5678 tube = CV2254 DF60 Röhre Audio subminiatur Pentode

5678 subminiature pentode for miniature battery preamps

The 5678 is a fully shielded, directly heated pencil-style pentode with five flexible wire leads. Its 1.25 V, 50 mA filament allows extremely compact and economical valve preamplifiers.

Published operating points use only 45 to 67.5 V and less than 2 mA of plate current. This makes the valve attractive for miniature preamps, portable instrument electronics and unusual low-voltage audio experiments.

Useful applications

• Miniature battery and instrument preamplifiers
• Compact guitar, microphone and effects circuits
• Portable low-voltage valve-audio projects
• Measurement, receiver and subminiature equipment

Technical specifications

Valve type

Directly heated, fully shielded subminiature pentode

Equivalent designations

CK5678, DF60, DF654, DF669 and CV2254

Connection style

Five flexible wire leads without a conventional valve base

Filament

1.25 V DC at 50 mA

45 V operating condition

Va and Vg2 45 V, Ia 0.8 mA, Ig2 0.22 mA and gm 0.82 mA/V

67.5 V operating condition

Va and Vg2 67.5 V, Ia 1.8 mA, Ig2 0.48 mA and gm 1.1 mA/V

Internal resistance

Approximately 1.2 MΩ at 45 V or 1 MΩ at 67.5 V

Maximum anode voltage

90 V

Dimensions

Maximum width approximately 9.8 mm and length approximately 38.1 mm

Wire connections

Lead 1

Anode, identified by the adjacent red dot

Lead 2

Screen grid g2

Lead 3

Negative filament, shield and part of g3

Lead 4

Control grid g1

Lead 5

Positive filament and second part of g3

Miniature audio design notes

The directly heated filament benefits from a clean DC supply and must be considered in the bias arrangement. Valves intended for sensitive inputs should be selected for low noise and low microphonics.

Its high plate resistance makes the 5678 primarily a voltage-gain device. Low-impedance outputs require a following buffer, transformer or semiconductor stage.

Questions about selection and audio use

Is the 5678 suitable for a miniature valve preamplifier?

Yes, its small size, full shielding and 45 to 67.5 V operating conditions make it highly attractive for miniature preamps. A suitable anode load can provide useful voltage gain with very low power consumption.

Can the 5678 operate in a battery-powered audio project?

Yes, its filament requires only 1.25 V at 50 mA and can be supplied from a suitable cell or regulated rechargeable battery. The anode circuit additionally requires approximately 45 to 67.5 V.

Is the 5678 useful for guitar, microphone and portable preamps?

Yes, its subminiature construction supports unusually compact valve modules for instruments, effects and portable audio equipment. Valves intended for sensitive input stages should be tested for noise, hum and microphonics.