November 2017 11
circuit. For the triode circuit in Figure 4, μ = –gm / RP. The
minus sign indicates that the output waveform is inverted.
That is, when input voltage increases causing an increase in
IP, the output voltage drops because of the higher current
flowing through the plate resistor, R3.
The Full Gallon
That’s the colloquial name for the full legal limit on
output power for US amateurs: 1,500 watts. (The reference
comes from before the “modern era” when power
limits were stated in terms of the DC input power to the
amplifier; back then, the limit was 1 k W. The slang term for
k W was “gallon,” so there you are.)
Until fairly recently, the only economical way of
generating that much power was by using big vacuum
tubes — both physically large and electrically outsized.
The typical amateur amplifier produces from 600 to
1,500 watts with one or two tubes. Construction varies
from custom homemade units to top-quality commercial
Amplifier building is common since it doesn’t take
special tools or techniques. You can build a perfectly good
amp with hand tools, and many hams have done just that.
The most popular tubes for HF builders are the
venerable 811 (developed in the late 1930s and still used
commercially for RF and audio), the 3-500Z triode, and the
8877 tetrode which is often available as a “pull-out” from
commercial equipment. A browse through a ham radio
equipment dealer’s website will show there is plenty of
interest in full power operation.
Figure 5 shows the result of a project by John
Stanley K4ERO to design a simple amplifier that could
use inexpensive components — even a microwave oven
transformer — in various configurations: The Everyham’s
Stanley designed the circuit so that almost any large
tube (or tubes) available could be used, including some
Russian military surplus tubes from amplifiers used in tanks.
All the parts can be found in old amplifiers, at hamfest flea
markets, or scrounged from friends.
Figure 6 is an example of what a skilled builder can
do. This full power design is for use at 50 MHz.
If you like the looks of these projects, there are
plenty of amplifier and suitable power supply projects in
the pages of the ARRL Handbook starting with editions
from the 1950s. There is lots of tube theory and design
n FIGURE 5. The “Everyham’s Amp” by John Stanley
K4ERO. Designed to use parts on-hand and available
used or from surplus outlets, the HF amplifier can be
built to use many common tubes, and produce power
from a few hundred watts to more than 1 k W of output
power. (Photo courtesy of the American Radio Relay League.)
n FIGURE 6. This beautiful example of home
construction by Dick Stevens W1QWJ produces the
“full gallon” on the 50 MHz band. The single tube
is a 4CX1600B tetrode. High power on VHF and
UHF frequencies requires great care in design and
construction because of the short wavelengths involved
and sensitivities to “stray” capacitance and inductance.
(Photo courtesy of the American Radio Relay League.)