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.

6688 Pentode Vacuum Tube

Also known as CK6688

Small-signal pentode for low-noise voltage amplification

PentodeHeater 6.3 V · 0.3 A7-pin MiniatureRCA 1958

Plate dissipation: 2.7W
Max plate voltage: 210V
Transconductance: 16.5mA/V
Plate resistance: 90kΩ

6688

Characteristics

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

HeaterIndirectly heated

Specifications of 6688 — Heater
Heater voltageVh	6.3 V
Heater currentIh	0.3 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of 6688 — Limiting values
Plate voltageVa max	210 V
Plate dissipationPa max	2.7 W

Mechanical

Specifications of 6688 — Mechanical
Base	7-pin Miniature
Rarity	Moderate
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of 6688 — Small signal
Transconductance	16.5 mA/V
Plate resistance	90 kΩ

Capacitances

Specifications of 6688 — Capacitances
Input	7.5 pF
Grid–plate	0.03 pF
Output	3 pF

Charactereditorial reading, not a measurement

Specifications of 6688 — Character
Gain	0%
Power	7%
Drive	10%
Linearity	75%

datasheet cited · model or editorial reading · catalogue value, no citation attached yet · nothing invented

Using this tube

Applications

Voltage amplifier

Generic pentode preamp stage

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Source : Radiomuseum 6688 / E180F · Philips OEMActive model : Koren pentode

Manufacturer datasheets

1 manufacturer datasheets cross-checked for 66881 verified

6688 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: Radiomuseum 6688 / E180F · Philips OEM
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Koren pentode

Context

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

History

Introduced: 1958
Manufacturer: RCA
Era: Golden Age

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