What is the KT99 vacuum tube?

The KT99 is a tetrode vacuum tube designed for power (pp) applications. Premium KT88 derivative. Nice tone, fragile..

What is a KT99 substitute or equivalent?

Direct equivalents to the KT99 include KT88, 6550. 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 KT99?

Transconductance gm ≈ 11 mA/V, max plate voltage 700 V, max plate dissipation 42 W, heater 6.3 V / 1.65 A.

Is the KT99 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the KT99 as Rare 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 KT99 sound like?

Premium KT88 derivative. Nice tone, fragile.. 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.

KT99 Tetrode Vacuum Tube

Also known as KT88 · 6550

Beam power tetrode rated at 42W for high-power output stages

TetrodeHeater 6.3 V · 1.65 AOctalEi·Chinese 1985

Plate dissipation: 42W
Max plate voltage: 700V
Transconductance: 11mA/V

KT99

Characteristics

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

HeaterIndirectly heated

Specifications of KT99 — Heater
Heater voltageVh	6.3 V
Heater currentIh	1.65 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of KT99 — Limiting values
Plate voltageVa max	700 V
Plate dissipationPa max	42 W

Mechanical

Specifications of KT99 — Mechanical
Base	Octal
Rarity	Rare
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of KT99 — Small signal
Transconductance	11 mA/V

Capacitances

Specifications of KT99 — Capacitances
Input	15 pF
Grid–plate	1 pF
Output	8.5 pF

Charactereditorial reading, not a measurement

Specifications of KT99 — Character
Gain	0%
Power	100%
Drive	50%
Linearity	75%

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

High-power output

Generic tetrode power (pp) stage

PA/public address

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 9, 2026Source : audit-we_transmit_gec_other (CLEAN — matches canonical datasheet)

Manufacturer datasheets

KT99 DatasheetFrank's Electron Tube Pages · PDF

SPICE models

Revision and verification status

Source: audit-we_transmit_gec_other (CLEAN — matches canonical datasheet)
Reviewed: 2026-05-09
Status: DataModelCalc
Model: under review

Context

The KT99 is a indirectly-heated beam tetrode from Ei·Chinese, introduced around 1985. Commonly used in power (pp) applications. Premium KT88 derivative. Nice tone, fragile.

History

Introduced: 1985
Manufacturer: Ei·Chinese
Era: Modern

METRIX TESTER