24-09-2016, 08:15 PM
Heath-Robinson style one valve receiver.
I did not really intend to build this. A few days ago, I wanted to test some old headphones that I had renovated, so I built a crystal set. It consisted of a centre-tapped coil wound on a piece of plastic drainpipe, 50mm diameter, mounted on the back of a variable capacitor. The germanium diode was contained inside an old glass fuse and holder mounted on the back of the tuning capacitor. A bracket was bolted to the front of the capacitor to take the 1/4 inch headphone jack socket. It did work, but not very loud.
I then decided to add a valve amplifier. I put an aluminium angle bracket between the variable capacitor and the coil. This was fitted with a 4-pin DIN socket for the valve. The valve is a CV122 triode that came out in the 1940s. I believe they were used in the electronic guidance systems of certain artillery shells. I have a number of these. They are wire-ended, but it was an easy job to fit a 4-pin DIN plug on one end, and use a 4 pin DIN socket as a valve-holder.
The completed receiver did not work at all! I therefore removed the germanium diode from the glass fuse, and replaced it with a 220pF capacitor, and added a 1M grid leak resistor between the grid and earth. The receiver then worked, but was not much louder than the original crystal set version.
So, I wound a reaction coil around the lower half of the grid coil. I did this without taking it to pieces, by threading it between coil and capacitor, and passing it round the coil - it was all very laborious!
Initially, I had a 47K pot and a 150pF feedback capacitor in the reaction circuit, and it then worked very well with quite strong reception, and very selective.
The only drawback was that the reaction was "crackly," so I dispensed with the 150pF capacitor and pot. and replaced them with a small, modern, variable capacitor. The reaction then became very smooth.
The reaction works back to front though, as the capacitor decreases as it is turned clockwise, but I quickly got used to that.
With 54 Volts HT, it draws 0.38mA anode current. When I increased it to the valve's maximum, of 100 Volts, it drew 1.46mA, and a significant gain in volume. The headphone volume is quite adequate, thanks to the addition of the reaction circuit.
It is something of a Heath-Robinson affair, but I enjoyed building it!
I did not really intend to build this. A few days ago, I wanted to test some old headphones that I had renovated, so I built a crystal set. It consisted of a centre-tapped coil wound on a piece of plastic drainpipe, 50mm diameter, mounted on the back of a variable capacitor. The germanium diode was contained inside an old glass fuse and holder mounted on the back of the tuning capacitor. A bracket was bolted to the front of the capacitor to take the 1/4 inch headphone jack socket. It did work, but not very loud.
I then decided to add a valve amplifier. I put an aluminium angle bracket between the variable capacitor and the coil. This was fitted with a 4-pin DIN socket for the valve. The valve is a CV122 triode that came out in the 1940s. I believe they were used in the electronic guidance systems of certain artillery shells. I have a number of these. They are wire-ended, but it was an easy job to fit a 4-pin DIN plug on one end, and use a 4 pin DIN socket as a valve-holder.
The completed receiver did not work at all! I therefore removed the germanium diode from the glass fuse, and replaced it with a 220pF capacitor, and added a 1M grid leak resistor between the grid and earth. The receiver then worked, but was not much louder than the original crystal set version.
So, I wound a reaction coil around the lower half of the grid coil. I did this without taking it to pieces, by threading it between coil and capacitor, and passing it round the coil - it was all very laborious!
Initially, I had a 47K pot and a 150pF feedback capacitor in the reaction circuit, and it then worked very well with quite strong reception, and very selective.
The only drawback was that the reaction was "crackly," so I dispensed with the 150pF capacitor and pot. and replaced them with a small, modern, variable capacitor. The reaction then became very smooth.
The reaction works back to front though, as the capacitor decreases as it is turned clockwise, but I quickly got used to that.
With 54 Volts HT, it draws 0.38mA anode current. When I increased it to the valve's maximum, of 100 Volts, it drew 1.46mA, and a significant gain in volume. The headphone volume is quite adequate, thanks to the addition of the reaction circuit.
It is something of a Heath-Robinson affair, but I enjoyed building it!
