Common Miniature 7pin (B7G) Battery tubes (valves)
Common Miniature 7pin (B7G) Battery tubes (valves)
Introduction
The RCA BP10 was one the first of the B7G based Miniature valve radio sets, called a “Personal Radio” along with the Sonora Candid (which may have the first). So compact for 1941 compared to the "boxes" in UK of 1950s. Over 250,000 made and sold before Pearl Harbour. Though was the Marconiphone P17b (1947) a design via HMV / RCA connection? It's a very similar outer shape though different chassis layout and single B114 combo 69V/1.5V battery pack (later called a 70V/1.5V). One version has used Marconi (MOV) X17 W17 ZD17 N17 and other DK91, DF91, DAF91 and DL92 (both essentially the RCA 1940 1R5 1T4 1S5 1S4 series used in the BP10).
Marconi appears to have made the Ever Ready Personal B (1947), or perhaps supplied the chassis with DK91, DF91, DAF91 and DL92 and box fitted by Ever Ready as the internal front panel is different. The 3S4 is a centre tapped 1S4, thus the later European equivalent is the DL92.
There are Mazda/Ediswan (AEI), RCA/USA, Mullard/Philips, Marconi/Osram (MOV) and Japanese part numbers. Five main schemes. The Russian part numbers are in Cyrillic and are not exact equivalents (1.2V @ 60mA vs 1.4V @ 50mA). The Russian Miniature Rod Pentodes are a completely different family, in this article we only consider the Domestic B7G tubes. Some of the Ediswan/Mazda numbers are the same as completely unrelated octal valves (tubes) or Ballast tubes.
There are two main kinds of 7 pin miniature Western Battery valves:
1) The RCA series released in 1940 (1R5 – 1T4 – 1S5 – 1S4 or 3S4 )
These are 50mA 1.4V filaments except the O/P which is 100mA, or a version which is 50mA + 50ma Centre tap, so 3V 50mA series and 1.5V 100mA parallel. The Mazda and Philips/Mullard are licensed copies, hence identical with a different part number. In parallel the 4 tube set is thus 250mA @ 1.4V (1.5V battery). In series it's 7.5V @ 50mA
2) The Philips Series released 1953 (DK96 – DF96 – DAF96 – DL96 )
These are 25mA 1.4V filaments except the O/P which is 50mA, or 25mA + 25ma Centre tap, so 3V 25mA series and 1.5V 50mA parallel. In parallel the 4 tube set is thus 125mA @ 1.4V (1.5V battery). In series it's 7.5V @ 25mA
Hence both systems are 7.5V ( 1.5V for 3 tubes and 3V for one tube) when used in series. If the LT 7.5V is from a single HT rectifier (usually about 105V on Mains, 90V is only a "nominal" HT volts), then there are a couple of large dropper resistors picked to drop 97.5V assuming a 105V HT to give 7.5V. This is often so a single tube (some use EZ41) is used as rectifier for LT & HT. Later models used two metal rectifiers, one for HT and one for LT so a bit "safer" for the filaments.
The Russians and East Europeans did variations with 1.2V filaments and higher filament current.
The Basic AM radio architecture is 1R5 Heptode Frequency changer: g1 & g2 form the Local Oscillator and the tuned RF loop aerial is fed in one of the later grids , the latter part of Heptode is a Mixer and the Anode is tuned to the IF frequency. The 1T4 is used as an IF amp. The 1S5 is a diode and Pentode sharing a cathode/filament. The diode detects the AM and DC level is filtered for AVC/AGC. The AC (audio) is amplified in the Pentode. Finally a Beam Tetrode (1S3 or 3S4 only differ in filament) is the audio output. It thus needs twice the filament current. Virtually every AM valve portable using B7G tubes from 1940 to 1960 uses the same basic circuit. The innovations were to switch to Ferrite rod instead of loop (ironically inferior to larger loops on Briefcase sized sets), the 125mA tubes from Philips after 1953 instead of RCA 250mA for radio set and finally adding VHF-FM, rechargeable LT battery and even switching to a Mains type audio out tube when on the AC power.
1st series 1940, RCA 1R5 – 1T4 – 1S5 – 1S4 or 3S4
IR5 Heptode Frequency Changer DK91, 1R5, VT-171, X17, 1C1, 1A2_1R5
DK92 (Improved 1R5) 1AC6, X18, X20, 1C2 (Improved 1C1) (1951)
1T4 RF/IF Amp DF91, 1T4, W17, 1K2, 1F3
1S5 Detector Diode + AF Preamp DAF91, 1S5, ZD17, 1B2_1S5, 1FD9
1S4 (100mA) Output Beam Tetrode DL91, 1S4 1947, 270mW
3S4 (50mA + 50mA Centre tap version of 1S4) DL92, 3S4, N17, 2P2, 1P10, CV820, VT-174 1947, 270mW
DL93, 3A4, 2P3, CV807 (related is DL191, DL193) 1950, 600mW
DL94, 3V4, N19, 1P11, CV1633 (DL101) 1950, 340mW
DL95, 3Q4, N18, CV818 (similar but different pinout to DL94) 1950, 340mW
Japanese: 1A2-1R5, 1K2, 1B2-1S5, 2P2
There are other “replaceable” equivalents that may vary in Filament rating (Russian (1.2V, 60ma) 1a1p is similar DK91) or pin connections. The listed types are believed to be identical licensed designs from RCA.
Lists order:
Heptode Frequency Changer – RF Pentode (IF) – Detector+ AF Amp – O/P Beam Tetrode
• RF 1940 Original RCA line up 1R5 – 1T4 – 1S5 – 1S4
and for series or parallel 1R5 – 1T4 – 1S5 – 3S4
• 1947 Philips version DK91 – DF91 – DAF91 – DL92
• 1951 Philips version DK92 – DF91 – DAF91 – DL94
• 1947 Ediswan/Mazda 1C1 – 1F3 – 1FD9 – 1P10
• 1951 Ediswan/Mazda 1C2 – 1F3 – 1FD9 – 1P11
• 1947 Marconi X17 – W17 – ZD17 – N17
• 1950 Marconi X17 – W17 – ZD17 – N18
• Emerson 1U6 1AF4 1AF5 3A4
Other Tubes
DM70 (Philips)“Magic Eye”, wire leads 25mA 1952 DM70 – 1M3
(Identical but cut for socket DM71 – 1N3 – Y25 – 1M1 1953) Frequently used in Mains Radio sets and portable Tape recorders more often than portable radio. Sometime on mains sets powered off the cathode resistor on output tube.
DC90 (Philips) VHF triode 50mA 1952 DC90
2nd series c. 1953, Philips DK96 – DF96 – DAF96 – DL96
DK96 Heptode Frequency Changer: 1AB6, X25, 1C3
DF96 RF/IF Amp: 1AJ4, 1F33, W25, 1F1
DF97 VHF/RF/IF Amp: 1AN5 (1954 VHF front end)
DAF96 Detector Diode + AF Preamp: 1AH5, 1AF33, ZD25, 1FD1
DL96 Output Beam Tetrode: 3C4, N25, 1P1 (1953) 200mW
• Philips AM Sets DK96 – DF96 – DAF96 – DL96
Early AM + VHF-FM DC90 – DF96 – DK96 – DF96 – DF96 – DAF96 – DL96
• Later AM + VHF-FM DF97 – DF96 – DK96 – DF96 – DF96 – DAF96 – DL96
• Marconi (MOV) X25 -- W25 -- ZD25 -- N25
• Ediswan/Mazda 1C3 -- 1F1 -- 1FD1 -- 1P1
By 1955 UK and other European sets very standardised on this series. US models appear to use a greater variety of tubes after the RCA 1st series. Some models (Ever Ready N3 vs Ever Ready N) are the same design as used the 1st RCA series with minor component changes. In the late 50s and early 60s there were magazine articles on how to replace the 1T4 / DF91 series with DF96 series tubes to double LT battery life. The most change being the series dropper resistors for LT on mains sets.
The post WWII Zenith Transoceanic All Band Radio
Sylviania did the 1L6 (1949) an improved version of the 1R5 Heptode for the Zenith Transoceanic. It is rare and the 1R5 may not work on the higher frequency bands. It's supposedly a miniature version of the 1LA6 Loctal-8 Heptode The later 1U6 (1955?) can be used as a substitute, but in series filament chains needs a parallel filament resistor as it is the 25mA 2nd Series Generation (but not interchangeable with DK96 as g3, g4 reversed and g5 on DK96 is to filament rather than g3 on 1U6). There is some suggestion that there are two versions of the DK92 and one of these will work in place of the 1L6 (but it also is different pin connections as it's like DK96 pins). The IL6 and 1U6 do have same connections.
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Sylvania's early introduction of the Octal-based tubes
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Pre-B7G Battery tubes (valves)
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 100mA 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.
Pinch and button bases (not to scale)
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.
I'm sure there are others better qualified than I to write accurately of pre-B7G battery tube development. But I think the above is at least the broad brush strokes of the history.
See attached RCA document on Development of B7G "button" base tubes.
Attachments:- RCA Minature Tubes (830 KB)
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