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.

TT21 Tetrode Vacuum Tube

Also known as KT88 (different pinout)

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

TetrodeHeater 6.3 V · 1.6 AOctal + Top CapGEC·MOV 1957

Plate dissipation: 45W
Max plate voltage: 1250V
Max screen voltage: 600V
Transconductance: 11mA/V

TT21

Characteristics

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

HeaterIndirectly heated

Specifications of TT21 — Heater
Heater voltageVh	6.3 V
Heater currentIh	1.6 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of TT21 — Limiting values
Plate voltageVa max	1250 V
Screen voltageVg2 max	600 V
Plate dissipationPa max	45 W
Screen dissipationPg2 max	6 W

Mechanical

Specifications of TT21 — Mechanical
Base	Octal + Top Cap
Rarity	Collector
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of TT21 — Small signal
Transconductance	11 mA/V
Plate resistance	12 kΩ
Amplification factor	8

Capacitances

Specifications of TT21 — Capacitances
Input	17 pF
Grid–plate	0.25 pF
Output	13.5 pF

Charactereditorial reading, not a measurement

Specifications of TT21 — Character
Gain	8%
Power	100%
Drive	88%
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	B+	Vg2	Ia	Ig2	Pout	THD	Source
Class AB1, push-pullpentode connection · fixed bias	560 V	300 V	60 mA	1.7 mA	100 W	2.5%	OCR, to be checked GEC / M-O Valve Company Limited · 1973 · p. 3 · 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 TT212 verified

TT21 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: GEC·MOV
Reviewed: 2026-05-29
Status: DataModelCalc
Model: under review

Context

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.

History

Introduced: 1957
Manufacturer: GEC·MOV
Era: Golden Age

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