Zenith Model 4-V-31 Tombstone Farm Radio (1936)
Zenith's model 4-V-31 (4V31) tombstone-style radio, intro-
duced in the summer of 1935 at $39.95, was designed to
operate from a 6V lead-acid battery. Many rural locations
at the time, especially farms, were not connected to the
electricity supply and as a result there was quite a niche
market for so-called "farm" radios. The 4-tube 4-V-31 cov-
ers standard broadcast from 500-1800kc. The tube line-
up is 15 (LO/ mixer), 15 (IF), 75 (2nd detector/AGC/1st
AF) & 38 (AF O/P).

At the hart of this set's power-supply is a vibrator, serving
to convert 6V DC from the battery to AC, whereupon it is
stepped-up in voltage and converted back to DC, using
the same vibrator, and smoothed to provide the B+ supp-
ly. The filaments in this set are supplied directly from the
6V battery.

Zenith and Farm Radios

In the early days, prior to the widespread electrification of
homes, tube radios used expensive and bulky A, B and C
batteries, some of which needed to be constantly re-char-
ed or replaced. The frequent handling of these batteries
meant that acid spills were common, ruining furnishings
and floorings.  As a result, many radios ended up largely
unused.  Something had to be done!

By the mid twenties, with the advent of more widespread
electrification, the first solution to this problem arrived in
the form of battery eliminator units. These were separate
from the radio and designed to plug into light sockets. A
great improvement! By the time 1927 came around, mill-
ions of such units had been sold. However, at that time
tubes designed for AC filament operation appeared and
radios having built-in power supplies, designed to be plug-
ged directly into a light socket, became available. Over-
night, the old battery radios, batteries and battery elimin-
ators were rendered almost (but not quite) obsolete. Deal-
ers and manufacturers arranged "radio burning" parties in
public places, encouraging the public to dispose of their
old radios and replace them with the latest socket-power-
ed models.

This was all possible except in remote rural areas, which
because of  the high cost of stringing power lines were the
last to be connected to the power grids. As a result, batt-
ery sets continued to be used in these areas, even as late
as 1950. However, largely because of the entrepreneur-
ship of the Zenith Corporation, all was not as bad in these
communities as it might first appear.

Sometime in the early 30s, the vibrator or chopper tube
was invented, probably driven by the burgeoning demand
for automobile radios. This was a mechanical device that
would rapidly reverse the polarity of a DC battery and gen-
erate an alternating voltage that could be applied to a
transformer, which in conjunction with a rectifier (a tube or
as for the 4-V-31, a second vibrator, operating synchron-
ously with the first) produced all the internal voltages req-
uired by a radio. This meant that a single low voltage batt-
ery was all that was needed to operate a  set. Even though
a beefy automobile battery was used, it would neverthe-
less need periodic re-charging, usually necessitating a
trip to the local auto shop (where there was power). This
whole process could sometimes take several days and
since the batteries were expensive, back-ups were often
unavailable. Soon, however, small wind-driven electric
generators began to be used to re-charge 6V batteries,
such as used to power the 4-V-31, when the radio (or oth-
er device) was not in use. Commander McDonald of Zen-
ith was early in recognizing the economics of these "Win-
chargers" and bought a majority share in a small company
producing them. He introduced mass-production techni-
ques and clever marketing, offering discounts on "Zenith
Winchargers" with every Zenith farm radio bought (& even
with radios of other manufacturers). Soon thousands of
the small windmills were being sold a month at prices as
low as $10 - $20, establishing Zenith as the radio of choice
for rural America.
Copyright TubeRadioLand.com
Click any thumbnail to enlarge
Ad clipping (both appeared together) from West Virginia, Nov
1934. Note that the 4-V-31 price had increased from the $39.95
it was when first introduced back in the early summer.
Zenith 4-V-31 Farm Radio (1935)