Originally all tubes had direct heaters and ran from battery, usually the Lead Acid accumulator. These were not really meant to be portable sets.
Development was from Bright Emitter, Thorated Tungsten and finally Oxide coatings to improve efficiency. We had introduction of indirect heaters for mains sets quickly standardising on 6.3V with some 12.6V. But at higher power levels some tubes in mains sets used direct heaters still. These would not be the sort of valves (tubes) used in Portable sets, and not usually in “Farm Radio”.
Naturally filaments or Radio sets designed to use nominal 2V, 4V, 6V or 12V Lead Acid supplies. Actually a fully charged Lead Acid cell is a little over 2.2V, hence 6.3V rather than 6V chose, Cars often in the past had 6V (3 cell) rather than 12V (6 cell) batteries. Modern Lead Acid has Calcium and rises sharply to 14.2V rather than 13.6V so as to cut off charging and loss of water by electrolysis, usually called Zero Maintenance cells.
Even in cities in later 1930s many people might not have had electricity or for various reasons had a Battery set.
My Father remembers prior to 1939 the battery being taken to the Cycle shop on the Lisburn Rd. in Belfast city for charging. Belfast certainly had electricity before the 1930s. On the left is a modern Lead Acid cell of the style used in many “wet” battery sets.
But RCA and Sylvania developed 2V 100mA and even some 50mA filament octal tubes These could feasibly run from dry cells (Carbon Zinc batteries) as used in ordinary flashlights. Even in 1930s a single D cell may have been about 5000mAH to 7000mAH and an F cell perhaps as much as 10,000mAH. So with 4 tubes at 100mA using 2 x D cells in parallel you get 30 hours listening. The F cells still exist inside the 6V spring top lantern batteries. Zinc Carbon about 10,500mA and Alkaline about 26,000.
Duracell marketing may suggest Alkaline last x5 longer, but that is only true under certain specialised higher drain conditions were an NiMH is better. At high current drain (1A on an AA pen cell) the NiCd and NiMH spectacularly beat Alkaline and Zinc Carbon. But at C cell or D Cell size with 50mA current the Zinc Carbon can beat NiMH, especially on self discharge. For Battery Radio Application the Alkaline is about x2 the life of Zinc Carbon.
The last stage of development of Octal and Loctal (or Loktal) 8 pin Battery Radio tubes was in 1938 to 1938 with the 50mA filaments for all except the Audio output beam Tetrode. The 100mA and 50mA tubes even though octal made not just “Farm Radio” sets usable on Dry cells, but resulted in more models designed to be used in a more portable fashion.
The RCA B7G series (R&D in 1938 and 1939) of 1940 was thus the advanced high efficiency filaments of the latest octal tubes coupled with the improved performance and lower manufacturing cost (and more automated) “button” base with pins. Sylvania was perhaps first to market with a “button” base on the Loctal-8 series vs the poorer performance and more costly “pinch” base of the octal tube. However not all Octal are “Pinch” base, some are “Button” base.
The Sylvania Octal of 1938
From Electronics 1938 (Supplied by Emilio Ciardiello)
These are all 50mA except for the 1C5G which is 100mA, The audio output power of the 1A5G can’t be very high as it’s only 50mA filament. The 1A7G — 1N3G — 1H5G — 1C5G line up would take exactly the same LT voltage and Current as the RCA B7G 1R5 – 1T4 – 1S5 – 1S4 used in the Personal BP10
The UK set makers from 1940 used the Osram (Marconi /MOV?) X14 Z14 HD14 N14 (or N15 / N16 ) Battery Tube line up which appears to be the same as the Sylvania 1A7G 1N5G 1H5G 1C5G (or 3Q5) line up used in USA models. The “European” number version line up (but only DL33 is listed as equivalent to 3Q5) is probably DK32 DF33 DAC32 DL33 perhaps from 1939 to 1949 (All 250ma 1.5V parallel or 50mA 7.5V series octal base). The 3Q5 output (N15/N16) or 3Q5G output beam tetrode is the 50mA 3V series Octal for 7.5V “Dry Cell” based battery (5 x D Cells would give 75 hours operation). The 1C5G (N14) is 1.4V 100mA parallel operation only.
Essentially the WWII delayed the widespread use of the miniature B7G tubes in Europe for almost 7 years.
Another aspect of B7G development was the more accurate diamond die drawing of tungsten wire. For series operation the filaments have to be better matched. Even so the Audio output has a higher cathode current and so typically it is the first in the series chain (allowing also for more negative grid bias) and the other series filaments typically have a capacitor to decouple the output audio and a parallel resistor to match the output beam Tetrode “cathode” bias (DC) current.
As well as Sylvania in USA, we have before WWII Philips in Europe developing “glass button” base, the most famous is called the Valve that won the War. The EF50 is a 9 pin Loctal “glass button” base. No pinch. Philips also developed the Rimlock with production shortly after WWII, it’s mostly Mains valves (tubes) very similar miniature Button base versions of the Philips octal and slightly earlier European Edge connector tubes. It’s not long after in the early 1950s that we get the 9 pin “button base” miniature tubes (Noval), like the Rimlock, mostly indirect mains application 6.3V heaters. Some European sets manage to have Rimlock and Noval tubes. Of course in USA having “mixed” tube bases in a radio set was more common.
Even after WWII 1945 in Europe we see some octal based “portable” sets, not just Farm Radio. During WWII in Europe most Portable or battery sets used the Octal tubes. The RCA B7G used in some receivers dropped by air for clandestine listing, The Norwegian designed “Sweetheart” using 3 x RCA 1T4 ( Miniature Receiver Type 31/1 from 1943) and the Polish designed Miniature Receiver OP-3 Type 30/1 which looks similar but is a dual band superhet based on RCA BP10 type of architecture.