What is the 833A vacuum tube?

The 833A is a triode vacuum tube designed for transmitting (set) applications. Monster transmitting triode. 1kW capable..

What is a 833A substitute or equivalent?

Direct equivalents to the 833A include FU-33. 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 833A?

Amplification factor μ ≈ 35, transconductance gm ≈ 4 mA/V, plate resistance rp ≈ 7.8 kΩ, max plate voltage 4000 V, max plate dissipation 450 W, heater 10 V / 10 A.

Is the 833A still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the 833A 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 833A sound like?

Monster transmitting triode. 1kW capable.. 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.

833A Triode Vacuum Tube

Also known as FU-33

Power triode delivering 450W for push-pull and SET designs

TriodeHeater 10 V · 10 ASpecialRCA·GE·Penta 1938

Plate dissipation: 450W
Max plate voltage: 4000V
Transconductance: 4mA/V

833A

Characteristics

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

HeaterDirectly heated

Specifications of 833A — Heater
Heater voltageVh	10 V
Heater currentIh	10 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of 833A — Limiting values
Plate voltageVa max	4000 V
Plate dissipationPa max	450 W

Mechanical

Specifications of 833A — Mechanical
Base	Special
Rarity	Sought-after
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of 833A — Small signal
Transconductance	4 mA/V
Plate resistance	7.8 kΩ
Amplification factor	35

Capacitances

Specifications of 833A — Capacitances
Grid–cathode	12.3 pF
Grid–plate	6.3 pF
Plate–cathode	8.5 pF

Charactereditorial reading, not a measurement

Specifications of 833A — 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 DevergneReviewed May 29, 2026

Manufacturer datasheets

4 manufacturer datasheets cross-checked for 833A4 verified

833A DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: RCA·GE·Penta
Reviewed: 2026-05-29
Status: DataModelCalc
Model: under review

Context

The 833A is a directly-heated triode from RCA·GE·Penta, introduced around 1938. It provides a gain factor (μ) of 35 with 7800Ω plate resistance. Commonly used in transmitting (set) applications. Monster transmitting triode. 1kW capable.

History

Introduced: 1938
Manufacturer: RCA·GE·Penta
Era: Pre-War

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