What is a 6C33C(2 cath) substitute or equivalent?

Direct equivalents to the 6C33C(2 cath) include 6C18S, 6S33S-V (EW). 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 6C33C(2 cath)?

Amplification factor μ ≈ 2.7, transconductance gm ≈ 39 mA/V, plate resistance rp ≈ 69 Ω, max plate voltage 250 V, max plate dissipation 60 W, heater 6.3 V / 6.4 A.

Is the 6C33C(2 cath) still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the 6C33C(2 cath) as Moderate 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.

6C33C(2 cath) datasheet — pinout & specs

Q: What is the 6C33C(2 cath) vacuum tube?

The 6C33C(2 cath) (6С33С — 6C33C-B — 6S33S-V) is a triode vacuum tube designed for otl / series regulator applications. Soviet power triode designed as a series-pass regulator (тube ballast) — two internal triode systems with paralleled anodes/grids/cathodes but separate filaments. Very low μ ≈ 2.7, very low Ri ≈ 80-120 Ω, transconductance gm ≈ 40 mA/V at 550 mA: the rare combination of low gain and high current that the OTL crowd (Atma-Sphere, Graaf, BAT, Lamm) bent the tube toward in the late 1980s. Built like a tank (200 g, internal bracing, oversized anode pin) with documented vibration/shock specs (6 g / 150-500 g for the -B/-V variant). Famously slow to reach thermal equilibrium (≥ 600 s) and prone to runaway if biasing isn't actively managed..

Characteristics

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

HeaterIndirectly heated

Specifications of 6C33C(2 cath) — Heater
Heater voltageVh	6.3 V
Heater currentIh	6.4 A
Series wiring	12.6 V · 3.3 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of 6C33C(2 cath) — Limiting values
Plate voltageVa max	250 V
Plate dissipationPa max	60 W

Mechanical

Specifications of 6C33C(2 cath) — Mechanical
Base	7-pin Soviet septar (РШ-3) — oversized anode pin 4
Rarity	Moderate
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of 6C33C(2 cath) — Small signal
Transconductance	39 mA/V
Plate resistance	69 Ω
Amplification factor	2.7

Capacitances

Specifications of 6C33C(2 cath) — Capacitances
Grid–cathode	30 pF
Grid–plate	31 pF
Plate–cathode	10.5 pF

Charactereditorial reading, not a measurement

Specifications of 6C33C(2 cath) — Character
Gain	3%
Power	100%
Drive	100%
Linearity	95%

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

Using this tube

Applications

SET output
Push-pull output
Headphone output

Generic triode otl / series regulator stage

Headphone ampPA/public address

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Source : Boele Rev 1.1 (2005) · Penta Labs · Original Soviet datasheetActive model : Norman Koren

Manufacturer datasheets

6C33C(2 cath) Datasheet3 variants · pick below

Manufacturer datasheets (PDF downloads)

Michael Boele (compilation of Soviet documentation)

Rev 1.1, 2005-02-03 · 7 pp · EN

PRIMARY consolidated source. Seven pages covering nomenclature, mechanical data with Septar 7 pinout, filament options, typical operating point, full small-signal parameters with capacitances, service-life criteria, absolute maxima, environmental specs, extended-life de-rating tables, and the parallel-operation de-rating table for multi-tube banks. Defines the modern -B / -V notation (Russian: 6C33C-B / Western: 6S33S-V) and the high-reliability -EB / -EV variant (3000 h).

Penta Laboratories, Inc., Chatsworth CA

Penta Laboratories commercial sheet (undated) · 2 pp · EN

Western re-issue of the Soviet datasheet for the US replacement-tube market. Carries the canonical Class-A push-pull audio operating point used by Atma-Sphere / Graaf / BAT OTL designers: Va = 200 V, Vg = −65 V, Ia = 480 mA, Raa = 800 Ω, Wo = 55 W for two tubes. Also publishes Ia/Va curves at Ef = 6.3 V from Ec1 = 0 down to −100 V in 10 V steps (Ua 0–200 V, Ia 0–1000 mA), and announces Va max 400 V (slightly more conservative than the Soviet 450 V / Pa ≤ 30 W ceiling).

Soviet electron-tube reference book (publisher TBD)

Original Soviet справочник, p. 363–364 (translated) · 2 pp · RU

Original-source Russian datasheet for the base 6С33С (pre-B variant) covering: nominal Ia = 540 ± 90 mA, gm = 39 ± 11 mS, Ri ≤ 130 Ω, Cfk ≤ 70 pF, end-of-life criteria after 1000 h (Ia ≥ 340 mA / Igrid ≥ 15 µA), and the less stringent environmental envelope of the base type (vibration 4 g, shock 35 g, ambient −60…+70 °C). The -B / -V uplift figures (6 g vibration, 100–500 g shocks, +100 °C ambient) come from Boele's compilation, not this card.

Datasheet limits (full)

Va0 (cut-off plate V)

600 V

max, valve biased to cut-off

Va max

250 V

Pa max

60 W

Ik max

600 mA

Rg max

0.2 MΩ

Vhk max

300 V (f−/k+) · 300 V (f+/k−)

Bulb temp

260 °C

Va_max 250 V applies when plate dissipation exceeds 30 W; relaxes to 450 V when Pa ≤ 30 W, and to 600 V cold (no plate current at power-up). Penta Labs prints a more conservative 400 V ceiling. Wa_max 60 W is for both internal triodes simultaneously (45 W if only one section is energized — single-filament operation). Ik_max 600 mA for both / 350 mA for one. Negative grid voltage range −0.5 V to −150 V; Rg max 200 kΩ for fixed bias, relaxes to 1.5 MΩ in active feedback regulator loops where the grid leak also serves as the plate load of the control amplifier. Bulb temperature: 260 °C at ϑamb = 20 °C, 300 °C at 100 °C (max 100 h), 320 °C at 150 °C (max 2 h). Filament voltage tolerance: 11.3–13.9 V series or 5.7–6.9 V parallel.

Per-section / per-manufacturer capacitances

Capacitance	boele_b_variant	penta_labs	soviet_original_legacy
Cin	30 pF	30 pF	30 pF
Cout	10.5 pF	10.5 pF	10.5 pF
Cga	31 pF	31 pF	31 pF
Cfk	60 pF	—	70 pF

SPICE variants per manufacturer

Each card below is a named subcircuit inside the downloadable .subckt file. Same physical tube, different manufacturer / measurement convention. The Ampera loadline calculator uses the Philips/1953 family; copy the Koren line from any card into your own simulator to bias the engine the same way.

6C33C_AMPERA_KOREN_FIT

Ampera engine — Koren synthesized fit

Va120 V

Vg0 V

Ia550 mA

μ3.27

gm40 mA/V

rp0.1 kΩ

Cag31 pF

Koren fit

MU=3.27 EX=1.397 KG1=150.9 KP=13.94 KVB=300

Default Ampera load-line and load-curve calculator fit, with mu pushed slightly above the 2.7 datasheet value to match the published Ia/Va curves over the full plate-dissipation envelope. Use this variant for in-page calculations.

6C33C_BOELE_KOREN_FIT

Boele / Penta — Koren-like fit (datasheet-matched mu)

Va120 V

Vg0 V

Ia550 mA

μ2.67

gm40 mA/V

rp0.1 kΩ

Cag31 pF

Koren fit

MU=2.67 EX=1.45 KG1=418 KP=14.6 KVB=5

Lower-mu fit that hews exactly to the datasheet typical value µ = 2.7. Better matches the steep, low-Va part of the family (the actual OTL working zone) at the cost of a slight roll-off near Pa_max. Available as the public SUBCKT '6C33C' shipped with Ampera.

Transfer curve comparison

Ia vs Vg at Va = 250 V — each manufacturer Koren fit overlaid. The de-rated French families (μ=30) read lower than the Philips/1966 high-gm family (μ=32) at the same grid voltage.

Ampera engine — Koren synthesized fitBoele / Penta — Koren-like fit (datasheet-matched mu)

Original descriptive texts

Verbatim excerpts from the four manufacturer datasheets. Expand each section to read the original text — the Philips technical article carries the richest application content.

SPICE models

Revision and verification status

Source: Boele Rev 1.1 (2005) · Penta Labs · Original Soviet datasheet
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Norman Koren

3 Koren fits

Context

The 6C33C(2 cath) is a indirectly-heated triode from Svetlana · Ulyanovsk, introduced around 1960. It provides a gain factor (μ) of 2.7 with 69Ω plate resistance. Commonly used in otl / series regulator applications. Soviet power triode designed as a series-pass regulator (тube ballast) — two internal triode systems with paralleled anodes/grids/cathodes but separate filaments. Very low μ ≈ 2.7, very low Ri ≈ 80-120 Ω, transconductance gm ≈ 40 mA/V at 550 mA: the rare combination of low gain and high current that the OTL crowd (Atma-Sphere, Graaf, BAT, Lamm) bent the tube toward in the late 1980s. Built like a tank (200 g, internal bracing, oversized anode pin) with documented vibration/shock specs (6 g / 150-500 g for the -B/-V variant). Famously slow to reach thermal equilibrium (≥ 600 s) and prone to runaway if biasing isn't actively managed.

History

Introduced: 1960
Manufacturer: Svetlana · Ulyanovsk
Era: Late Production

METRIX TESTER

6С33С — 6C33C-B — 6S33S-V Triode Vacuum Tube