Re: How does this reflex receiver work?
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.
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.
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?
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.