What is the FU33 vacuum tube?

The FU33 (FU-33 / 833C) is a triode vacuum tube designed for transmitting (set) applications. Chinese 833A. Affordable transmitting triode power..

What is a FU33 substitute or equivalent?

Direct equivalents to the FU33 include 833A, 833C. 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 FU33?

Amplification factor μ ≈ 35, transconductance gm ≈ 4.5 mA/V, plate resistance rp ≈ 7.8 kΩ, max plate voltage 3000 V, max plate dissipation 350 W, heater 10 V / 10 A.

Is the FU33 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the FU33 as Common 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 FU33 sound like?

Chinese 833A. Affordable transmitting triode power.. 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.

FU33 FU-33 / 833C Triode Vacuum Tube

Also known as 833A · 833C

Power triode delivering 350W for push-pull and SET designs

TriodeHeater 10 V · 10 ASpecialChinese 1965

Plate dissipation: 350W
Max plate voltage: 3000V
Transconductance: 4.5mA/V

FU33

Characteristics

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

HeaterDirectly heated

Specifications of FU33 — Heater
Heater voltageVh	10 V
Heater currentIh	10 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of FU33 — Limiting values
Plate voltageVa max	3000 V
Plate dissipationPa max	350 W

Mechanical

Specifications of FU33 — Mechanical
Base	Special
Rarity	Common
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of FU33 — Small signal
Transconductance	4.5 mA/V
Plate resistance	7.8 kΩ
Amplification factor	35

Capacitances

Specifications of FU33 — Capacitances
Grid–cathode	9 pF
Grid–plate	7 pF
Plate–cathode	6 pF

Charactereditorial reading, not a measurement

Specifications of FU33 — Character
Gain	35%
Power	100%
Drive	92%
Linearity	83%

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

SET output
Push-pull output

Generic triode transmitting (set) stage

PA/public address

Replace or compare

Sources

By Jean-Michel Falligan Devergne

Manufacturer datasheets

FU33 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: Chinese
Reviewed: —
Status: DataModelCalc
Model: under review

Context

The FU33 is a directly-heated triode from Chinese, introduced around 1965. It provides a gain factor (μ) of 35 with 7800Ω plate resistance. Commonly used in transmitting (set) applications. Chinese 833A. Affordable transmitting triode power.

History

Introduced: 1965
Manufacturer: Chinese
Era: Late Production

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