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Most present day owners of the popular Radiola III or IIIA assume that the only "correct" type of tube to be used is the WD- 11. Not necessarily so! In 1925, the sales department of RCA issued a bulletin describing how the efficiency of the Radiola III family could be improved by the use of the Radiotron UX-120. The bulletin detailed how the receiver could be modified to accommodate this newly-introduced output amplifier tube.
The III/IIIA were the first receivers to use the bakelite version of the WD-11, the dry cell tube developed by Westinghouse. The earlier brass-based WD-11 had been first used in the Aeriola Senior receiver, introduced in 1922. The tube's oxide-coated platinum filament was rated at .25 amperes at 1.1 volts, and was intended for use primarily as a detector.
It was for this reason that the RCA Sales Department issued Bulletin No. 6 dated December 11, 1925 -- barely a year after the introduction of the Radiola III. Owners of these sets could upgrade the performance of a receiver
which otherwise would soon be rendered rather obsolete, with
the superior performing sets appearing on the market. Dealers,
likewise, could use the bulletin as a sales tool for disposing
of unsold Radiola III/IIIA inventory, The eight page bulletin offered two methods of modifying the sets. Method 1 (Figure 1) required no internal wiring changes, but rather the addition of a 6-ohm rheostat in the filament circuit. Battery setting controls, on the set, were already kept fully 'on' and the filament voltage was subsequently controlled with the external rheostat. The detector stage used a UX-I99 which has the same 3.3 volt filament rating as the UX120. Note the higher battery voltages required for the filament and plate supply. Method II (Figure 2) involved
two internal wiring changes in the Radiola III sets likewise could be modified in either of two ways. The first method (Figure 3) used two UX-199s and was recommended for reception of signals from distant stations, where higher voltage amplification was desired. In stronger signal areas an alternate method (Figure 4) used the UX-120 in combination with a UX199, which boosted loudspeaker output. The original +40 V amplifier plate supply has been increased to +135 V along with an increased "C' supply to properly bias the UX-120 tube. These factory-recommended changes may easily account for Radiolas that occasionally turn up with adapter sockets and UX-I99 and UX-120 tubes instead of WD-11s. Instead of promoting obsolescence, RCA, with its bulletin, actually extended the useful life of a popular receiver.  BACK TO ARCHIVES |
Radiola
III/IIIA Modification
Sales
of the Radiola III began in mid 1924. This widely advertised
set was no doubt one of the most popular of the Radiola series.
Its affordable price of $24.50 brought radio into the homes of
thousands of families. An optional balanced amplifier was available
if the owner wished to obtain loudspeaker volume. Also marketed
at the same time was the Radiola IIIA, which combined the III
and balanced amplifier in one unit.
Not
long after the introduction of the Radiola III an increased emphasis
was being placed on higher audio output power from receivers.
To satisfy this need, General Electric developed the UX-120,
intended as a final audio stage for sets equipped with UV-199/UX-I99
dry cell tubes, also produced by General Electric. The Radiola
25 and 28 were among the first sets to use the UX-120. The improved
output power was substantial.
which was still considered a most
economical set.
set.
As originally wired, filament control of the balanced amplifier
tubes and detector/first audio were independent of one another.
The change placed both rheostats in series with all four paralleled
tube filaments. Voltage on all tubes was controlled with either
rheostat.