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.

C3m Pentode Vacuum Tube

Also known as C3g

Small-signal pentode for low-noise voltage amplification

PentodeHeater 20 V · 0.125 A8-pin LoctalSiemens·Telefunken 1950

Plate dissipation: 4W
Max plate voltage: 300V
Amplification factor: 19
Transconductance: 6.5mA/V
Plate resistance: 250kΩ

C3m

Characteristics

Datasheet values, once each. Every value cites its sheet.

HeaterIndirectly heated

Specifications of C3m — Heater
Heater voltageVh	20 V
Heater currentIh	0.125 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of C3m — Limiting values
Plate voltageVa max	300 V
Screen voltageVg2 max	60 V
Plate dissipationPa max	4 W
Cathode currentIk max	30 mA

Mechanical

Specifications of C3m — Mechanical
Base	8-pin Loctal
Rarity	Sought-after
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of C3m — Small signal
Transconductance	6.5 mA/V
Plate resistance	250 kΩ
Amplification factor	19

Capacitances

Specifications of C3m — Capacitances
Input	8.5 pF
Grid–plate	0.018 pF
Output	6 pF

Charactereditorial reading, not a measurement

Specifications of C3m — Character
Gain	19%
Power	10%
Drive	0%
Linearity	91%

datasheet cited · model or editorial reading · catalogue value, no citation attached yet · nothing invented

Using this tube

Interactive loadline calculator not available for this tube yet — it needs calibrated Koren or Ayumi parameters.

Applications

Voltage amplifier

Generic pentode preamp / driver stage

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026

Manufacturer datasheets

3 manufacturer datasheets cross-checked for C3m3 verified

C3m DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: Siemens·Telefunken
Reviewed: 2026-05-29
Status: DataModelCalc
Model: under review

Context

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.

History

Introduced: 1950
Manufacturer: Siemens·Telefunken
Era: Golden Age

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