Homemade Regenerative Radio.
Homemade Regenerative Radio.
This one looks just a little on the ugly side, at least from the front. But they tell me that back in them thar olden days, homemade sets looked like that. But behind that black face lurks a thing of beauty! This radio is the closest that I have come so far to the original sets of the day. I found a very nice variable capacitor with brass plates at the Rochester Hamfest. I bought the tube socket and the grid leak capacitor from Play Things of the Past. The coil is described on one of my Variocoupler page.
This radio uses a single type 30 tube. I really like the shape of those old tubes. The #30 tube requires 2 volts to operate the filament. So far I have been running it on a 1.5 volt D cell. When I find a 2 volt lead acid cell, I will use that. Another way to light the tube is to use two D cells with a 20 ohm resistor in series.
The type 30 is a triode tube, meaning that the tube has 3 elements. A filament, a grid and plate. The electron flow is from to the filament to the plate with a grid or wire in between that will control this electron flow. This is how a vacuum tube amplifies. Controlling the regeneration, or feedback is simpler with a tetrode or pentode tube as the voltage can be varied on the screen grid with a pot. With a triode, I found the nicest way to control the amount of regeneration is by changing the coupling between two coils.
The RF choke I used was a 5 mh type made by Miller. It looked really cool so I used it. A 2.5 mh or even smaller choke can be used in it's place. As you see, this radio doesn't have many parts.
The front panel is made from a piece of 1/8 inch black Garolite®. a4 Garolite® can be cut on a table saw, but sometimes the edges tend to chip, so a fine tooth blade would probably work better.
The connections to the radio are as follows: Looking at the back, from left to right are, 2 connections for headphones, then two for filament, then two for the 45 volt "B" battery and finally the ground and antenna connections. I used fahnenstock clips for these connections. They are very cheap and work very well.
When operating a regenerative radio, frequent adjustment of the regeneration control is required when tuning the band. Try to keep the set out of oscillation as there is no isolation between the tube and the antenna. You can end up with a miniature transmitter that will disturb your neighbors reception (if anyone except for me listens to AM radio anymore).
I have two developments to tell you about. First, if you can't find a 2 volt battery, you can use two 1.5 volt C or D cells in series if you put an 18 ohm resistor in series with the battery. This will cut the filament voltage to just about 2 volts on a fresh battery.
While we are talking about filament supplies, I discovered something rather important. It matters which way the "A" battery is connected to the radio. With the type 30 tube (and the 1T4 and other directly heated tubes in a grid leak detector circuit) the return of the grid leak resistor (through the coil) should go to the + voltage. With my receiver that means the + terminal of the battery has to be grounded. The page from the tube manual mentions this. My circuit reflects this change. When I build my upcoming receiver with two tubes, I will make sure that this step is taken care of.
73, Dave - N2DS