How does this reflex receiver work?

[neazoi]

Hello,
I have found this receiver circuit and I am interested in how does it work.
I mainly consider the first triode stage and I need to know the flow of RF and AF signals (where do they pass from, feedback, rectification etc).
Thanks a lot.

 

[jiripolivka]

The only reflex thing is that V1A serves as a RF preamplifier, then after D2 diode detector also amplifies the audio signal. V1B is a pure audio amplifier, SP1 must have >1 k impedance.
Operating such radio with 12 V plate voltage allows only to receive quite strong local signals.
Many years ago I used a RV12 P2000 as a low-voltage audion, with G1 connected to +12 V and G2 used as the control grid. Such one-stage receiver with a feedback allowed to receive even weak signals with earphones
With your schematic, try add a RF feedback (a 2-3 turn coil in series with L1 choke, coupled to L2, this may allow a better sensitivity.

 

[neazoi]

The only reflex thing is that V1A serves as a RF preamplifier, then after D2 diode detector also amplifies the audio signal. .

So V1A amplifies the RF signal ONLY once and the audio signal ONLY once too?
I thought that it amplifies the RF signal only once and then the audio is rectified by the diode and re-applied to the V1A many times (continuous loop feedback), so that it is amplified more than once.

With your schematic, try add a RF feedback (a 2-3 turn coil in series with L1 choke, coupled to L2, this may allow a better sensitivity.

How will this work? My guess is that:

C2/L2 selects the frequency.
L3 picks up this frequency and V1A amplifies it (RF, first time).
After this amplification RF (as well as AF) appears at the V1A anode.
The small 2-3 turn coil you suggest, re-couples this RF signal into the ferrite.
L3 picks up this amplified signal and V1A re-amplifies it.

With your suggestion you add RF feedback as well to the circuit.

Are my thoughts correct?

 

[BigBoss]

V1A is a RF amplifier..
V1B is an Audio amplifier..
C4,D2,R1,C3 combination supplies necessary negative bias obtained from AM modulated signal for tube V1A and at the same time a simple AGC.
If the signal level increases, negative bias also increases and obviously the gain of V1A decreses..

 

[neazoi]

V1A is a RF amplifier..
V1B is an Audio amplifier..
C4,D2,R1,C3 combination supplies necessary negative bias obtained from AM modulated signal for tube V1A and at the same time a simple AGC.
If the signal level increases, negative bias also increases and obviously the gain of V1A decreses..

So it is there only for supplying AGC? Where is the reflex (feedback) action takes place then?

 

[jiripolivka]

So V1A amplifies the RF signal ONLY once and the audio signal ONLY once too?
I thought that it amplifies the RF signal only once and then the audio is rectified by the diode and re-applied to the V1A many times (continuous loop feedback), so that it is amplified more than once.



How will this work? My guess is that:

C2/L2 selects the frequency.
L3 picks up this frequency and V1A amplifies it (RF, first time).
After this amplification RF (as well as AF) appears at the V1A anode.
The small 2-3 turn coil you suggest, re-couples this RF signal into the ferrite.
L3 picks up this amplified signal and V1A re-amplifies it.

With your suggestion you add RF feedback as well to the circuit.

Are my thoughts correct?

Yes, correct

 

[THINKER_KAM_MAN]

just a suggestion

the art of a good reflex radio receiver is to apply controlled
positive feed back, to increase the gain on the amplifying
device. because at near the joint where the device may just
goes into oscillation the gain can be very large.
and this gives the sensitivity. the draw back is that the stage may oscillate and cause interference if the signal is fed back up the antenna
also of course the recovered modulation is fed back to the RF amplifier
for amplification.

 

[neazoi]

just a suggestion

the art of a good reflex radio receiver is to apply controlled
positive feed back, to increase the gain on the amplifying
device. because at near the joint where the device may just
goes into oscillation the gain can be very large.
and this gives the sensitivity. the draw back is that the stage may oscillate and cause interference if the signal is fed back up the antenna
also of course the recovered modulation is fed back to the RF amplifier
for amplification.

To be honest, I have not a clear reply in this thread of how this specific circuit near V1A work. I can see the feedback path, but is it detected audio that is fed back to the tube or both audio and radio?
In other words, is audio the thing that is amplified many times by the tube in this circuit, or radio, or both?
According to this article https://en.wikipedia.org/wiki/Reflectional_receiver a reflex RX amplifies ONCE only the RF, the diode rectifies the amplified RF and the rectified audio, passes through the tube again for amplification. The article does not specify if AF is amplified more than once in this configuration (although I think this is the case, because of the HPF capacitor), but is states that RF feedback is not present in the reflex. It is only AF feedback that is present and this happens only ONCE (that probably explains the absence of feedback adjustment).
This is the main point of consideration, is this the case in this receiver?

- - - Updated - - -

With your schematic, try add a RF feedback (a 2-3 turn coil in series with L1 choke, coupled to L2, this may allow a better sensitivity.

What if I connect a 100nF capacitor directly from the V1A anode to one end of the L2/C2, through a potentiometer (to controle the feedback). The other end of L2/C2 will be grounded to complete the RF path.

Whould this trick work as a controlled RF feedback instead of the simple series coil?

 

[betwixt]

I agree with the article. The circuit is very crude and I would not expect good results but as I see it, the way it works is:

V1A works as amplifiers at RF and AF. Initially, it uses coupled RF from the tuned circuit and uses the leakage through R1 and D2 to hold the grid at near ground potential. Amplified RF at the anode is isolated by L1 which is an RF choke and coupled via C4 to D2 which acts as the demodulator. Any recovered signal is filtered by R1 and C3 which ensures only LF (Audio) content reaches the grid again. The audio output path is from the filtered side of L1 and RF is filtered by C5. V1B is an audio amplifier stage.

There are a number of deficiencies in the design but they are a trade-off to reduce component count. The most obvious improvement would be to remove L3 and connect the L2/C2 tuned circuit in it's place. The impedance at that point is high so why a coupling coil is used is a mystery, it would be better to use it as a regeneration winding instead which would improve selectivity and sensitivity.

Without negative grid bias V1A already works as a 'leaky grid' detector which is probably detrimental to the operation of the audio feedback. I would add a low value cathode resistor in V1A and bypass it with a suitable capacitor so the triode works in a more linear mode.

The volume control could also be improved, it is bad practice to pass DC through a volume control so why not replace it with a fixed resistor and put the potentiometer where R5 is now.

I think the originator took liberties with the design, relying on the low supply voltage to run V1 in a way that isn't optimal but wouldn't cause damage. A few more volts on the anodes wouldn't go amiss!

Brian.

 

[jiripolivka]

I agree with the article. The circuit is very crude and I would not expect good results but as I see it, the way it works is:

V1A works as amplifiers at RF and AF. Initially, it uses coupled RF from the tuned circuit and uses the leakage through R1 and D2 to hold the grid at near ground potential. Amplified RF at the anode is isolated by L1 which is an RF choke and coupled via C4 to D2 which acts as the demodulator. Any recovered signal is filtered by R1 and C3 which ensures only LF (Audio) content reaches the grid again. The audio output path is from the filtered side of L1 and RF is filtered by C5. V1B is an audio amplifier stage.

There are a number of deficiencies in the design but they are a trade-off to reduce component count. The most obvious improvement would be to remove L3 and connect the L2/C2 tuned circuit in it's place. The impedance at that point is high so why a coupling coil is used is a mystery, it would be better to use it as a regeneration winding instead which would improve selectivity and sensitivity.

Without negative grid bias V1A already works as a 'leaky grid' detector which is probably detrimental to the operation of the audio feedback. I would add a low value cathode resistor in V1A and bypass it with a suitable capacitor so the triode works in a more linear mode.

The volume control could also be improved, it is bad practice to pass DC through a volume control so why not replace it with a fixed resistor and put the potentiometer where R5 is now.

I think the originator took liberties with the design, relying on the low supply voltage to run V1 in a way that isn't optimal but wouldn't cause damage. A few more volts on the anodes wouldn't go amiss!

Brian.

Any "reflex" circuit utilizes one active device (tube, transistor) for both RF and audio amplification. This comes from history when those devices were expensive, so people tried to best use their tubes or transistors.

Your idea to use a capacitor for RF feedback may work but with the coil you can adjust the positive feedback as desired, which with capacitor may not work with a correct phasing. For RF feedback use rather 10-100 pF, otherwise the feedback will include audio, too.

I can see too much talking above. I prefer to take a soldering iron and try it for yourself if you wish. Talking would not solve problems, experiments do.

 

[FvM]

To be honest, I have not a clear reply in this thread of how this specific circuit near V1A work. I can see the feedback path, but is it detected audio that is fed back to the tube or both audio and radio?
In other words, is audio the thing that is amplified many times by the tube in this circuit, or radio, or both?

IMHO, you got clear answers in this thread. The problem is that you are chasing an idea which can't be found in this receiver. There's neither feedback implemented nor the same signal (AF or RF) passing the same amplifier multiple times. V1A is amplifying RF once and AF once. That's it.

Feedback could be optionally used for RF to increase Q of the input LC circuit.

 

[neazoi]

IMHO, you got clear answers in this thread. The problem is that you are chasing an idea which can't be found in this receiver. There's neither feedback implemented nor the same signal (AF or RF) passing the same amplifier multiple times. V1A is amplifying RF once and AF once. That's it.

Feedback could be optionally used for RF to increase Q of the input LC circuit.

Allright, I initially thought AF re-follows the same path and re-amplifies more than one times (positive AF feedback). But there is C4 (HPF) which do not allow audio to re-amplify more than once.

I will try the ideas mentioned in this thread to see how it goes.
I think I could replace the diode by using a 12DW8 tube, still low +B and an internal tube diode connected to the tube cathode could replace this solid state device. I do not know how well this will behave based on betwixt explanation for biasing the grid, but it is worth to try it.
Also the audio potentiometer is a worthy addition without increasing the amount of components.
The use of the 12DW8 will actually save a power resistor component (12v for the filament and the anode).\
I think he used inductive coupling to avoid killing the Q of the resonant LC, by loose coupling (although a ferrite antena is used). Or maybe C3 would affect the resonance of the tuned LC if it was connected at it's end to the ground. For some unknown reason, all reflex RX designs I have seen (transistorized) use an L to couple RF to the transistor.

 

[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)

 

[neazoi]

Thanks I will definitely try this, it will improve selectivity and sensitivity

 

[betwixt]

For some unknown reason, all reflex RX designs I have seen (transistorized) use an L to couple RF to the transistor.

Transistorized designs would typically have much lower input impedance so a coupling coil would be appropriate. A vacuum tube would have higher impedance despite miller effects so direct connection to the tuning elements should give better results. RF feedback to improve gain and selectivity and any antenna coupling would be lower impedance so in those circumstances an additional winding would be the way to go.

If you use a vacuum tube diode I suggest you add a resistor of say 2.2M from V1A grid to ground to avoid possible floating grid problems.

You should also note that V1B runs with no grid bias so it will tend to clip the positive side of the audio waveform. I suggest adding a resistor of say 220 Ohms in it's cathode and bypassing it with a 100uF capacitor. You woud get more volume if you used an output transformer to match a standard loudspeaker. A pure guess would say you would get rasonable volume level if you did that, otherwise you need a high impedance loudspeaker which would be difficult to find.

Brian.

 

[neazoi]

Thanks very much for your replies. I will try these points!

 

[neazoi]

There are a number of deficiencies in the design but they are a trade-off to reduce component count. The most obvious improvement would be to remove L3 and connect the L2/C2 tuned circuit in it's place. The impedance at that point is high so why a coupling coil is used is a mystery, it would be better to use it as a regeneration winding instead which would improve selectivity and sensitivity.
Brian.

Shall I ground the bottom end of the L2/C2, if I connect it's top end to the tube grid directly?

Without negative grid bias V1A already works as a 'leaky grid' detector which is probably detrimental to the operation of the audio feedback. I would add a low value cathode resistor in V1A and bypass it with a suitable capacitor so the triode works in a more linear mode.
Brian.

What short of values should I use for the parallel cathode RC?

 

[betwixt]

No, that would stop the reflex action. Simply put the tuned circuit where L3 is at the moment. Connect the 'chassis' side of the tuning capacitor to C3 to minimize hand capacity effects when you adjust it.

The RC network values are a problem because the value of the volume control isn't shown and it doubles as the anode load resistor. I would guess 4.7K would be a good value to choose. The intention is to lift the cathode of V1A just enough that any incoming signals can't make the grid more positive than it. As the incoming signals are tiny and some negative is produced by the detector diode (when a signal is present) I would guess you want to make the cathode sit at about 75 - 100mV. Calculating the resistor to do that in such a strange circuit and with such a low supply would be difficult and I think I might be tempted to wire a 1K variable resistor there and adjust it experimentally. The capacitor just has to bypass it to RF so 10nF to 100nF should be fine.

Brian.

 

[neazoi]

No, that would stop the reflex action. Simply put the tuned circuit where L3 is at the moment. Connect the 'chassis' side of the tuning capacitor to C3 to minimize hand capacity effects when you adjust it.

Brian.

Thanks! I did so, before I read your post. The Receiver worked at once! I have also tried the dual triode, with embedded diode inside the tube, and the circuit worked as well. This is great if an all tubed design is to be produced.

One thing that worries me is that, if I connect L2/C2 where the L3 is, then the tuned circuit resonance will change and so the frequency coverage will change.. Now C3 will be part of the tuned circuit I think, is that right?

 

[betwixt]

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.