What is the 6550 vacuum tube?

The 6550 is a tetrode vacuum tube designed for power (pp) applications. American powerhouse..

What is a 6550 substitute or equivalent?

Direct equivalents to the 6550 include KT88. 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 6550?

Amplification factor μ ≈ 8, transconductance gm ≈ 11 mA/V, plate resistance rp ≈ 12.0 kΩ, max plate voltage 600 V, max plate dissipation 35 W, heater 6.3 V / 1.6 A.

Is the 6550 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the 6550 as Common 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 6550 sound like?

American powerhouse.. 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.

6550 Tetrode Vacuum Tube

Also known as KT88

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

TetrodeHeater 6.3 V · 1.6 AOctalTung-Sol·GE 1955

Plate dissipation: 35W
Max plate voltage: 600V
Transconductance: 11mA/V

6550

Characteristics

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

HeaterIndirectly heated

Specifications of 6550 — Heater
Heater voltageVh	6.3 V
Heater currentIh	1.6 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of 6550 — Limiting values
Plate voltageVa max	600 V
Plate dissipationPa max	35 W

Mechanical

Specifications of 6550 — Mechanical
Base	Octal
Rarity	Common
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of 6550 — Small signal
Transconductance	11 mA/V
Plate resistance	12 kΩ
Amplification factor	8

Capacitances

Specifications of 6550 — Capacitances
Input	14.5 pF
Grid–plate	0.82 pF
Output	11 pF

Charactereditorial reading, not a measurement

Specifications of 6550 — Character
Gain	8%
Power	88%
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+	Va	Vg2	Vg1 / Rk	Ia	Ig2	Load	Pout	THD	gm	μ	rp	Source
Class A, single-endedpentode connection · fixed bias		250 V	250 V	-14 V	140 mA	12 mA	1.5 kΩ	12.5 W	7%	11 mA/V		12 kΩ	OCR, to be checked RCA · 1962 · p. 2 · transcript
pentode connection, single-endedclass not stated · fixed bias		250 V	250 V	-14 V	140 mA					11 mA/V			OCR, to be checked Tung-Sol Electric Inc. · 1963 · p. 2 · transcript
Class A, single-endedtriode connection · fixed bias		250 V		-14 V	140 mA					11 mA/V	8	12 kΩ	OCR, to be checked RCA · 1962 · p. 1 · transcript
Class A, push-pullpentode connection · fixed bias · values for two tubes	400 V		275 V	-23 V	180 mA	9 mA	3.5 kΩ a-a						OCR, to be checked RCA · 1962 · p. 2 · transcript

Using this tube

Applications

High-power output

Dynaco Mark IIIMcIntosh MC275SVT bass amplifier

Guitar ampPA/public address

Featured in these amplifiers

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Source : Tung-Sol·GE datasheetActive model : Koren v1

Manufacturer datasheets

2 manufacturer datasheets cross-checked for 65502 verified

6550 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: Tung-Sol·GE datasheet
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Koren v1

Context

The 6550 is a beam power tetrode developed by Tung-Sol in 1955 specifically for high-power audio amplifier use, with a plate dissipation of 35W. It shares the KT88's octal base and general operating envelope but has a distinctly different sonic character: tighter, more controlled, and more linear than the KT88, with slightly less midrange bloom but superior bass definition. The 6550 is the original output tube of the McIntosh MC275 and many Ampeg bass amplifiers, where its clean headroom and damping characteristics are valued. Tung-Sol 6550 (original and reissue), GE 6550A, and Svetlana/SED 6550C are the notable variants.

History

Introduced: 1955
Manufacturer: Tung-Sol·GE
Era: Golden Age

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