode has been cut on one side. This is how the diode is formed from an otherwise idendical plates twin triode. The tube is a space charge one and it's specified max +B is 16V! The filament varmth of this tube is great (after all that is why tubes are used) and the tube runs really hot, so a ceramic socket is recommended.

Of course I have tried it at 12V to feed both the plate and the filament in a single (12v (convenient for car battery operation) power source. The only changes to the original circuit I have made, is at the volume control as you suggested and at the grid to the resonant circuit as suggested.

To further improve the audio volume, I am thinking of how could I apply controlled positive audio feedback to the second triode V1B. I have not seen this technique at any receiver. Careful control must be needed to avoid oscillation. Have you got any ideas of how to try this?

 

[betwixt]

Feedback in V1B wont work. You can do it in V1A because it's easy to separate the RF and AF paths but to do it in the audio amplifier stage will do nothing but make it oscillate. You would be trying to feed the same frequency through the amplifier twice rather than ones with widely different frequencies.

The other problem is that even if you could keep feedback below the threshold where oscillation starts, the bandwidth would reduce, you would get extra gain but over a very narrow audio frequency range, maybe only a few tens of Hz.

Brian.