What is the 6688 vacuum tube?

The 6688 is a pentode vacuum tube designed for preamp applications. Selected pentode..

What is a 6688 substitute or equivalent?

Direct equivalents to the 6688 include CK6688. 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 6688?

Transconductance gm ≈ 16.5 mA/V, plate resistance rp ≈ 90.0 kΩ, max plate voltage 210 V, max plate dissipation 2.7 W, heater 6.3 V / 0.3 A.

Is the 6688 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the 6688 as Moderate 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 6688 sound like?

Selected pentode.. 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.

By Jean-Michel Falligan Devergne · Reviewed May 29, 2026

←6688

← 6550 6888 →Compare → CK6688

6688 Pentode Vacuum Tube

Pentode6.3V / 0.3A7-pin MiniatureSPICE

SourceRadiomuseum 6688 / E180F · Philips OEMReviewed2026-05-29StatusDataModelCalcModelKoren pentodePDF

Small-signal pentode for low-noise voltage amplification

Datasheet ↗SPICE

6688 — RCA — pentode vacuum tube — (1958)

90.0kΩ

16.5mA/V

7.5pF

0.0pF

3.00pF

GAIN0%

POWER7%

DRIVE10%

LINEARITY75%

SPECIFICATIONS

Heater6.3 V · 0.3 A

Max Plate Voltage210 V

Pa max2.7 W

Gain Factor—

gm16.5 mA/V

rp90.0 kΩ

Inter-electrode capacitances

Cgk7.5 pF

Cgp0.03 pF

Cpk3 pF

Pinout7-pin Miniature

Era1958

RarityModerate

HEATER6.3V / 0.3A

MAX PLATE210V / 2.7W

BASEnoval

Source datasheets

1 manufacturer datasheets cross-checked for 6688

1 verified

EXECUTIVE SUMMARY

The 6688 is a indirectly-heated pentode from RCA, introduced around 1958. Commonly used in preamp applications. Selected pentode.

APPLICATIONS

Voltage amplifier

Generic pentode preamp stage

VS CK6688

Rp +0%Pd -10%Full comparison →

HISTORY

Introduced1958ManufacturerRCA

Golden Age

6688 Datasheet

Local mirror · PDF

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METRIX TESTER

Test this tube on a Metrix

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