C3G

The C3g is a professional high-transconductance pentode for wide-band amplifiers. Siemens specifies telecommunications, RF and IF uses, emphasizing low noise, long life, reliability and tight tolerances. Its oxide cathode is indirectly heated; Siemens and Telefunken specify parallel AC or DC heating at 6.3 V, 370 ±20 mA. Keeping voltage within ±5% is an express condition of the life guarantee, not a minor supply detail.

As a pentode, the Siemens and Telefunken characteristic point uses plate 220 V, screen 150 V, suppressor zero and cathode resistor 115 Ω. Central values are plate current 13 mA, screen current 3.3 mA, transconductance 14 mA/V and internal resistance 300 kΩ. Siemens also gives spreads: plate current 10–16 mA and transconductance 12–16.3 mA/V. Nominal current must not be taken from an extreme column. Equivalent noise resistance is 650 Ω in those two files; the earlier Lorenz sheet dated 1 June 1955 instead gives less than 600 Ω, retained as that source’s figure.

Triode connection is explicitly documented: g2 joins the plate and g3 the cathode. At 200 V with cathode resistor 180 Ω the figures are current 17 mA, transconductance 17 mA/V, amplification factor 40 and internal resistance 2.3 kΩ. Siemens and Telefunken give equivalent noise resistance 200 Ω, Lorenz less than 200 Ω. The triode curves describe the sum of plate and screen currents; they must not be confused with plate-only pentode curves.

Small power-amplifier service is not merely an extrapolation: Siemens and Telefunken provide a single-tube class A example. With plate 220 V, screen 150 V, g3 zero, load 15 kΩ and cathode resistor 115 Ω, drive 0.85 V RMS produces 1.2 W at 10% distortion. Plate current remains listed as 13 mA, while screen current rises from 3.3 mA at rest to 4.7 mA driven. The 3.5 W maximum rating is plate dissipation, not power delivered to the load.

Reliability data show the concern for prolonged service. Telefunken’s explicitly tentative sheet guarantees 10,000 hours averaged over one hundred tubes, not identical life for each specimen. Its failure factor p is about 1.5 per thousand per 1,000 hours. The special cathode is described as avoiding an interface layer even during long heated periods without plate current. End-of-life criteria are also defined, such as current reduced to 8.3 mA, transconductance to 9.8 mA/V or negative grid current increased to 1 µA.

Siemens and Telefunken limits include operating voltage 220 V for plate, g2 and g3, distinct from cold 550 V; plate dissipation 3.5 W, g2 and g3 dissipation 0.7 W each, cathode current 30 mA, grid leak 0.5 MΩ, heater-cathode voltage 120 V and maximum envelope temperature 120 °C. Lorenz 1955 instead specifies cathode current 25 mA; the tables are not merged into a universal limit. The 100 MHz figure is also a conditional measurement: Siemens and Telefunken list input resistance 2 kΩ with both cathode connections joined, not a guaranteed maximum frequency.

The base is continental keyed, called Loctal by Telefunken. Its numbered connections are heater 1 and 8, g3 and screen 2, plate 3, g2 4, cathode 5 and 7, g1 6; central contact m is shown separately. Our archive also identifies earlier versions under the same designation with different connections: check the specimen against its own sheet rather than relying solely on the name C3g. Available photographs show a screened Siemens specimen numbered 091197; the retained description connects the outer screen to the central spigot.

Dimensions are recorded separately by maker: maximum diameter 32 mm and body above the pin plane maximum 55 mm. Lorenz also specifies maximum overall 69 mm; Telefunken and Siemens separately dimension the lower keyed projection up to 14.4 mm. No new overall length is derived by adding maximum part dimensions. Siemens and Telefunken cold capacitances include input 9.5 ±1 pF and output 3.5 ±0.5 pF; effective input at cathode current 16.3 mA is about 13.8 pF. Cold capacitance alone therefore does not describe dynamic behaviour. Documentation: Lorenz, C3g, seven pages, 1 June 1955; Siemens, C3g, RÖK 3222, thirteen pages, 1 January 1960; Telefunken, C3g, tentative data and curves, twelve pages, pp. 65–76.