What is the TT21 vacuum tube?

The TT21 is a tetrode vacuum tube designed for power (pp) applications. KT88 with top cap. Transmitter heritage..

What is a TT21 substitute or equivalent?

Direct equivalents to the TT21 include KT88 (different pinout). 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 TT21?

Amplification factor μ ≈ 8, transconductance gm ≈ 11 mA/V, plate resistance rp ≈ 12.0 kΩ, max plate voltage 1250 V, max plate dissipation 45 W, heater 6.3 V / 1.6 A.

Is the TT21 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the TT21 as Collector 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 TT21 sound like?

KT88 with top cap. Transmitter heritage.. 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.

By Jean-Michel Falligan Devergne · Reviewed May 29, 2026

←TT21

← T110-1 TT22 →Compare…

TT21 Tetrode Vacuum Tube

Tetrode6.3V / 1.6AOctal + Top Cap

SourceGEC·MOVReviewed2026-05-29StatusDataModelCalcModelunder reviewPDF

Beam power tetrode rated at 45W for high-power output stages

Datasheet ↗

TT21 — GEC·MOV — tetrode vacuum tube — (1957)

12.0kΩ

11.0mA/V

17.0pF

0.3pF

13.50pF

GAIN8%

POWER100%

DRIVE88%

LINEARITY95%

SPECIFICATIONS

Heater6.3 V · 1.6 A

Max Plate Voltage1250 V

Pa max45 W

Gain Factor8

gm11 mA/V

rp12.0 kΩ

Inter-electrode capacitances

Cgk17 pF

Cgp0.25 pF

Cpk13.5 pF

PinoutOctal + Top Cap

Era1957

RarityCollector

HEATER6.3V / 1.6A

MAX PLATE1250V / 45W

BASEoctal

Source datasheets

2 manufacturer datasheets cross-checked for TT21

2 verified

EXECUTIVE SUMMARY

The TT21 is a indirectly-heated beam tetrode from GEC·MOV, introduced around 1957. It provides a gain factor (μ) of 8 with 12.0kΩ plate resistance. Commonly used in power (pp) applications. KT88 with top cap. Transmitter heritage.

APPLICATIONS

High-power output

Generic tetrode power (pp) stage

• PA/public address

HISTORY

Introduced1957ManufacturerGEC·MOV

Golden Age

TT21 Datasheet

Local mirror · PDF

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METRIX TESTER

Test this tube on a Metrix

SPICE model not yet available. Loadline calculator requires calibrated Koren parameters.

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