2-11
UHF TUBE TYPES
Uhf tubes have very small electrodes placed close together and often are manufactured without
socket bases. By reducing all the physical dimensions of the tube by the same scale, the interelectrode-
capacitance and transit time effects are reduced, without reducing the amplification capability of the tube.
A disadvantage to this type of tube construction is that the power-handling capability of these tubes is
also reduced due to the close placement of the tube elements.
Uhf tubes are placed in three broad categories based on their shape and/or construction; ACORN,
DOORKNOB, and PLANAR tubes.
Acorn and Doorknob Tubes
ACORN TUBES, as shown in figure 2-10, are available for use as diodes, triodes, or pentodes.
Acorns are very small tubes that have closely spaced electrodes and no bases. The tubes are connected to
their circuits by short wire pins that are sealed in the glass or ceramic envelope. Because of their small
size, acorn tubes are usually used in low-power uhf circuits.
Figure 2-10.—Acorn tubes.
The DOORKNOB TUBE is an enlarged version of the acorn tube. Because of its larger physical
size, it can be operated at higher power than the acorn tube.
Planar Tubes
PLANAR TUBES are so named because of their construction. The ordinary (conventional) tube you
studied earlier uses concentric construction. This means that each element (cathode, grid, and plate) is
cylindrical in shape. The grid is placed over the cathode, and the plate, which is the largest cylinder, is
