Two Valve TRF

[Variometer]

I have just completed my latest project. It is a two-valve TRF using a couple of EF91s. Incorporated in the circuit is my reaction unit described lower down in this forum section. It is just a grid leak detector, transformer coupled to the second EF91. The band covered is 700kHz - 2Mhz. The transformers are both 12-0-12 mains types & work very well. Quite loud output & drives a loudspeaker on local stations. The physical construction gives me as much pleasure as anything else using polished wood, brass strips etc in a style that is generally old-fasioned, but bears my own individual style. I had considered putting a front panel on it, but decided against as a lot of the appeal is visual as well as performance. It seems to work best at 100 Volts HT, but will work with correspondingly less volume down to quite low voltages. Current drain about 7mA. A great "fun" project!"
Bob

[John M0GLN]

What an excellent looking job you've made, congratulations Bob.

John

[Station X]

A joy to behold.

[mickjjo]

An excellent piece of work, Well done!

Regards, Mick.

[Variometer]

Thanks, there is a bit of camera distortion in the pictue, it is actually quite square. As we seem to be in a month of compulsory darkeness, the picture is not as good as I would wish, but when we do get a bit of sun, I will take some more from different angles.
Bob

[Aerodyne]

You've done it again, Bob. My admiration grows exponentially!
Superb bit of handiwork, maybe the way some home-built radio receivers should look: a glance lovingly backward at past glories, yet new and fresh in approach. The connecting strips (in lieu of chassis, very neat idea) look like flat brass. Is this glued to the baseboard, or thick enough to support itself? It's a little hard to tell.
More please!
-Tony

[Variometer]

It just brightened up a bit, so got another photograph from another angle. The other picture shows it in the "design" stage. I began with extending the prototype of the "Paint Tine One." It was largely trial & error. The "messy" picture was taken just as I got it working. Then I drew out the circuit & made each unit neatly & fitted it all to the wooden baseboard. I can seldom think up a circuit & find that it works first time, so it usually takes quite a lot of "messing about." Never been much of a scholar as far as electronics are concerned, but what I can't carry, I usually manage to drag quite well.
Bob

[Variometer]

Hi Tony,
The brass strips are quite soft & springy (I got a box full on a car boot sale some time ago). They are initially fixed to the baseboard with double-sided tape. Once the wires are soldered on though, the tape becomes superfuous, so even should it eventually fail, the wiring will still hold the strips in position. You can buy thin brass strips from model shops. The black bases for the various sections are 3mm black acrylic sheet which is quite cheap & easy to obtain.
Bob

[paulsherwin]

A very nice job indeed, it makes my efforts look as if they were put together by a 3 year old

Do you have the circuit that you used?

Paul

[Variometer]

Thanks for comments. I haven't drawn the circuit out neatly yet, but it is very basic. Both valves have their screen grids strapped to their anodes & their supressor grids to their cathodes. The coupling transformer has its 240V winding in the anode of V1. Both the 12 Volt windings are in series with one end down to earth & the other to the grid of V2. Cathode bias in V2 is a 330 Ohm resistor, with a 33uF bypass capacitor. I tried the coupling transformer both ways round, but this configuarion was by far the best. The reaction unit is as described elsewhere in the Homebrew section. Grid capacitor is 220pF & grid leak is 2.2M. I can't find the rough circuit at the moment, maybe even threw it away. I will post if after drawing it out again. Oh - aerial coil was two 47uH RF chokes in series, centre junction going to aerial via 100pF. Reaction choke (rotating) 100uH & reaction capacitor 220pF.
Bob

[Variometer]

Here is the circuit. L3 rotates physically next to L2, thus giving a very smooth reaction control.
Bob

[Aerodyne]

Thanks for sharing the circuit with us, Bob. I love the simplicity of it and I also love the way you have of using components specific for a given purpose in entirely different ways. Your mains transformers used as coupling and output are a case in point. I've used output transformers as mains transformers but never the other way around - it simply hasn't occurred to me. BTW what are you sourcing the LT and HT from?
And what's next...?
-Tony

[Variometer]

Hi Tony,
Yes, the circuit is simple enough. I just wanted something to put my reaction unit in. The HT & LT come from a variable power supply that I made a few months ago. All the details & circuit are on the forum somewhere. Basically, it is a mains transformer with a 250V & 6.3V outpit. The 6.3 is used for the heaters. The 250V goes into a variable auto-transformer (From Maplin). That varies the output from zero up to about 275V. That goes into a standard rectifier & smoothing circuit consisting of a FW silicon bridge rectifier, reservoir/smoothing capacitor & smoothing choke, plus the usual fuses. I find it very handy because I can select my output volts from zeo to about 300V (after rectification & smoothing).
Don't know what radio project I will build next, but I will be doing other things for a while. I only come back to radio at intervals when the spirit moves me.
This is usually after a few weeks. But I still like to come & see what is going on here even if I am not doing much.
Bob

[Al (astral highway)]

Hi Bob, that's a very pleasing project indeed. I wish I had that ability to craft things to such a standard.

Where can one get those very pleasing brass strips, btw?

[Variometer]

It doesn't really require any great deal of skill. The baseboard is a piece of quite common wood with a dark veneer stuck on it with Evo-Stik contact adhesive. After smoothing with wet & dry paper. it is painted with Evostik wood adhesive (white glue) diluted with water. This fills the grain & after smoothing, the final polish is applied using an aerosol lacquer from Halfords. I cut the acrylic bases using a small hobby bandsaw. You can get brass strip from model shops, but as I said, I got a large quantity on a car boot sale. Brass nuts & bolts from model engineering firms. Aluminium angle from DIY stores. I suppose the hardest thing is the fact that if you throw it together, it works fine. Re-build it neatly & it doesn't work at all! Takes a fair amount of experimentation to find a workable layout.
In the field of shipmodelling, I often have people saying "I could never build anything like that!" My reply is normally: "Saying or even thinking something like that is half the battle - the half that is already lost, so you may as well not bother!" The correct attitude is "I would like to do something like that, and what's more, I am going to find out how, & do it myself!"
Bob

[Aerodyne]

It doesn't really require any great deal of skill.

You do yourself a dissservice, Bob. You may not think much skill is required - others amongst us know differently. I do like your positive attitude though, and could say much about that but this forum isn't the place to get all philosophical, so I'll just say that wonders can be achieved with the right sort of thinking.

Tony

[Variometer]

Hi Tony,
I would agree with that, it is really just a matter of wanting to do something. My interst always lay in the practical side of radio & to be honest, the theory doesn't interest me all that much. I spent so long getting to understand the workings of valves & when I did understand them, they dumped them in favour of the transistor & IC that never really interested me all that much, even though I had to work with them for many years.
Bob

[Norm_N_Tam]

Originally Posted by Variometer

It seems to work best at 100 Volts HT, but will work with correspondingly less volume down to quite low voltages. Current drain about 7mA. A great "fun" project!"

Bob,

I have seen examples of radios like this one running on as little as 12 volts. Of course, you can't expect loud speaker volume with just 12 volts of HT! Please see the link below:

http://www.qsl.net/kl7h/12v.htm

Note: the North American 6BA6 is the same as the EF93 in the U.K.

Norman

[Aerodyne]

the transistor & IC that never really interested me all that much, even though I had to work with them for many years.

...As so many of us, Bob.
-Tony

[Variometer]

Further experiment. I took two empty 1 1/4 inch fuses. The first one I wrapped the glass section full of 35 swg enamelled wire. On the inductance meter, this came out at 17.7uH. I placed it in the holder next to the reaction choke (leaving the 47uH in the other holder). I found that the set then operated well into the sw bands, but nowhere near as loud!
The second fuse, I removed the caps & filled with iron powder, sealing off the ends with plastic padding before replacing the metal caps. I then wound it full of 35 swg enamelled wire exactly like the first fuse. It tested at 55.6uH & worked quite well in the circuit. Slightly different coverage than with the original choke, but all on good volume.
Bob