In 1938 Sylvania in the USA realises that the best 50mA 2V tube designs could be adapted for 1.4V 50mA. During the early 1930s many 2V Tubes had already become efficient enough to run with 100mA filaments. With 1.4V a light weight Zinc Carbon battery could be used instead of a heavy “wet” or Gel lead Acid Accumulator. For low audio power a set of four tubes would take only 200mA 1.4V LT and about 8mA 90V HT, the “All Dry” radio was born. For louder audio a 100mA filament output tube could be used. The filament drawing dies and improved processing meant the tubes could be operated in series from a nominal 7.5V battery or smoothed mains supply (about 6.8V recommended), so a version of the output tube with a centre tapped filament 50ma @ 2.8V series or 100mA @1.4V parallel. Sylvania produced these in Octal and Loctal tube formats initially with pinch bases and later button base.
RCA was a second source. These also were attractive to the military. But in 1939 RCA developed their button base b7G miniature tube and essentially miniaturised the the Sylvania Loctal design.
Sylvania All Dry Octal and Loctal
The Octal & Loctal line up was Pentagrid converter (Oscillator and Mixer), Pentode RF/IF amp, Audio triode with RF diode on single 50mA filamentary cathode and the three Beam Tetrode audio output tubes. Philips and the UK produced CT8/P8 base versions of these as DK1, DF1, DAC1 and DL2 used in the UK from 1939 in “All Dry Portables”, replaced during 1942 to 1946 with US import Octal versions due to wartime shortages. See DK1 DF1 DAC1 DL2 (Ever Ready/Lissen All Dry) vs USA 1A7 1N5 1H5 1C5 (Detrola 282) vs 1A7G 1N5G 1H5G 1C5G or DK32 DF33 DAC32 DL35 in 1946 McMichael 463 at the Radiomuseum (the USA types are variously listed as G or GT as well as no suffix, there is no metal envelope version)
RCA Miniature B7G 1940
The initial RCA B7G lineup was 1R5 Pentagrid converter, 1T4 RF variable mu Pentode, 1S5 Diode / Audio Pentode and 1S4 100mA Audio output, The 3S4 was the centre tapped filament version for 50mA 2.4V or 100mA 1.4V operation. Very many radios sets including the first “personal” models that could fit a large coat pocket. These US tubes didn’t appear in UK models till 1946 (“Romac Personal Portable”) and European/Philips part numbers 1946/1947 as 1R5 = DK91, 1T4 = DF91, 1S5 = DAF91 and 1S4 = DL91 with 3S4 as DL92. See 1R5 1T4 1S5 1S4 in 1940 RCA BP10 Personal vs DK91 DF91 DAF91 DL92 in 1947 UK Ever Ready Personal B. Though RCA developed the tubes the Sonora Candid appears to have been the first “Personal” set to market in 1940.
For some people the Second World War can be said to have started in 1934. The UK entered in 3rd September 1939 and the USA in 7th December 1941, so there were considerable sales in the USA of the Miniature tube based All Dry Portables. The valve family saw extensive military use from 1940 to the early 1960s. During the War Years in UK Domestic battery set production quickly switched from CT8/P8 edge base and Mazda Octal to IO8 (International Octal, mostly US imports). The miniature tubes were only used for the covert receivers for Polish and Norwegians made in the UK.
Telefunken took a slightly different approach. Instead of designing for series use (6.8V or 7.2V nominal) from mains supply making all filaments 50mA as well as parallel battery use. DEAC had developed the NiCd battery which not only being rechargeable, also acted as a voltage regulator on an LT supply solving the issue of low voltage supply. The 50mA series types allowed simply a large resistor from the 90V supply during mains operation. So each Y8 low profile metal case tube used different currents.
DCH11 1.2V @ 75mA! (But a Triode Hexode rather than Pentagrid or Octode)
DAF11 50mA Diode Pentode
DC11 20mA Triode
DDD11 100mA Dual triode for push pull Audio.
DF11 25mA RF Pentode
DL11 50ma Audio output
In the late 1940s and early 1950s Telefunken added parts to have an entire 50mA series, but the Radio manufacturers had changed to USA / Philips B7G all glass types. In 1953 Philips introduced the 25mA series family thus a basic superhet was then only 125mA 1.5V battery rather than 250mA. But after 1945 most of the German & Philips Battery Portables had mains option on the B7G types by using the DEAC (NiCd) for LT on battery and LT regulator on Mains.