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B438 tube: datasheets and technical data
Explore available pinout, electrical data and related types for the B438 vacuum tube.
The Philips Miniwatt B438 is a triode for detection and resistance-coupled low-frequency amplification. The Philips table explicitly names audion and resistance-coupled amplifier service, more precisely than the generic RF category in some secondary descriptions. Our historical sources place its appearance in 1927. Directly heated filament: 4 V, 0.1 A; the FP summary specifies parallel heating on AC or DC.
The primary table gives maximum anode voltage 200 V, amplification factor 38, maximum transconductance 2 mA/V and anode-grid capacitance 4 µµF, equivalent to 4 pF. “Maximum” matters: this transconductance does not automatically describe every low-current operating point. With large anode resistors, supply voltage, resistor voltage drop and actual anode voltage must be distinguished.
Philips gives two cases. With external resistance 0.3 MΩ: normal anode current 0.2 mA, bias about −2.5 V, normal internal resistance 170 kΩ. With external resistance 1 MΩ: current 0.05 mA, the same approximate bias, internal resistance 400 kΩ. The FP summary of 26 August 1999 instead groups them under Vb = 200 V, with first resistor 320 kΩ and second 1,000 kΩ. Its 320 kΩ remains distinct from Philips’ 300 kΩ; Vb is not silently turned into anode voltage after the resistor drop.
Philips curves show anode current versus grid voltage at anode voltages 100, 150 and 200 V. They supplement detection/amplification data without defining output power or distortion by themselves. Dimensions also differ: the original specifies maximum overall length 78 mm and diameter 38 mm; FP simply states “Dimensions 83 × 42 mm”. The difference is retained, registering the explicit Philips maxima without assuming certain axes for secondary measurements.
Philips references base A35 and diagram S.I. FP draws a four-contact Base A: 1 anode, 2 f2, 3 f1, 4 grid. Viewing direction is not explicitly stated and must be checked against the drawing before real wiring. The documents identify B438 as a small-signal, resistance-coupled valve without assigning power-output performance. Documentation: Philips Miniwatt B438, trilingual table and curves, two undated pages; B438 Philips, FP technical summary, one page, 26 August 1999.
Pinout and connections
Base: British 4-pin (B4)
Electrical data
Heater
- Heater voltage Vh
- 4 V
- Heater current Ih
- 0.1 A
Other available variants
Datasheet PDF
EL84 ECC83 12AX7 EL34 6L6GC 300B KT88 GZ34
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