What is the C3m vacuum tube?

The C3m is a pentode vacuum tube designed for preamp / driver applications. Predecessor to C3g. Very linear..

What is a C3m substitute or equivalent?

Direct equivalents to the C3m include C3g. These share the same pinout and operating envelope, so they can usually be swapped without circuit changes — though batch-to-batch variations between manufacturers (NOS vs current production) may shift the sonic character noticeably.

What are the key specifications of the C3m?

Amplification factor μ ≈ 19, transconductance gm ≈ 6.5 mA/V, plate resistance rp ≈ 250.0 kΩ, max plate voltage 300 V, max plate dissipation 4 W, heater 20 V / 0.125 A.

Is the C3m still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the C3m as Sought-after availability. NOS (new-old-stock) examples from RCA, Mullard, Tung-Sol, GE, Sylvania and Telefunken trade among collectors; modern production where it exists comes mostly from JJ Electronic, Electro-Harmonix, Tung-Sol (New Sensor), Genalex, and a handful of small-batch makers.

What does the C3m sound like?

Predecessor to C3g. Very linear.. Sonic character also depends heavily on the circuit operating point, the surrounding tubes in the stage, and the output transformer when present — use the interactive loadline calculator on this page to explore the trade-offs.

By Jean-Michel Falligan Devergne · Reviewed May 29, 2026

←C3m

← C3g CK5654 →Compare → C3g

C3m Pentode Vacuum Tube

Pentode20V / 0.125A8-pin Loctal

SourceSiemens·TelefunkenReviewed2026-05-29StatusDataModelCalcModelunder reviewPDF

Small-signal pentode for low-noise voltage amplification

Datasheet ↗

C3m — Siemens·Telefunken — pentode vacuum tube — (1950)

250.0kΩ

6.5mA/V

8.5pF

0.0pF

6.00pF

GAIN19%

POWER10%

DRIVE0%

LINEARITY91%

SPECIFICATIONS

Heater20 V · 0.125 A

Max Plate Voltage300 V

Pa max4 W

Gain Factor19

gm6.5 mA/V

rp250.0 kΩ

Inter-electrode capacitances

Cgk8.5 pF

Cgp0.018 pF

Cpk6 pF

Pinout8-pin Loctal

Era1950

RaritySought-after

HEATER20V / 0.125A

MAX PLATE300V / 4W

BASEspecial-8

Source datasheets

3 manufacturer datasheets cross-checked for C3m

3 verified

EXECUTIVE SUMMARY

The C3m is a indirectly-heated pentode from Siemens·Telefunken, introduced around 1950. It provides a gain factor (μ) of 19 with 250.0kΩ plate resistance. Commonly used in preamp / driver applications. Predecessor to C3g. Very linear.

APPLICATIONS

Voltage amplifier

Generic pentode preamp / driver stage

VS C3g

μ -54%Rp -17%Pd +14%Full comparison →

HISTORY

Introduced1950ManufacturerSiemens·Telefunken

Golden Age

C3m Datasheet

Local mirror · PDF

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METRIX TESTER

Test this tube on a Metrix

SPICE model not yet available. Loadline calculator requires calibrated Koren parameters.

SIMILAR PENTODES

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