Can I swap a KT88 for a 6550?

Yes — the KT88 and 6550 are direct equivalents (same pinout and electrical envelope) and drop into the same socket without rebiasing in nearly every circuit. Manufacturer and batch differences usually matter more than the type number.

What is the difference between the KT88 and the 6550?

The KT88 has μ≈8 with about 42 W maximum plate dissipation; the 6550 has μ≈8 with about 35 W. The higher μ is on the KT88, the higher plate dissipation rating on the KT88.

Which is better, the KT88 or the 6550?

Neither is universally better — it depends on the circuit and the tone you want in a hi-fi system. Compare the side-by-side specifications and the comparison by objective on this page (gain, load, current, dissipation margin, heater, base) to choose for your build.

KT88 vs 6550

KT88 versus 6550 — two tetrodes compared.

KT88 vs 6550: what differs

The KT88 and 6550 are directly equivalent in most applications — same pinout, same electrical envelope, same general sonic signature. KT88 is the American/RETMA designation; 6550 is the European Mullard-Philips name (or vice versa). You can swap one for the other in nearly every circuit without rebiasing, with the caveat that production-batch differences between manufacturers (Mullard vs Sylvania vs JJ vs current Russian production) often matter more than the type-number itself.

Comparison by objective

Choose what the stage has to do: the quantities below are reordered and weighted for that objective. Every value comes from the catalogue and from the datasheet typical operating point; a missing value shows “—” with its reason. No score, no ranking — only measured differences.

Voltage amplification of a small signal — common cathode with a resistive plate load.

Quantity

KT88

6550

Difference

Available gain (μ)Weight 3/3 · decisiveμ is the ceiling of the voltage gain of a common-cathode stage.Av ≈ μ · Ra / (Ra + rp) — with Ra = 2–3 × rp the stage reaches about 2/3 to 3/4 of μ. As a follower the gain is close to 1 whatever μ.

KT88

6550

Differenceidentical

Load to drive (rp)Weight 2/3 · importantPlate resistance: the lower rp, the lower the load the stage can drive at the same gain and linearity.Ra ≥ 2–3 × rp for a common-cathode stage. As a cathode follower the output impedance is about 1 / gm.

KT88

12 kΩRa ≥ 24 kΩ – 36 kΩ

6550

12 kΩRa ≥ 24 kΩ – 36 kΩ

Differenceidentical

Available current (Ia typical)Weight 1/3 · to checkPlate current at the datasheet typical operating point — what the stage can deliver into a capacitance or a low load.Read from the published typical point (Va, Vg). No operating point is derived from a model here.

KT88

—no datasheet typical operating point in the catalogue for this tube

6550

140 mAat Va 250 V, Vg −14 VTypical point: Tung-Sol Electric Inc., Electron Tube Division, Bloomfield, New Jersey · 1963 (JULY 1, 1963 / MARCH 1, 1963; PLATE #6755/6679/6680/6681)

Difference—

Dissipation margin (Pa max − Pd typical)Weight 1/3 · to checkDifference between the maximum plate dissipation and the dissipation at the typical point (Pd = Va · Ia).Pd at idle < 70 % of Pa max.

KT88

—no datasheet typical operating point in the catalogue for this tubePa max 42 W

6550

0 WPa max 35 W · Pd typical 35 W (100 % of Pa max)Typical point: Tung-Sol Electric Inc., Electron Tube Division, Bloomfield, New Jersey · 1963 (JULY 1, 1963 / MARCH 1, 1963; PLATE #6755/6679/6680/6681)

Difference—

Heater (V × A)Weight 1/3 · to checkHeater voltage and current, hence the power drawn from the heater winding.Same voltage required; the winding must supply the summed current of the tubes in parallel (or the same current in series).

KT88

10.1 W6.3 V × 1.6 A

6550

10.1 W6.3 V × 1.6 A

Differenceidentical

Base compatibilityWeight 1/3 · to checkBase and pin-by-pin function according to the catalogue pinouts.Same base and identical pinout → same socket. Same base, different pins → rewiring. Different base → adapter or new socket.

KT88

octalOctal · 8 pins

6550

octalOctal · 8 pins

Differencesame base, pins differ

Rules: Amplifier topologies guide, §07 “Design rules of thumb”

Physical compatibility

KT88

6550

Base

octal

Pinout

Octal

differs on 4 pin(s): 1: Shell / NC, 2: H / G1, 5: G1 / K, 8: K+G3 / H

Sections

Bulb diameter (max)

—

— no mechanical data in the catalogue

Heating

KT88

6550

Heating

Indirect (IHT)

Heater voltage

6.3 V

same voltage — the same winding can feed both · KT88: no center tap · 6550: no center tap

Heater current

1.6 A

same current

Heater power

10.08 W

Bias

Datasheet typical operating point (one per tube, SEQ-02) and the cathode resistor it implies.

KT88

6550

Va typical

—no datasheet typical operating point in the catalogue for this tube

250 V

Vg typical

—

−14 V

Ia typical

—

140 mA

Indicative Rk = |Vg| / Ia

—

100 Ω

Pd typical = Va · Ia

—

35 W

6550 — Typical point: Tung-Sol Electric Inc., Electron Tube Division, Bloomfield, New Jersey · 1963 (JULY 1, 1963 / MARCH 1, 1963; PLATE #6755/6679/6680/6681)

Performance for the objective

Catalogue values, weighted as in the objective table (Gain input stage). rp is a design parameter, not a merit.

KT88

6550

Difference

identical

11.5 mA/V

11 mA/V

ratio ×1

12 kΩ

identical

Pa max

42 W

35 W

difference −7 W

Va max

800 V

600 V

difference −200 V

Substitution possible under conditions

KT88 and 6550 list each other as equivalents in the catalogue.

Substitutes for KT88 Substitutes for 6550

Side-by-side specifications

Side-by-side specifications of KT88 and 6550
Spec	KT88	6550
Full name	KT88	6550
Type	Tetrode	Tetrode
Primary use	Power (PP)	Power (PP)
Era	1956	1955
Heating	indirect	indirect
Heater voltage	6.3 V	6.3 V
Heater current	1.6 A	1.6 A
Amplification factor (μ)	8	8
Transconductance (gm)	11.5 mA/V	11 mA/V
Plate resistance (rp)	12 kΩ	12 kΩ
Max plate voltage	800 V	600 V
Max plate dissipation	42 W	35 W
Base	octal	octal
Manufacturer	GEC	Tung-Sol·GE
Rarity	Sought-after	Common

Sonic character

Listening folklore — not electrical data

KT88

Commanding authority.

6550

American powerhouse.

Need an interactive deep dive?

The Amperatubes comparator visualises loadlines for both tubes side-by-side, simulates the SPICE response, and lets you adjust the operating point in real time.

Open KT88 vs 6550 in the interactive comparator →

KT88 vs 6550

KT88 vs 6550: what differs

Comparison by objective

Physical compatibility

Heating

Bias

Performance for the objective

Substitution possible under conditions

Side-by-side specifications

Sonic character

Need an interactive deep dive?

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