diy IF transformers: first attempt.

[Amie]

Hi Adrian.

I suppose I like things that were made to be kept in the true sense of consumer durables, not throw away planned obsolescence, people my age say they are worried about waste, but can't wait to get the latest piece of tecnojunk just proves that its collective hypocrisy to my mind.

Mum does keep an eye on the account in case I'm being a nuisance to anyone.

GVR members seem a good bunch for the most part, I've picked up loads of useful information and generous practical help, not to mention real friendship.

Amie.

[Murphyv310]

We are happy to help.
Being fair Amie I think you help us more than we do to you. You are far more advanced than some including myself..... Other things put paid to that bringing up three kids, fixing cars, home DIY and my late wifes health that had its ups and downs in her last few years.

[Amie]

Thanks for the comments Trevor, but I think I'm by far a net consumer of knowledge! It may be all very well knowing theory (or thinking so),  but here's where I come to quietly read up on some of the techniques of how to actually do stuff.

Back to the axial chokes as transformers:

I've been doing some experimenting with different oscillator topologies with a little success in the form of the cathode coupled Hartley using two of the chokes, but have run into if simply adding the values of the chokes together, having to make sure they have no inductive coupling. ( a 180 and 22uH choke pair that inductively couple are of course more than the sum of the two, though they don't need to be coupled in this form of oscillator). The waveform purity is fairly good but the amplitude stability with change of frequency is abysmal

Tuned grid oscillators that I've tried with the method have had dire waveform purity, that is down to too much gain and also the self resonance of the choke used for a coupling coil, the amplitude distortion in this case could be controlled by a simple AGC system controlling the grid bias, (best with a sharp cut off valve ironically )

A shunt fed tuned anode oscillator is yet to be tried, as least with that the amplitude variation can be minimised by balancing the anode load resistance....

I wanted to get the best performance from the local oscillator as is possible, as I've noticed a lot of commercial radios are bad in this respect and the conversion efficiency suffers badly in the FC stage as a result...

I wanted to do this whole front end and IF using those chokes as it would make the design reproducible.

More later...

Amie.

[Amie]

Hi

Played a bit more and have come up with a basic idea for something compatible with using those chokes as an oscillator transformer, scribbled circuit attached. The difference from normal is that the feedback is coupled into the cathode rather than the grid, it gives a much cleaner waveform and constancy of amplitude than other topologies I've tried. To run this circuit with a DH valve it would need the filament power fed through two coupling windings I'm assured....

The phasing of the two chokes in the circuit is important...
   
Sorry about the picture, a clearer one will follow when I've optimized a little more

[Kalee20]

That is a most unusual and interesting configuration. I can't recall seeing anything like that, it deserves its own name

It seems to me that feedback at low frequencies will be greater than at high, because (with small coupling between coils) the cathode coil will have smaller reactance at low frequencies, so will be able to feed a larger signal into the low-resistance grounded-cathode, than is the case at high frequencies.

Will be interested to read observations of others. In the meantime, :clap :clap

[Murphyv310]

The circuit certainly works, very stable.
I remember seeing this configuration in an Eastern European set in the late 70s it may have been DDR or Czechoslovak, a comms set and used in the BFO. I have a sneaky feeling it was in the LO as well. I really cannot remember the make but why I remember it was the valve count of 15 and the diagram was inside the set.

[Kalee20]

Reference the 'scope probe capacitance...

Following on from 'scope probe capacitive loading, my own activities are at the point where a low-capacitance probe would be rather useful.

As they cost stupid money - and are rather easily damaged - I had a go at rolling my own yesterday.

Choice was, junction FET or valve. FET would be easier, and might even be slightly lower capacitance. On the other hand, valve would be more robust, and transients on the input would be shrugged off.

I used a 1j24b as cathode follower, circuit is below. It's operating as a pentode, so g2 has to be bypassed to the cathode. With both the cathode and screen-grid following the voltage on the grid, inter-electrode capacitances are bootstrapped so input capacitance is as low as it could be.

The filament supply is an AAA alkaline cell, down to 1.3V which is OK for the 1.2V of the 1j24b.

Performance? The probe works to 5MHz OK. Input capacitance? Adding 180kΩ in series with the input reduces amplitude as seen on the 'scope from 0.6V to 0.2V. A quick calculation then shows that the grid input shunt capacitance is around 2pF.

Is it worth it? Well, standard x10 passive probe has input capacitance around 14pF, so this gives about 7x reduction. On the other hand, the input swing, particularly at higher frequencies, is limited (not fully assessed as yet). The floating battery probably does no favours as it will have a significant capacitance of its own to 0V: possibly a small button cell might be better (the valve's filament consumes only 12mA). Layout right now is something of a birds-nest. Probably a peaking choke between the output, and the 'scope lead may help.

Comments invited!

[Amie]

Reference the 'scope probe capacitance...

Following on from 'scope probe capacitive loading, my own activities are at the point where a low-capacitance probe would be rather useful.

.........

I used a 1j24b as cathode follower

.........

Performance? The probe works to 5MHz OK. Input capacitance? Adding 180kΩ in series with the input reduces amplitude as seen on the 'scope from 0.6V to 0.2V. A quick calculation then shows that the grid input shunt capacitance is around 2pF.

.........
Is it worth it? Well, standard x10 passive probe has input capacitance around 14pF, so this gives about 7x reduction.

.........

Comments invited!

Hi Peter.
That seems to be a worthwhile project, it may become a necessity for me, it hasn't just been a nuisance to have to just bear in mind the extra capacitance in a circuit, its having to alter the circuit to make it work while being measured in some cases. Its easy then to forget its been done and then get into a right muddle. And this all happening at low RF.

Would results more qualitative than quantitative, regards measurements? It should at least show resonance of a tuned circuit pretty near where it should be if capacitive loading could cause a shift.

My oscillator project has morphed into one with amplitude regulation, the output being sampled, rectified, then the DC subtracted from a reference and used to bias the grid of the valve. It still remains quite simple, though the additional diodes are sand not hollow, as the regulation was a bit soggy when I tried with an EB91 in place of the two 1N4148's.... The output is 18volts PP....regulation settles at 1% across the tuning range as against 10% without regulation. The technique is to trim the anode load resistor to give as level a response as possible before putting the regulation in place....

The circuit as posted works well, despite being built from bench sweepings.

One point that I've found important to note here about those axial chokes is this, they don't seem to be wound with any regards to hand, I can't really express easily, but some seem to be "left hand thread" others "right hand thread" basically where phasing is important, you can't be certain that they are correct just from orientation..... The two I've used in this circuit are an example of this.

Thanks.
Amie.

[Kalee20]

Would results more qualitative than quantitative, regards measurements? It should at least show resonance of a tuned circuit pretty near where it should be if capacitive loading could cause a shift.

Hopefully reasonably quantitative. I haven't attempted any sort of calibration check, but will do next day or so. I'd expect it to have a constant attenuation, as a cathode follower, gain input to output will be about 0.95.

What I'm not confident of is, signal handling capability up to more than a few volts, especially at higher frequencies.

Your oscillator...

Sinewave looks really good and pure

Stabilisation method is novel, and 1% variation over the band, shows it's doing its job well.

What happens if you up the HT and use a higher value load, such that the average anode voltage or the supply current is the same?

Surprised at the chokes being wound in random direction, reason being, I'd expect them to be machine wound and all the same. Unless Epcos are running two kinds of machines, which rotate in opposite directions...

And, those valves which are decorating that birthday card (friend of yours I guess?), the pair of smaller ones look like the 1j24b! Which seems to have become my go-to small-signal battery pentode, sort of thermionic BC107. (And as no valve deserves to be compared with sand, I'm now off to wash out my keyboard!)

[Amie]

Would results more qualitative than quantitative, regards measurements? It should at least show resonance of a tuned circuit pretty near where it should be if capacitive loading could cause a shift.

Hopefully reasonably quantitative. I haven't attempted any sort of calibration check, but will do next day or so. I'd expect it to have a constant attenuation, as a cathode follower, gain input to output will be about 0.95.

What I'm not confident of is, signal handling capability up to more than a few volts, especially at higher frequencies.

I suppose the small signal handling will be OK, but "slew" with large inputs could be limited?

Your oscillator...

Sinewave looks really good and pure Clap

Stabilisation method is novel, and 1% variation over the band, shows it's doing its job well.

What happens if you up the HT and use a higher value load, such that the average anode voltage or the supply current is the same?

  Very pleased with what it does, don't really know about the increased HT, some of the components are voltage limited so would need changing before I tried. Apart from that, the output is controlled by the zener (15volts), the two diodes (1.2volts), the output has to overcome these before there is sufficient to start biasing the valve off. Increased HT and anode load resistance would suggest more gain and therefore an easier time for the valve to create the required output, as it stands, removing feedback completely allows the output to go up to around 30volts....

Surprised at the chokes being wound in random direction, reason being, I'd expect them to be machine wound and all the same. Unless Epcos are running two kinds of machines, which rotate in opposite directions...

Strange but true!

And, those valves which are decorating that birthday card (friend of yours I guess?), the pair of smaller ones look like the 1j24b! Which seems to have become my go-to small-signal battery pentode, sort of thermionic BC107. (And as no valve deserves to be compared with sand, I'm now off to wash out my keyboard!)

Yes very good friend, actually the same person let me have a bag of surplus TO18 transistors a while back, I like valves more but if there's no easier way I will use soot or sand. The late fifties valve/transistor hybrid portable radios must make you cringe 

Actually, I'm loth to disassemble the card just yet, but those 1j24b may end up in a superhet front end yet!

I do have a few 1j29b left also that dads friend sent him but they are more suitable for small speaker amplifiers, the Vanguard has one in the output stage in place of the DL96.

Amie