Miniature Tubes

Here is a list of the most common types with cross reference. There are other types that differ in base connections or exact filament current or exact characteristics. These are the common directly replaceable even in series filament sets.


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

DL94 = 3V4 = N19 = 1P11 = CV1633 Great power output than the DL92 (different connections) for same filament power. The 3Q4 = DL95 = N18 are electrically similar but same pin connections as DL92 (3S4 = N17 = 2P2 = 1P10 = CV820 = VT-174)

DK92 (Dec 1951) = 1AC6 = X18 = X20 = 1C2 is different connections and improved High Frequency performance to the original 1R5 (DK91), but same filament current. Basically used on most European Shortwave sets till 1960 (The USA used the 1L6).

Some UK models 1949 to 1952 changed DL92 to DL94 then  DK91 to DK92 without changing model number! Considering that neither valve is pin compatible, a bit awkward today if valve is missing!

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. or in parallel with a DAF96 on a Radio otherwise using 50mA filament valves.

DF92 (1949 also 1L4 = CV1758 = 1F2)  is a sharp cut off version of the DF91 / 1T4, was common in Military equipment. Where a radio or other amplifier needed an extra audio stage it was more common the use the DAF91 (1S4) or DAF96 (1AH5)

DC90  (Philips) VHF triode 50mA 1952  DC90

The 1U5 (1945) is similar to 1S5 (DAF91) but not pin compatible. The idea was improved diode & pentode isolation  as well as reduced microphony. There are no pin compatible alternatives. It’s used in the Zenith Transoceanic and “clones”.

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 basically  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  The DK92 and one of these will work in place of the 1L6 without rewiring despite apparent differences as the electrode arrangement isn’t the same, but he capacitance is higher which means it may not be possible to align the highest bands on the Zenith Transoceanic. The IL6 and 1U6 do have same connections, but the 1U6 if anything is rarer.

The original version of this article was published by me at the 17th February 2012. This version has some errors corrected and details added.