6J4 Röhre Triode B7G Uf 6,3V

6J4 high-gm B7G triode for line stages and audio drivers

The 6J4 was designed as a wideband UHF triode, but its high transconductance, low plate resistance and useful operating current are also attractive for audio. It can provide an unusual basis for line-level stages, voltage amplifiers and drivers.

The valve has three external connections to the same control grid. These reduce lead inductance in RF service and should be connected together correctly when the valve is used in a conventional audio stage.

Suitable applications

• Low-output-impedance line and preamplifier stages
• Driver stages for small and medium output valves
• Guitar amplifiers and experimental audio circuits
• Wideband measurement, oscillator and RF amplifiers

Technical specifications

Valve type

High-transconductance single triode for grounded-grid service

Related designations

A2209; 8532 and CV1763 are rugged or selected relatives

Base

Miniature B7G, 7-pin

Heater

6.3 V at 400 mA

Typical operating condition

Va 150 V, Rk 100 Ω and Ia approximately 15 mA

Amplification factor

µ 55

Transconductance

Approximately 12 mA/V

Plate resistance

Approximately 4.5 kΩ

RCA maximum ratings

Maximum Va 150 V and maximum plate dissipation 2.25 W

Dimensions

Maximum diameter approximately 19 mm and overall height approximately 54 mm

B7G connections – bottom view

Pins 1, 5 and 6

Common control grid

Pin 2

Cathode

Pins 3 and 4

Heater

Pin 7

Anode

Audio design notes

For a conventional common-cathode audio stage, the three grid terminals are joined at the socket. High transconductance calls for short wiring, a suitable grid stopper and effective power-supply decoupling.

Its approximately 15 mA operating current gives the 6J4 more drive capability than many high-resistance preamp triodes. A buffer or transformer is still required for loudspeakers and other low-impedance loads.

Questions about selection and audio use

Is the 6J4 suitable for a high-quality line stage?

Yes, its approximately 12 mA/V transconductance and 4.5 kΩ plate resistance are attractive for fast, wideband line and preamplifier stages. A suitable load can provide a lower output impedance than many conventional high-gain triodes.

Can the 6J4 drive an audio output valve?

Yes, its typical 15 mA plate current provides useful current reserve for a purpose-designed driver. The required voltage swing, output-valve input capacitance and 6J4 dissipation limit must be considered in the final design.

Is the 6J4 attractive for affordable stereo and audio DIY?

Yes, the single high-gm triode supports compact channel layouts and unusual circuit concepts. Two valves are required for stereo, and BTB's selectable matching option may be useful in channel-matched or balanced designs.