Golborne Vintage Radio

Hi.
I've been playing with the 1Ж42А Rod pentodes again.
This time for a simple superhet radio for medium wave. Perhaps pointless but again a little fun during the restrictions.
I remember Michael suggesting to me that without a balanced input to the two grids this wont work so I was hoping to get this to work with Michaels suggestion.
What I did do though was to make firstly a tunable oscillator. I simply had a medium wave Toko transistor radio oscillator coil and tuning cap wired between G2 & one G1 via a 220pf cap, the G2 fed from 9v via a 2mh choke the G1 has a 1.5m resistor to filament minus, anode initially again connected to 9v via another 2mh choke. G3 grounded to filament minus. Surprisingly the oscillator ran immediately, with 4v p-p on the anode. Substituting the anode choke with an IF transformer resulted in no output on the anode but the oscillator was still running as it was seen on the G2 on the scope.
After I added in a MW coil on a ferrite rod and cap and tuned this to my local, connected to the other G1 and filament minus (ground) with no coupling cap, observing the output from the IF transformer on careful tuning of the oscillator cap suddenly the I could see a carrier. the level was higher than that off the ferrite bar. The level was around 2.5 times higher. Now don't ask me if this is a fluke or not but I certainly got an output. I will post up pictures in time as the whole thing is a pigs breakfast during "the development stage"
To be fair I doubt this would really be a workable radio as the 1Ж42А doesn't have a lot of gain so I may need three IF stages unless I can add in regeneration. Perhaps I should go for the 1Ж37Б instead but I don't have any.

My question is why is this working as the 1Ж42А really shouldn't work like this just like in my pantry transmitter it worked and very well indeed.
I'd be happy to be enlightened.

I'd need a schematic to see what you are doing.
Note ANY amplifier or oscillator that's non-linear or driven out of a normal linear region will work as a mixer. However there are sometimes more efficient ways to use a device.

So OF COURSE it will work, if it oscillates at all. It's not taking advantage of the 1j42a other than low voltage (Shouldn't have more than 18V HT max).

After all a regular valve, rod tube transistor or FET wired as an oscillator will also always act as mixer if you also feed in RF at the grid/base/gate. The disadvantage is that a lot of the local oscillator leaks out on the RF in and it's not got good conversion gain. So a transistor tends to be wired as a common base mode oscillator (oscillator input/feed back is emitter) and the RF fed to the base. A dual gate FET is not at all like the dual grid 1j37b or 1j42a as it behaves a bit like two FETs in series, there is some isolation of signal between the gates.
Compare a single gate FET and a dual Gate FET.
This is also why g2 was used as the Local Oscillator input on Pentode Mixers before pentagrid/heptode/octode mixer oscillators developed, or Triode Hexodes.

Now a dual gate FET doesn't work the same way as a 1j42a or 1j37b. All the Rod Pentodes actually have a grid plate either side of the filament. So as those becomes more negative the dual beam of electrons emerging at both sides is "pinched" and eventually extinguished.
So someone thought (his name is documented), what if instead of internally tying the two g1 plates together we brought out an extra pin?
Thus the plates are like the X or Y plates on an electrostatic scope:
1) Move the voltage the same on both and it's like a normal rod tube.
2) Have a differential voltage on the g1a and g1b and it's like half of a 7360 beam tube. https://www.radiomuseum.org/tubes/tube_7360.html

So if you tied the two anodes together on a 7360, and drove the deflection plates suitably, it would act like a doubler or rectifier as any positive OR negative DIFFERENCE in the signal on the deflection plates  puts the beam on to either anode. The 1j37b or 1j42a has the opposite characteristic.
If you have the same voltage (any -Ve to 0V where there is still anode current) on g1a and g1b, then it behaves normally.
If there is a differential voltage on g1a or g1b, then the two beams are deflected and less electrons fall on the anode plates, which  are more cunningly shaped on the 1j42a, the 1j37b is similar to regular rod tubes. They used g1 because in a rod tube those are already a pair of plates very close to the filament. The g2 and g3 are electrostatic  focussing, pairs of rods, so they could have split the internal connection on g2 and/or g3. But g2 and g3 are just pairs of rods and further from the centre of the beam, so any deflection would have needed a much higher differential voltage.

So while connecting up a 1j42a in ANY oscillating circuit can work as a mixer (and using a separate triodised one as a LO and driving g2 works better), the Russians never ever used it that way. A separate oscillator is always better as otherwise the RF input or strong out of the IF band signals can cause FM and thus intermodulation of the desired IF. Only cheap simple transistor superhets use a mixer/oscillator. Better ones always, like good valve communication receivers, used a separate mixer and oscillator.
Actually the disadvantage of the Pentagrid/Heptode/Octode etc mixer osc compared with a triode-pentode or triode-hexode (or separate valves) is that while either will work in the actual circuit of the other, the heptode or octode leaks LO out the RF in and also suffers from intermodulation due to a strong RF signal causing FM on the oscillator. It's why having a second tuned RF amp was often done on USA MW only domestic sets, not just for more sensitivity. Blocking LO leakage on the aerial is the main reason for an RF preamp using EF80 (almost no gain) or 1/2 of ECC85 etc on VHF.

So the Russians only used ONE circuit idea with the 1j37b or 1j42a  at an RF front end.
1) Separate oscillator always.
2) Use a balanced transformer for the RF in. The LO is fed to a centre tap, so if carefully designed and laid out, there is minimal LO leakage out of the RF in.

Note if you look at battery valve DC90 or DF97 (always triodised) VHF mixer/oscillators (and often single transistor VHF mixer/oscillators) you'll see it is quite complex transformer arrangement and service information (see Philips Annette or Colette 1954 to 1958) warns of the tricky alignment to null out the LO being emitted on the aerial. It's easily received on a second set at 10.7 MHz offset from tuned frequency.

So the LO signal is equal on g1a and g2a. But with no LO, the RF would effectively be frequency doubled and somewhat rectified. There would be little actual RF at the output compared to driving both grids with the same signal. Obviously it's so non-linear that it will multiply the common mode signal (the Local Oscillator from a separate triodised oscillator).
Thus the output on the anode:

1. Some of any RF input.
   
2. Lots of the L.O. especially when there is no RF.
   
3. Lots of RF - LO
   
4. Lots of LO + RF
   
5. Lots of LO - RF
   
6. Some DC component of the AM of the RF
   
7. RF x2
   

We'd use LO + RF for very much lower than IF. Normally we'd use LO-RF for the IF, though on VHF (40 MHz to 110 MHz) we might want RF - LO for the IF if the valve can only oscillate up to 100 MHz or less.

You could feed RF via capacitors to both grids and then also feed the LO to only either but only one grid. Or vice versa if you wanted a mixer / osc, but how to stop both signals driving both grids almost equally without series resistors? You'd need a transformer. So it makes sense that the transformer is tuned on the primary for RF and the LO is fed to a centre tap. Obviously with a ferrite rod aerial you could simply have a centre tap for the LO, but then you need a floating RF tuning cap, or double gang with the two fixed vanes to either end of the ferrite rod. Then you'd want a triple gang to tune the LO, so a primary on the rod to tune and then a centre tapped secondary on the same rod to feed g1a and g1b.

Almost ANY arrangement will work as an osc/mixer. However the best way so as to not either radiate the LO and have avoid strong signals doing FM intermod and appearing on the IF is the separate oscillator (only another 11 mA filament) and balanced input for RF with a centre tap.

I'll leave it as an exercise as to how you do it at 38 kHz with two valves and have an FM multiplex decoder. Grundig did manage a single valve decoder. You need a 19 kHz pilot tone filter and it is fed to the 38 kHz LO, in a fashion to cause frequency lock. Since the 1j42 nicely frequency doubles, you just need weak feedback at 38 kHz for the LO. The other 1j42a is fed with high pass filter via a balanced transformer fed at centre tap with the 38 kHz LO and a low pass filter on the output gives L-R.

Thank you Michael for the explanation. Very informative and useful
I intend to very much tidy the work up and will draw out the circuit in due course.
Being fair I'm just fiddling around due to us having lots of time during lockdown.
Again thank you for your input.

Joe Sousa has a write-up on the Radiomusuem as to how it works in the intended way.

You'd not normally think of a DF97 being used on FM as a 1st IF but on AM as Pentode Mixer, or a DF97 g2 being used as an anode for audio as well as Pentode 1st IF. (Vidor and Ever Ready Battery valve AM/FM sets). Lots of things work, but it's good to first understand the designer's intention.

That's very true Michael. I suppose I'm a bit of a rebel and like to get things to work when it shouldn't. I'm not into using computer simulation of whether a particular circuit will do the task I've thought about, I tend to experiment and see what will work, perhaps it does or doesn't, again learning from experience and trial and error. A lot of the things I've made for many have worked contrary to the theory. Right now I've time to play about, after lockdown I'll be back to the charity work we did beforehand.

Trial and error and experimenting is fine and brilliant once you know how something is meant to be used.

Simulation is very tricky and you need to know what the parts really are, not just the values on them. Inductors, transformers and active devices other than op-amps are tricky to hard. Simulation doesn't design anything. It's a useful tool for complex and/or expensive to build designs. Also for things like inverters, SMPSUs, complex filters etc where you absolutely know the real resistances, coupling, capacitances, core properties etc of inductors or transformers. If you were expert enough to simulate a superhet properly you likely wouldn't need to unless going into volume production. It's very valuable for production designs that already work on the bench to see the effects of component tolerances, e.g. Monte Carlo simulations. But the model needs to take account of all the real world resistances, inductances and capacitances of parts that are not simple resistors or near perfect capacitors.

When it comes to an active device it's not hard to get it to oscillate and nearly any active device works as mixer. Those are the two most critical aspects of a superhet, so it makes sense to start with the best known design for the kind of active device you are using: Pentode, Heptode, Triode-Hexode, Bipolar transistor or dual gate FET. some devices such as a ring diode mixer, Pentode, Hexode, Dual Gate FET or dual g1 Rod Pentode are inherently only sensible as a mixer if using a separate oscillator.

Also if building your own superhet a separate mixer and oscillator is ALWAYS better. A Triode-Hexode, though a single envelope, is really a separate mixer and oscillator. A Heptode or Octode is really a sort of triode and pentode or hexode IN SERIES, and thus is really really inferior to the Triode-Hexode (or Triode Pentode) in even the same circuit. Note any Triode-Hexode circuit works fine with a separate Pentode (LO into g2) and Triode oscillator (or Triodised pentode). A single Pentode , triode or bipolar transistor as a combined mixer oscillator is cheap but terrible performance. At the VHF a single triode (or triodized Pentode), transistor or FET was often used simply because of the cost and difficulty of a separate mixer and oscillator. Almost professional gear from the 1980s onward used separate mixers and oscillators. It's now standard in all VHF, UHF and higher gear other than cheap domestic radios.

Hi.
I attach a quick drawing of the circuit, there's a couple of corrections as this was done quick.

I concur with Michael, although this works to an extent it has some major drawbacks.
The big one is the tuning gang, it has to be totally isolated from each other, you could use two separate capacitors that were mechanically connected via say plastic gears or a toothed belt but this is OTT, or even varicap diode tuning. Also it is insensitive, it needs an RF stage and even with two stages of IF amplification it's not great with only the stronger stations being acceptable. So yes it does work but it's severely limited. I'm sure other rod Pentodes would work and much better than trying this approach. I've used these little 6v valves for other projects and they work very well indeed and intact using them in a TRF configuration with both control grids strapped the performance is excellent.
I wanted to try an infinite impedance detector but as yet I've not worked out the best way to do this with a filamentary valve, suggestions are welcome. So this superhet project is now shelved.

Addendum. The 150pf cap is wrongly drawn, it should be in series with the oscillator coil not in series with the choke to the G2.

Hi Trevor,
I read your original post a day or two back and was a bit confused at the time, thinking a diagram would be brilliant. I turn on today and find one

It has made things a lot easier to see and I understand your thinking. It's not a million miles off my pantry transmitter oscillator and modulator but with both functions combined. I shall spend some time looking and thinking at what you have done and Mike's useful replies.

With these type of valves I have tried various options for filament with cathode resistors. The best way for me seems a separate supply as I tend to use mains operation rather than battery, although the principle is the same with both. I am reminded that I do have a differential amplifier arrangement in which I used a constant current cathode load. Initially I tried a mini voltage isolator supply module (tiny) which worked but I was making a MW RF amplifier and I found some AM interference, mainly to other equipment. I replaced the module with a normal mini regulated isolated supply which did work well, but the project was put to one side as the amplifer did not have great gain. It's just extra work obviously. 

I have come across some Russian methods (see some in the attachment). I show this as it also includes another use of the valve being used here. From the Russian circuit using the эм7 with transistors I have been playing with constant current feeds to the filament. Theoretically this should limit the flow via the filament circuit but it gets a bit thought intensive. The эм7 has a 1V 15mA or so filament. From this work I have been increasingly using constant current supplies as I use them often and the currents below 100mA are perhaps easier to handle than low voltages but it's not a real concern (although with my electrometer circuits a well regulated anti surge filament supply is useful).

I have bought some little transformers purely for filament supply although have occasionally in the past used capacitor tap off on normal secondary supplies to provide isolation (and scratched my head as to what might happen if a capacitor fails and I lose isolation). There are some standard methods of doing things but as you are now thinking - it's what is the best easy to implement and cheap method. Perhaps just using batteries with simple regulation? Really it is just a case of isolation of the filament.

Here's hoping it gives you a few ideas! Keep up the good work.

Tracy

(29-06-2020, 01:51 PM)BusyBee Wrote: [ -> ]I have come across some Russian methods (see some in the attachment). I show this as it also includes another use of the valve being used here. From the Russian circuit using the эм7 with transistors I have been playing with constant current feeds to the filament. Theoretically this should limit the flow via the filament circuit but it gets a bit thought intensive. The эм7 has a 1V 15mA or so filament. From this work I have been increasingly using constant current supplies as I use them often and the currents below 100mA are perhaps easier to handle than low voltages but it's not a real concern (although with my electrometer circuits a well regulated anti surge filament supply is useful).

I have bought some little transformers purely for filament supply although have occasionally in the past used capacitor tap off on normal secondary supplies to provide isolation (and scratched my head as to what might happen if a capacitor fails and I lose isolation). There are some standard methods of doing things but as you are now thinking - it's what is the best easy to implement and cheap method. Perhaps just using batteries with simple regulation? Really it is just a case of isolation of the filament.

How about a single NiMH cell with trickle charge via a capacitor. Need a schottky diode to drop the 200mV from the cell to the 1V heater. It's hardly a new idea. DEAC rechargeable batteries were used on some valve portable sets for this purpose.

If you're worried about the capacitor then put 2 in series and consider clamp diodes across the filament. 2x silicon in series will clamp at about 1.4V. Otherwise do something with schottky diodes to get closer. If you're using AC on the heater then use diodes in inverse parallel.

While I was writing that I thought of the battery idea, but really went over the things I have done which may be useful. It's an approach I have not used but is in the back of my mind.

With my heater supplies I always use DC even though it may be less efficient. With these rapid heat valves I would wonder about induced hum from the direct filament anyway. Clamp diodes do seem a good idea as it's likely, at least during experiment, that more valves are wrecked through accidental HT on the filament than any component failure. I know I have destroyed at least one. When I was thinking about the capacitor it was perhaps more in relation to my HT supplies I once used to make. I used to make parallel fed multipliers with high(ish) voltage electronic smoothing and regulation. There I may be talking of a couple of hundred volts per stage so a capacitor failure would maybe have catastrophic consequences whereas my current thinking on low voltages should be far easier to sort but good thinking - thank you.

Tracy