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It's not a problem having some mismatch, it just means some LO is radiated on the aerial or rod. That will still be far less than the LO radiation on the RF in of any single device combined mixer/oscillator like an Octode, Heptode or transistor. The ECH81 or any similar idea, even with a Pentode-triode is really a separate mixer and oscillator in one envelope, so it radiates far less LO.
Here is the wikipedia article on the ring diode mixer.
https://en.wikipedia.org/wiki/Ring_modulation
Often the IF connections are shown where Wiki has "Carrier" (= Local Oscillator), and then the input or output port would be Local Oscillator.
It's used for superhet RF to IF, baseband Audio to Transmit IF, Transmitter IF and synthesiser to RF out, for 10.7 to 455 KHz IF conversion, or 68MHz IF to 6MHz IF , for BFO mixing, for 455 kHz to 12.5 KHz for sound card SDR. It's been used in the 1930s with copper oxide diodes for Telephone multicarrier systems on a coax cable. Largely replaced by ICs, but CBs often still use them for 10.7 MHz IF <> RF conversion. The better the centre tap windings and diodes are matched, the better the suppression of the un-mixed signals between ports, such as LO or RF in on the IF port (receive), or LO on RF in (receive) or IF and LO on RF out (transmit). The 1j42a or 1j37b circuit does give good LO to input rejection, but not as much rejection of unwanted products on the anode. It will however have a better rejection of RF in on the output than a simple mixer using a non-linear device or amplifier as only common mode signals have decent gain on the g1a and g1b and differential signals have poor gain on the g1a and g1b, this is the opposite performance to a differential amplifier like a long tailed pair, two triodes with one cathode resistor, or an opamp.
This is why the RF in has to feed a differential signal to the 1j42a (or 1j37b), it means out of band and signals in or near the IF bandpass are poorly amplified. The LO is amplified, but that's a single known clean signal well outside the IF.
Your LO should on ANY radio design be noise free, have low phase noise and no FM or harmonics, for best performance. This is why early PLL synthesised radios are noisier than types with multiple crystals for the bands and a very stable 5MHz to 5.5MHz or similar mechanical variable oscillators.
I like the video's. It's interesting with the frame aerial. I was impressed with a transistor radio I have at how well it was picking up Caroline until I realised that it was sitting underneath my frame aerial which I keep tuned to Caroline lol. Obviously I am a lot closer and in some ways it can be regarded as local and a standard for checking. It has made me wonder about multi element aerials but, partly due to lack of space, that is probably unlikely. I think a limitation with the 1ж42а is how scarce they are but I have some 1ж37б which can be used in similar ways.
Thanks for more useful information Mike.
Tracy
Hi.
Here is the circuit I am using.
The first stage is an RF amp with positive feedback to the ferrite rod via a winding of two turns, ideally the 4.7k pot should be returned to HT and then to the coil rather than the way I did it, the 2Mh choke from the G2 to HT keeps the gain of the valve up as the HT stays constant with adjustment of the reaction pot when the feedback is reduced, this makes the reaction far more progressive than without it.
There is no grid resistor on the AF amp even using a 10 meg reduces volume somewhat, I have also tried a little forward bias on the diode to see if weak stations come in any stronger with no improvement.
The 2nf decoupler on the anode of the AF amp is important as is the HT decoupler. Results are quite spectacular with 12 stations on medium wave last night three foreign stations were listenable last night with no external aerial.
I've made a few TRF sets over the years and I can in all honesty say this performs far better than any. The AF gain is far higher than I thought you would get from these little valves. A simple and easy project to do.
Without reaction I still can receive three stronger stations at good volume so there is very good gain between the stages.
I also agree with Tracy these valves are getting scarce.
HT is 9v
You'd be better with a 470K to 2M resistor to ground at the diode input and 1M at the grid as grid leak and DC load. The DC conditions are undefined on the diode and grid. In practice there is likely a negative charge on g1a and g1b, so diode is conducting when AC voltage is above that voltage. Thus with the two resistors it will be more sensitive.
The only AM diode detectors that work well with no DC path on the input is the two diode peak detector, which needs a DC load. So you could add a 2nd 1N60 rather than an input resistor and only add the grid leak/diode load.
A trick I've done to make a simple transformer for RF or IF is two resistor style inductors side by side or random wind fine wire on one as a secondary.
The 1j42a was one of the last Rod Tubes and was difficult to make. For years no-one was selling them. The 1j24b is the next best if you don't need dual grid.
Hello all.
I find these rod valves fascinating.
Just one question though, I've seen diodes used as in Trevor's diagram occasionally, But what is its purpose? Is it acting as a clamp?
(There is no DC continuity at its anode so is therefore just floating?)
Would the circuit still work if it were not there?
A suggestion would be either another diode anode to ground, cathode to first diode anode, Mikes peak detector, or a choke in the same position to provide DC continuity and block RF.
Thanks.
Amie.
All rectifiers / diode detectors have a DC circular path.
A peak detector:
input cap is charged by D1, then Input cap is discharged by D2 into load. Input cap has only AC, circular DC path is D1, D2, Load resistor, earth, D1 etc.
As shown it's working due to reverse DC leakage in the diode and -ve Space charge accumulating on the grid. As drawn, the grid capacitance would gradually charge to near HT via D1 if there was no negative space charge. That's why it either needs a transformer, an input resistor to 0V, or a 2nd diode at the input of the diode and a high value load resistor on the diode and 100pF cap, so maybe 1M or 2.2M and the cap could be 220pF or even 470pf to reduce RF on the output valve.
So it works due to parasitic capacitances and resistances and grid space charge. The bias and low pass RF filter time constant are not well defined. All diode detectors/rectifiers are either charge pumps (more than one diode) or else have a DC path on both sides, otherwise performance is undefined.
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