6BQ5 / EL84 — Miniature High-Sensitivity Audio Output Pentode

An indirectly heated nine-pin power pentode for single-ended Class A, push-pull Class AB and B, triode-connected, and ultra-linear audio amplification.

Overview

The 6BQ5, also designated EL84, is a miniature audio-frequency power output pentode developed for radio and television receivers, home music systems, and medium-power high-fidelity amplifiers. The manufacturer sheets repeatedly emphasize high power sensitivity, efficient operation, modest audio drive requirements, and the ability to produce useful output from a small glass envelope.

Amperex describes the EL84 as a true pentode designed for high-fidelity service, specifically discussing its output swing and distortion behavior at low instantaneous plate voltage. Westinghouse highlights high plate efficiency and power sensitivity. Tung-Sol, General Electric, RCA, and Ferranti provide extensive operating tables for single-ended and push-pull amplifier service. Later JJ Electronic, Sovtek, and Tung-Sol-branded documents continue to describe the EL84/6BQ5 as an audio output pentode.

The best-known manufacturer operating examples are 5.7 W from one tube in Class A and 17 W from a pair in Class AB push-pull, each under specified supply, bias, transformer-load, and distortion conditions. These are operating-point results, not unconditional output guarantees.

Construction and Nine-Pin Basing

The 6BQ5 has a coated, indirectly heated unipotential cathode, control grid (grid 1), screen grid (grid 2), suppressor grid (grid 3), and plate. It uses the E9-1 small-button nine-pin miniature/noval base with 9CV basing. The suppressor grid is internally connected to the cathode.

RCA explicitly warns against using the internal-connection pins as external circuit terminals. Some newer documents label pins 1, 6, and 8 as free; the safest interpretation for the original historical construction is to respect the manufacturer-specific base diagram.

The envelope is identified as T-6½ glass. Ferranti gives a maximum overall length of 78.5 mm, seated height of 71.5 mm, and diameter of 22.2 mm; later JJ and Sovtek dimensions are broadly similar. Any mounting position is permitted. RCA specifically warns that the bulb becomes hot during operation and requires free circulation of cooling air.

Heater and Cathode

The 6BQ5's 6.3 V heater is the defining heater difference from the 8BQ5 and 10BQ5 family members. The latter are documented with 8.0 V / 0.60 A and 10.6 V / 0.45 A heaters respectively. Their differing warm-up and heater-current requirements matter particularly in series-heater receiver designs.

Design-Center Maximum Ratings

The principal historical rating set is:

Tung-Sol and Ferranti also list 550 V for plate or screen under no-current conditions; that value must not be confused with a continuous loaded operating rating. Several manufacturers impose 250 V plate and screen limits and 9 W plate dissipation where positive supplies are produced from a storage battery through a vibrator.

These limits are expressed under manufacturer rating conventions and should not be indiscriminately combined. The 12 W plate limit and 2 W continuous screen limit are especially important in output-stage thermal design.

Typical Pentode Characteristics

At the widely published operating point of 250 V plate, 250 V screen, and −7.3 V control-grid bias, the manufacturer data give:

This amplification-factor figure refers to the grid relationship stated in the pentode sheets, not to the ordinary plate-voltage amplification factor of a triode. The high transconductance contributes to the tube's sensitivity: substantial plate-current change is available with a relatively small control-grid voltage swing.

Single-Ended Class A Amplification

A single 6BQ5 can drive an output-transformer primary in Class A1 service. The published examples use 250 V plate and screen supplies, approximately −7.3 V grid bias, and a 135 Ω cathode resistor.

Amperex discusses an important measurement distinction: a fixed-bias power measurement at the specified operating point may not equal the sustained sine-wave result with a cathode resistor, because increasing plate and screen current changes the cathode bias. The brochure notes an approximately 10% reduction in available output under the described sustained-signal condition.

The Tung-Sol and General Electric sheets provide additional Class A examples with 7,000 Ω loads and changed screen voltage or bias, producing lower rated output under their stated conditions. These demonstrate that output power and harmonic distribution depend on screen supply, quiescent current, drive, and effective transformer load.

Two-Tube Class AB Push-Pull Amplification

The 6BQ5 is extensively documented for Class AB1 push-pull audio output. The output transformer load is specified plate to plate.

These figures appear across Amperex, Tung-Sol, Westinghouse, General Electric, and RCA documentation. Tung-Sol specifically describes the 17 W result without feedback. Amperex emphasizes the tube's suitability for high-fidelity amplifiers using such push-pull arrangements.

Screen current rises markedly under maximum signal, an important consideration for screen supply impedance, dissipation, and power-supply design.

Class B Push-Pull Operation

Tung-Sol and General Electric also document Class B push-pull operation at 250 V and 300 V, with 8,000 Ω plate-to-plate loads and control-grid biases of approximately −11.6 V and −14.7 V. The maximum-signal output examples are again 11 W and 17 W, with approximately 3% and 4% distortion.

The similar maximum output figures do not mean Class B and Class AB have identical quiescent currents or bias arrangements. The manufacturers provide separate operating tables because idle current, bias method, and signal-current distribution differ.

Triode-Connected Operation

The 6BQ5 may be connected as a triode by tying the screen grid to the plate. This changes the effective characteristic of the existing pentode; it does not change its physical electrode construction.

The common single-ended Class A triode-connected example uses 250 V, a 270 Ω cathode resistor, and approximately 3,500 Ω load. It gives 34 mA zero-signal plate current, 36 mA maximum-signal plate current, approximately 1.95 W output, and 9% total harmonic distortion.

For two triode-connected tubes in Class AB push-pull, the published examples use a 10,000 Ω plate-to-plate load and 270 Ω common cathode resistor, with output of approximately 3.4 W at 250 V and 5.2 W at 300 V, both at about 2.5% distortion.

Ferranti additionally lists triode-connected characteristics near 2 kΩ internal plate resistance and amplification factor 19 under its stated conditions. Triode connection is therefore a distinct amplifier option with different drive, impedance, output, and distortion characteristics from conventional pentode operation.

Ultra-Linear Operation

Sylvania documents a Class AB1 ultra-linear connection with 300 V plate voltage, 270 Ω cathode resistance per tube, 8,000 Ω plate-to-plate load, and 11 W output. The screen-grid connection in an ultra-linear stage is distinct from a conventional fixed screen supply and from a direct screen-to-plate triode connection.

Interelectrode Capacitances

Historical manufacturers specify approximate direct capacitances:

Ferranti lists approximately 11 pF input, 6.5 pF output, and less than 0.5 pF anode-to-control-grid capacitance. JJ Electronic publishes a different modern capacitance set, including approximately 10 pF grid-to-cathode, 5.1 pF output, 0.6 pF grid-to-anode, and 0.15 pF grid-to-heater. Such figures should retain their manufacturer measurement definitions rather than being treated as a contradiction requiring a single forced value.

Characteristic Curves and Manufacturer Variants

The extended General Electric, RCA, Ferranti, and Tung-Sol sheets contain plate-current, transfer, and audio output characteristics. Their graphs illustrate the effects of plate voltage, control-grid bias, screen voltage, output load, and drive on plate current, screen current, power, and distortion.

Later JJ Electronic EL84, Sovtek EL84/6BQ5, and Tung-Sol-branded 6BQ5/EL84 sheets continue to specify a miniature audio output pentode with a 6.3 V heater and operating data centered around the familiar 250 V plate/screen conditions. They should be regarded as individual manufacturer implementations with their own published limits and production tolerances, rather than assumed numerically identical in every respect.

A classification nuance is worth preserving: most manufacturer documents explicitly call the 6BQ5 a power pentode or output pentode, whereas the Sylvania technical-manual entry uses the phrase beam pentode. The differing wording should not be silently converted into a universal claim of beam-power construction; the shared published electrical application is audio output amplification.

Technical Summary

The 6BQ5 / EL84 is a 6.3 V, 0.76 A, nine-pin miniature audio power pentode with 9CV basing, a 12 W plate-dissipation rating, a 2 W continuous screen-dissipation rating, and approximately 11.3 mA/V transconductance at its principal 250 V operating point.

Its manufacturer-documented applications include single-ended Class A audio output, Class AB and Class B push-pull amplifiers, triode-connected audio stages, and ultra-linear output circuits. Representative output ranges from 5.7 W single-ended to 17 W for a push-pull pair, with each output tied to its stated supply, load, bias, and distortion conditions.

The tube's high power sensitivity, modest required drive, compact construction, and multiple supported output-stage configurations explain its extensive use in radio, television, home music, and high-fidelity audio equipment.