Self-contained reaction unit for valve circuits.

[Variometer]

Thought you may be interested in this. I wanted a self-contained reaction unit that I could easily change from one circuit to another with a minimum of fuss. Also required easy band change. This is the result. I have just tested it and it works superbly well, despite its unorthodox configuration. I didn't want a variable reaction capacitor because they are rather hard to find these days and can be quite expensive. Neither did I want a potentiometer-controlled circuit because sometimes they are not as smooth as they might be. The main part of this little unit is mechanical. The brass spindle has a piece of dowell on the back end and a slice of brass tubing over it forming a single slipring. On the end of the spindle is a small circle of acrylic with two holes drilled in it to take a 100uH miniature choke. This is the reaction coil. One end of the coil goes to the slipring, whilst the other end is connected to a 220pF reaction capacitor, the other end of which is soldered to the spindle.
behind the reaction choke is a 1 1/4 inch fuseholder. I removed the centre from a fuse and replaced it with a 47uH miniature choke. This is the lower half of the aerial coil. The single brush is a piece of springy brass. The connections to the circuit are quite simple. One tag to earth and the other to the detector anode. The other half of the aerial coil is permanently in the receiver. This is another 47uH choke inside a fuse mounted in another fuse holder. In operation, the fact that the reaction choke revolves next to the aerial coil moves it from out of phase to in phase. The reaction is very smooth and the volume even seems to be louder than the set (1 valve) was before with a normal reaction. The fact that the anode goes to the reaction coil, then the reaction capacitor and finally to earth, seems to improve performance.
I am very pleased with this unit and will soon be incorporating it in my next project.
Bob

[kalee20]

This is swinging-coil reaction control. The mechanical rrangements are very nice - congratulations!

[Variometer]

Thanks, the two spindle bushes came from a couple of rusty old potentiometers. I had thought of making it like a conventional variometer, but thought if I got the turns wrong, it would be a bit tedious sorting it all out. I am still surprised at how smoothly it works. On the prototype, the reaction capacitor was not included, that was still on the main part of the receiver board. Then I wondered if it would work by putting it between one end of themoving choke & the spindle. That actually seemed to improve the performance. Having recently bought a job lot of high amp 1 1/4 inch fuses from a hardware shop going out of business, I think I will fill them up with different value chokes. It is more convenient when they are plug in, it allows for more experimentation (never did like winding coils).
Bob

[Al (astral highway)]

Ingenious and pleasing.

[Mike Phelan]

A nice piece of engineering and recycling, Bob, and I really like the idea of the "fuse"

[Station X]

Well Done Bob.

That's home construction at its best.

[OErjan_S]

a "fuse" could be used for both coils giving een more options, nice idea

[Variometer]

If a fuse was used for both coils, I dont know if they could be got close enough for it to work properly! As it is, the aerial coil can be lifted out vertically. It would complicate matters trying to make the reaction choke removeable as well, but the reaction choke value is less critical. The whole thing is open to experimentation though & I would be interested to hear of any improvements. This is only my second one. The prototype worked OK, but did not have a slipring. The connection was made using a flexible wire, but it kept getting in the way of the end stops. With the above version, I was able to dispense with the stops as the slipring allows the spindle to turn indefinitely. This does not matter as the best point of reaction is found in less than a 180 degree movement anyway.
Bob

[Aerodyne]

I like this. I've been thinking for some time about how I might construct a variometer to do away with a variable capacitor or variable resistor in the construction of an ultra-simple 1-valver, but haven't so far got around to practical matters. I think your swinging coil arrangement is a good way to go as it solves the difficulty of winding turns 'balance' as you've pointed out.
Nice idea, neatly carried through.
Chokes from RS, presumably, or perhaps Farnell?
-Tony

[Variometer]

Hi Tony,
I get the chokes from the local Maplin store. Not long ago I purchased a bag full from some obscure component supplier (mail order) very cheap, but found that most of them were miles off value, so now I only buy from reputable suppliers. I have only just started putting them inside fuses & find it very convenient. I get the caps off by heating the end up with a soldering iron & they just push off. The inside can be cleaned by shoving a "cotton bud" through them. After the choke is inserted, no need to glue the ends back on as the solder holds them in position. I leave a short stub of wire protruding at each end so I can easily check the inductance with my little component analyser. Before I got the analyser, chokes could drive me mad sometimes as they look too much like resistors for my liking. Despite their small size, when used as tuning coils they are equal in performance to the coils we could once buy for home projects. I have even used them in frequency changer stages in superhets.
Bob

[Aerodyne]

If there's one drawback to the use of chokes as tuning coils it is their vulnerability to damage. You've avoided this by your mounting method - an ingenious way to get around the problem and make them sturdier in use.
Must look in to my local Maplin store one of these days. I tend to walk straight by it, always in a rush to get somewhere else! I did check on RS and they have the chokes, however.
-Tony

[Variometer]

Thanks, I was fortunate in being able to buy a huge box of 1 1/4 inch fuses for about £2. They were all unpopular values in excess of 20A or more, so I had no qualms about pulling the ends off & putting chokes inside.
Today, I put the unit in a one valve circuit using an EF91 & it worked fine. Then I added another EF91, transformer coupled, & it now drives a loudspeaker at fairly adequate volume on the local stations with good tone. Now I need to re-build it neatly.
Bob

[Norm_N_Tam]

Here is a little crystal radio that appears to use one of those little chokes that Bob likes so much:

I've been thinking these little chokes would be great for making very small radios, now here's an example!