What is the 7581A vacuum tube?

The 7581A is a tetrode vacuum tube designed for power (pp) applications. Ruggedized 6L6GC. Balanced, detailed..

What is a 7581A substitute or equivalent?

Direct equivalents to the 7581A include 6L6GC (ruggedized). 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 7581A?

Amplification factor μ ≈ 8, transconductance gm ≈ 6 mA/V, plate resistance rp ≈ 22.5 kΩ, max plate voltage 500 V, max plate dissipation 35 W, heater 6.3 V / 0.9 A.

Is the 7581A still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the 7581A 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 7581A sound like?

Ruggedized 6L6GC. Balanced, detailed.. 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.

7581A Tetrode Vacuum Tube

Also known as 6L6GC (ruggedized)

Beam power tetrode rated at 35W for high-power output stages

TetrodeHeater 6.3 V · 0.9 AOctalTung-Sol·GE 1960

Plate dissipation: 35W
Max plate voltage: 500V
Transconductance: 6mA/V

7581A

Characteristics

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

HeaterIndirectly heated

Specifications of 7581A — Heater
Heater voltageVh	6.3 V
Heater currentIh	0.9 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of 7581A — Limiting values
Plate voltageVa max	500 V
Plate dissipationPa max	35 W

Mechanical

Specifications of 7581A — Mechanical
Base	Octal
Rarity	Sought-after
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of 7581A — Small signal
Transconductance	6 mA/V
Plate resistance	22.5 kΩ
Amplification factor	8

Capacitances

Specifications of 7581A — Capacitances
Input	10 pF
Grid–plate	0.6 pF
Output	6.5 pF

Charactereditorial reading, not a measurement

Specifications of 7581A — Character
Gain	8%
Power	88%
Drive	78%
Linearity	95%

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

Typical operation

Conditions published by the manufacturers, one row per condition. Nothing is computed.

Typical operation
Setup	Va	Vg2	Ia	Ig2	gm	Source
pentode connection, single-endedclass not stated	250 V	250 V	72 mA	5 mA	6 mA/V	OCR, to be checked General Electric · 1963 · p. 2 · transcript

Using this tube

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

Applications

High-power output

Generic tetrode power (pp) stage

PA/public address

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026

Manufacturer datasheets

2 manufacturer datasheets cross-checked for 7581A2 verified

7581A DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: Tung-Sol·GE
Reviewed: 2026-05-29
Status: DataModelCalc
Model: under review

Context

The 7581A is a indirectly-heated beam tetrode from Tung-Sol·GE, introduced around 1960. It provides a gain factor (μ) of 8 with 22.5kΩ plate resistance. Commonly used in power (pp) applications. Ruggedized 6L6GC. Balanced, detailed.

History

Introduced: 1960
Manufacturer: Tung-Sol·GE
Era: Late Production

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