Homemade 4 Tube Radio
Whew! This one was a big job. I wouldn't recommend this one for the beginner. Welcome to my 4 tube radio page. I suspect that I won't be building a 5 tube set, but you never know. At least it won't be a battery operated set. I think that this one will flatten the batteries pretty well. But it is done, except for the tweaking.
This set uses 4 of the miniature battery tubes that were developed in the early forties. They were meant for superhet and other types of receivers and some military walkie-talkie type transceivers. My homemade radio operates on the broadcast band only. There is an RF amplifier (1T4 tube), a regenerative detector stage (1T4 tube), an audio amplifier, (1S5 tube) and an audio output stage (3S4 tube). The set runs on a single D cell with a current draw of 250ma. Nearly the current draw of a flashlight bulb. The audio output stage runs on 90 volts. the rest of the radio uses 45 volts. There is also a grid bias battery which is a 9 volt battery. I use one that is weak as 9 volts is a bit high for the bias and there is no current required.
There are two power switches. One for the filament and one for the 45 volt supply. The grid bias and 90 volt supplies don't have to be switched off. The bottom small control is the volume pot while the top one is for regeneration. Each has an on-off switch.
The first tuned circuit is made with the external loop antenna and the left hand side tuning capacitor. The right hand tuning capacitor is for the detector tuning. Now days, these are ganged together with one capacitor (as if anyone builds these kind of sets anymore). Getting the capacitors to tune together can be quite a problem (tracking), so I kept them separate. So, this is a "two handed set"!
I put up a hand drawn circuit diagram just to give you all an idea of what goes inside one of these sets. I am not sure if I will do one on the computer. They take a long time to make, and I don't want to do it until I am happy with the design. Right now, I am not overjoyed with the performance. The audio could use more steam. I will work this out and put up a better circuit. Again, I don't recommend that this be built as shown. My next idea is to take a more modern (semiconductor) circuit radio but keep the old knobs and the old look. That way, you don't have to spend a fortune on batteries. Any thoughts on this?
By the way, I bought the box at Wal-Mart. I saw it in their craft section. It is about 15x6 inches. It was put together with nail brads. I took out the brads and put in some flat head wood screws, then sanded it and finished it with mahogany Min-Wax stain. After that, I sprayed some glossy deft on the box. Not bad for a 5 dollar box.
Let me know if you like this set and what I might do to improve it.
73, Dave - N2DS