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
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 available
from McMaster-Carr. 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 base is from Wood Box Works. It was the left over lid from my sixth crystal set. You see, I don't throw much away! I used Mixwax natural stain
on this box.
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