■ FIGURE 4. A circular nichrome
hot-wire was used to fracture the
glass envelope near the base.
■ FIGURE 5. The inner grid (grid1) wires are
hard to see because they are obscured by grid 2.
In case you’re not familiar with the parts of a tube,
please take a look at Figures 4-6. In Figure 5, the cathode
is the silvery hollow tube in the middle, with a folded
alumina coated tungsten filament tucked inside, but
peeking out the top. The two grids are the fine wires
which are spiral-wrapped around the shiny support rods
and nested inside of each other. In Figure 4, the plate is
the hollow black ribbed rectangular structure and the
round piece in front of it is a piece of thin mica — an
insulator. During assembly, the plate is carefully slipped
down over the grids and held in place by mica disks at the
top and bottom. The whole thing is an intricate structure
all nested inside of each other, but not touching.
Each of the elements is spot-welded to the pins in the
glass base, and a glass envelope is slipped down from the
■ FIGURE 6.
envelope of the
tube can be over
100°C, so be
top. The envelope is sealed to the base with a torch, and
then the air is sucked out through a small glass tube at the
top. Another torch melts the small tube closed, sealing the
vacuum inside. You can see the melted tip in the upper
left of Figure 4.
In operation, 12 volts is applied to the filament and
heats it up to over 1,000 deg C, as seen in Figure 6. The
hot filament, in turn, heats the oxide-coated cathode
which boils off a cloud of negative electrons — called the
space charge. Grid 1 — which is hooked to + 12 volts — is
located very near to the space charge region and attracts
most of the electrons to it. However, some do get
The accelerated electrons that get through grid 1
continue on through grid 2, and are attracted to the plate
which is also hooked to +12V. The net result is that more
electrons get to the plate as a result of grid 1 than if there
was just the plate to attract them. Plus, these space charge
tubes run the filament at a hotter temperature and have
an increased cathode area in order to produce even more
electrons than usual.
Now, if a small negative voltage (-4V) is applied to
grid 2, it will repel the electrons back towards grid 1 and
the cathode, shutting off some or all of the flow to the
plate. In other words, a tiny change in the negative voltage
on grid 2 makes a big change in the current flowing to the
plate. This is how a tube can amplify a signal. However, in
a practical amplifier circuit, additional components are
needed to convert the plate current into a voltage and to
bias the tube into the desired operating region.
The bottom line is the 12K5 is suitable for certain
applications, but it’s not as powerful as tubes that use high
voltage. In the Retro-Shield, the 12K5 serves a dual
purpose. L1, C11, C12, and R6 form a radio frequency
(RF) oscillator tuned to the AM radio band. In addition,