Can I swap a KT88 for a KT66?

They share a base and envelope so they fit the same socket, but the operating point differs — expect to re-set the bias rather than perform a blind drop-in.

What is the difference between the KT88 and the KT66?

The KT88 has μ≈8 with about 42 W maximum plate dissipation; the KT66 has μ≈— with about 30 W. The higher μ is on the KT88, the higher plate dissipation rating on the KT88.

Which is better, the KT88 or the KT66?

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 KT66

KT88 versus KT66 — two tetrodes compared.

KT88 vs KT66: what differs

The KT88 and KT66 share a family but trade gain for headroom: KT88 has the higher amplification factor (μ ≈ 8), making it the typical choice for high-gain preamp stages. KT66 runs lower μ (—) and is the standard cathode follower / phase-splitter / line-stage triode. They share the same envelope and base — wire them in the same socket and only the operating point changes.

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

KT66

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

KT66

—no μ published (rectifier or value missing from the catalogue)

Difference—

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

KT66

22.5 kΩRa ≥ 45 kΩ – 67.5 kΩ

Differenceratio ×1.9

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

KT66

85 mAat Va 250 V, Vg −15 VTypical point: Marconi's Wireless Telegraph Company Limited · JUNE 1956

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

KT66

8.75 WPa max 30 W · Pd typical 21.25 W (71 % of Pa max)Typical point: Marconi's Wireless Telegraph Company Limited · JUNE 1956

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

KT66

8.2 W6.3 V × 1.3 A

Differencedifference −1.89 W

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

KT66

octalOctal · 8 pins

Differencesame base, pins differ

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

Physical compatibility

KT88

KT66

Base

octal

Pinout

Octal

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

Sections

Bulb diameter (max)

—

— no mechanical data in the catalogue

Heating

KT88

KT66

Heating

Indirect (IHT)

Heater voltage

6.3 V

same voltage — the same winding can feed both · KT88: no center tap · KT66: no center tap

Heater current

1.6 A

1.3 A

current difference −0.3 A (−19 %)

Heater power

10.08 W

8.19 W

Bias

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

KT88

KT66

Va typical

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

250 V

Vg typical

—

−15 V

Ia typical

—

85 mA

Indicative Rk = |Vg| / Ia

—

176 Ω

Pd typical = Va · Ia

—

21.25 W

KT66 — Typical point: Marconi's Wireless Telegraph Company Limited · JUNE 1956

Performance for the objective

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

KT88

KT66

Difference

—

11.5 mA/V

7 mA/V

ratio ×1.6

12 kΩ

22.5 kΩ

ratio ×1.9

Pa max

42 W

30 W

difference −12 W

Va max

800 V

550 V

difference −250 V

Substitution possible under conditions

No direct substitution listed in the catalogue.

What rules it out as a drop-in:

same base but pins differ: 1: Shell / NC, 8: K+G3 / K

Substitutes for KT88 Substitutes for KT66

Side-by-side specifications

Side-by-side specifications of KT88 and KT66
Spec	KT88	KT66
Full name	KT88	KT66
Type	Tetrode	Tetrode
Primary use	Power (PP)	Power (PP/SE)
Era	1956	1937
Heating	indirect	indirect
Heater voltage	6.3 V	6.3 V
Heater current	1.6 A	1.3 A
Amplification factor (μ)	8	—
Transconductance (gm)	11.5 mA/V	7 mA/V
Plate resistance (rp)	12 kΩ	22.5 kΩ
Max plate voltage	800 V	550 V
Max plate dissipation	42 W	30 W
Base	octal	octal
Manufacturer	GEC	GEC·Marconi
Rarity	Sought-after	Sought-after

Sonic character

Listening folklore — not electrical data

KT88

Commanding authority.

KT66

Warm British beam tetrode.

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 KT66 in the interactive comparator →

KT88 vs KT66

KT88 vs KT66: 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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