Electron-Coupled 1 Valve Receiver

[Variometer]

I have not done any radio for some time now, but yesterday, the urge came over me to build something. This little receiver uses a 1T4 pentode and is rather unusual. The coils are my usual method of a couple of little RF chokes inside two old 1 1/4 inch fuses (with fusewire removed, of course). The unusual feature is that the reaction choke does not need to be in close proximity to the aerial choke. I therefore mounted them at right-angles. The aerial choke is behind the tuning capacitor, whilst the reaction choke is down the right-hand side of it. On completion, the reaction didn't work, but that was only because I had incorrectly wired the in-set earth lead. As soon as I put that right, it worked very well. Reaction is controlled by the small potentiometer varying the screen voltage. The LT is the normal 1.5V and the HT can be anything from about 18V up to 90V, with a current drain of less than 2 mA. Great fun, but it took me all day to build and get working properly. The transformer is to match the output so that standard low impedance headphones can be used!
Bob

[kalee20]

Photo looks lovely, 1920's wiring techniques, 1940's valve and holder, 1980's knobs perhaps and 2000's transformer?

Any chance of a circuit diagram? And what aerial and earth do you use?

[Variometer]

The valve is a 1T4 that was commonly used as IF amplifier in those little "suitcase" style battery sets that were still being produced in the early 1960s. The valveholder was made by myself. It is a flat round acrylic disk with a standard B7G fitted in it. The wiring underneath solders directly to the underneath of the brass screws round the edge. The knobs are from Maplin Electronics, current production, and the transformer is a PCB mounting miniature mains type, 250V in 12V out, the 250V winding in the anode circuit and the 12V to the headphone jack. Earth is a copper rod knocked in ground outside window and aerial is just thin plastic insulted stranded wire trapped in window frame at bottom end and leading across to peak of garage roof and on to a flagpole further down garden. I haven't made a diagram yet, but it is a bit strange as you can see the aerial connection (far left) goes across to filament positive and then to the junction between the two 47uH chokes. The bottom of the right hand choke goes to earth. reaction control, bottom of 47K pot goes to earth. .01uF cap between earth and slider. Slider goes to grid 2 and other end of pot to HT positive. You can see most of the wiring on the picture. Centre of the three left hand valve connections is grid and goes to 100pf grid cap with 1M resistor across it. The two valve connections on either side of grid are F+ and F- The two brass strips under transformer go to anode and grid 2 of valve on the right of holder.

I was messing about with the original circuit for several days before it was ready to build neatly. Final day was spent on final build and then getting it to work due to me wiring the earth wrong.

The chokes don't need to be in proximity, which helped a lot in layout as I could have one down the side of the capacitor.

I will post a circuit when I have sorted one out.

Bob

[kalee20]

Hi, yes,

When I meant the 'valve and valveholder' being 1940's I meant late 1940's (and as a good design the 1T4 was still used in productio radios in the 1960's) And the valveholder, I meant the commercial B7G socket, not your own superb enhancement to make it into a base-board component with screw terminals!

Thanks for info on aerial and earth. I'm awaiting the circuit diagram with interest!

[Variometer]

Here is the circuit. I first built is as short wave receiver with 8 turns and 4 turns respectively on a 1 1/2 inch former and it worked well. Chokes are not suitable for short waves. This circuit is medium wave. I believe this type of arrangement was initailly developed over 60 years ago.
Bob

[Neil Purling]

8 turns and 4 for the cathode tap if you are doing short wave?

I stick with mains bottles. As this thread is about a battery receiver please send me a PM so we don't upset Mr Moderator.

[Variometer]

Have sent PM, but why should it upset moderator?
Bob

[Brian R Pateman]

The discussion of valve types is perfectly OK for this thread.

In fact we would prefer the discussion to take place in the thread rather than by PM. After all one of the purposes of the forum, especially this section, is to spread information and encourage other members to experiment.

Regards,

Brian,

"Mr Moderator" (One of them).

[Variometer]

That is what I thought. What I said in the PM was that the circuit is supposed to work even better with indirectly heated valves, although I haven't tried one yet. 8 turns and 4 turns worked OK for me. I have found that mains valves such as EF91, EF80, EF95 etc will all work with 54 Volts or even less. I haven't tried one yet, but the 1T4 works OK in this circuit.
Bob

[daviddeakin]

That valve socket is beautiful! (As is the whole thing).

Out of interest, why do you put the inductors inside fuses? Just to make them easier to swap for different values?

[Variometer]

It is partly to make them easier to change. Initially, I found the chokes were a bit expensive at about 44p each and tended to have a high mortality rate when moving from circuit to circuit, espcially if I cut the wires short. Then I bought a large box of 1 1/4 inch glass fuses of quite useless (to me anyway) values, 25A and that sort of thing from a shop that was closing down and tried the chokes out inside them and found they worked fine. The glass did not prevent the correct coupling to reaction choke or aerial choke etc. Eventually, it didn't matter as much because I purchased 60 100uH chokes online for a very low price and he sent 150 of them! I asked if it was a mistake and he said no, without further comment. So I can now connect them in series or parallel etc to get all manner of values.
The valveholder was a piece of 3mm black acrylic. First I stuck a drawing of it (made with a pair of compasses) on a piece of acrylic with the holes marked out. I drilled the valveholder hole first and then the small fixing and connection holes. Finally, I cut out the outer circle using a small hobby bandsaw. The valvholder connections were wired up underneath, soldering direct to the brass bolts. The valve holder still requires a shallow hole drilled in the basebaord to take the projecting valveholder and pins, but it is the neastest type of baseboard mounting type I have ever devised. It is based on the old ceramic acorn valveholders.

Bob

[daviddeakin]

Originally Posted by Variometer

The glass did not prevent the correct coupling to reaction choke or aerial choke etc.

I really love the valve holder, would be ideal for prototyping...


I must admit I know practically nothing of radio (audio's my thing). Are you saying that the inductors used in a typical radio often need to be physically positioned in a specific way, relative to other components? What I mean is, what was your initial fear about using the glass tube that prompts you to make the above comment?

[Variometer]

They usually need to be positioned close to each other for the prupose of mutual inductance. The reaction choke normally couples RF back into the grid circuit by its proximity to the lower part of the aerial coil (like a transformer). In this circuit though, it doesn't matter, so I was able to have the two chokes apart.
Bob