Three Valve TRF. 16th May, 2016.
The receiver is now complete, and working extremely well with more than adequate volume, and receiving a number of stations with good selectivity. The circuit is complete with all the component values. It is a very conventional arrangement, apart from the rather unusual configuration of L1, L2, and VC1a that, as I said, was discovered by accident!
The total HT current drain at 100 Volts is about 10mA. 2mA for V1, 2mA for V2 and 6mA for V3.
The valves may often be found on Ebay, but other similar types will work just as well! Of the components, the rarest are L6, RF choke in the anode circuit of V1 and the intervalve transformer, T1.. I used a vintage components, but I did try a modern 2.2mH RF choke in place of L6, that performs just as well. The intervalve transformer, T1 will probably be rather harder to find, and can be quite expensive these days, but I hope to experiment with a more modern substitute shortly. The terminals of this transformer are labelled HT, P, G and GB, standing for High Tension, Plate, Grid and Grid Bias. In times long gone, the anode was referred to as the "plate," hence the P! Newer intervalve transformers are labelled IP, OP, IS, OS. The equivalent designations are:
HT = OP
P = IP
G = OS
GB = IS
The next task is to rebuild it, but in a much neater manner. (This one looks a mess!) This will probably take some time, because I will need to either find, or make, components that fit in with the 1930s era.
VC1a and VC1b are a double gang tuning capacitor. VC2 is a separate reaction capacitor.
In the prototype, the loudspeaker matching transformer T2, was not mounted on the board, but on the separate loudspeaker. When I build the set properly, I will put it on the board!
In the circuit, bear in mind that the black dots indicate where wires are connected. If there is no dot where lines cross, they are not connected at that point! Note that the polarity of the grid bias battery is very important, the positive must go to the earth line, and the negative to GB on the transformer.
I will be pleased to answer any questions (may not know the answers though).
Incidentally, I drew the circuit myself, it is not copied from anywhere!