10BQ5 — Miniature Audio Output Pentode

An indirectly heated, nine-pin power pentode for medium-power high-fidelity audio amplification, including single-ended Class A, push-pull Class AB and Class B, and triode-connected output stages.

Overview

The 10BQ5 is a miniature audio-frequency power output pentode intended for the output stage of audio amplifiers. Tung-Sol describes its role in medium-power high-fidelity amplifiers and documents 5.7 W single-ended output and 17 W push-pull Class AB output at the stated operating conditions. RCA and Sylvania publish corresponding amplifier examples and identify the 6BQ5/EL84, 8BQ5, and 10BQ5 as electrically related versions with different heater requirements.

Unlike a small-signal voltage-amplifier pentode, the 10BQ5 is designed to deliver appreciable audio power into an output-transformer load. Its operating data include plate and screen-grid dissipation limits, high transconductance, plate-load resistance, screen current, harmonic distortion, and output power. The manufacturer's examples cover conventional pentode operation and operation with the screen grid connected to the plate to obtain triode characteristics.

Construction, Heater, and Base

The tube has a coated, indirectly heated unipotential cathode, a control grid (grid No. 1), a screen grid (grid No. 2), a suppressor grid (grid No. 3), and a plate. It is housed in a miniature glass envelope with a nine-pin button base. Tung-Sol identifies E9-1 base construction, 9CV basing, and a JEDEC miniature outline; Sylvania gives a T-6½ miniature envelope and permits any mounting position.

The heater specification distinguishes the 10BQ5 from its 6.3 V and 8 V relatives. The 11-second warm-up figure is particularly relevant to equipment using coordinated heater warm-up behavior. Tung-Sol expressly notes that the 10BQ5 is otherwise identical to the 6BQ5 apart from heater characteristics and heater warm-up time; RCA likewise groups these output pentodes as corresponding electrical types differing in heater ratings.

Sylvania lists maximum dimensions of approximately 0.875 inch diameter, 2.812 inches seated height, and 3.062 inches overall height for the family.

Electrical Ratings

The published conventional pentode ratings use a design-center rating system. The limits must not be treated as a license to operate all electrodes at their individual maxima simultaneously.

Tung-Sol also lists 550 V maximum plate and screen voltages without current, which are not ordinary loaded operating-voltage ratings, and a 4 W peak screen-dissipation limit. It gives a maximum grid-current starting-point condition of approximately −1.3 V at 0.3 µA control-grid current.

The Tung-Sol sheet adds a specific restriction when heater and positive voltages are obtained from a storage battery using a vibrator supply: maximum plate and screen voltages become 250 V and maximum plate dissipation 9 W. This is a special supply-condition rating, not a replacement for the general ratings in other circuits.

Typical Single-Ended Class A Pentode Operation

In a single-ended output stage, one 10BQ5 drives the primary of an output transformer. The control grid receives the audio signal, the cathode resistor establishes operating bias, and the screen grid receives its specified DC supply.

RCA documents the following representative Class A1 operating condition:

Tung-Sol also publishes a 250 V plate / 250 V screen Class A condition using a 135 Ω cathode resistor and a 5,200 Ω load, with 48 mA zero-signal plate current. Its tables show that changes in output-transformer load, cathode bias, and screen voltage alter the available output and harmonic distribution.

For example, its single-ended tables include a 7,000 Ω load with lower screen voltage or changed bias, producing approximately 4–5.6 W depending on the specified signal condition. This illustrates why the 5.7 W figure is a particular rated operating example, not a guaranteed output at every supply voltage and load.

The reported transconductance is comparatively high for an audio output stage, allowing significant plate-current modulation from a relatively modest grid-voltage swing. The plate resistance is considerably greater than the recommended output load because the output transformer establishes the effective AC load under the specified conditions.

Push-Pull Class AB Audio Amplification

A major application of the 10BQ5 is a two-tube push-pull Class AB1 amplifier. The two output tubes operate into opposite halves of an output-transformer primary, with the effective load specified from plate to plate.

RCA and Tung-Sol provide two representative operating conditions:

These figures are published operating examples without a feedback-dependent reduction of distortion. Tung-Sol specifically characterizes the 17 W Class AB push-pull result as approximately 4% distortion without feedback.

The data also illustrate the importance of screen-grid current under signal drive. Screen current rises substantially at maximum output, so screen supply arrangements and dissipation must be evaluated alongside plate current and plate dissipation.

The 8,000 Ω specification refers to the full plate-to-plate output-transformer load, not an 8,000 Ω resistor connected independently to each plate.

Class B Push-Pull Operation

Tung-Sol also includes Class B, two-tube operating data, with fixed control-grid bias. Representative cases use 250 V or 300 V plate and screen supplies and an 8,000 Ω plate-to-plate load.

The corresponding published grid-bias values are approximately −11.6 V and −14.7 V, with audio power outputs of 11 W and 17 W at approximately 3% and 4% total harmonic distortion, respectively.

Although the maximum output figures resemble the Class AB examples, the bias method and idle currents differ. The operating class, bias, and load conditions must therefore be preserved when using the published values in amplifier design.

Triode-Connected Operation

The 10BQ5 may be operated with grid No. 2 connected to the plate, converting the pentode electrode arrangement into a triode-connected output stage. This is an explicitly documented application, not a claim that the tube is internally constructed as a triode.

Sylvania and Tung-Sol give a single-tube Class A triode-connected example:

In triode connection the effective electrical characteristics differ substantially from conventional pentode operation, including the relationship between plate current, plate voltage, and control-grid voltage. The manufacturer curves separately document this connection.

For two triode-connected tubes in Class AB push-pull, Tung-Sol and Sylvania specify a 10,000 Ω plate-to-plate load and a 270 Ω common cathode resistor. Their examples produce approximately 3.4 W at 250 V and 5.2 W at 300 V, with approximately 2.5% total harmonic distortion under the published conditions.

The choice between conventional pentode and triode-connected operation is therefore a substantial circuit-design decision affecting output power, drive, loading, and distortion.

Ultra-Linear Output Operation

Sylvania additionally documents a Class AB1 ultra-linear connection for the family. Its example uses 300 V plate voltage, a 270 Ω cathode resistor per tube, an 8,000 Ω plate-to-plate load, and reports approximately 11 W output.

Ultra-linear operation is distinct from both ordinary pentode connection and direct triode connection. The manufacturer presents it as another output-stage operating arrangement, rather than as a different physical tube type.

Interelectrode Capacitance

The manufacturers give approximate direct interelectrode capacitances:

These values are relevant to amplifier input loading, high-frequency stability, feedback paths, and the relationship between the output-stage wiring and its driving stage. The published input and output capacitances include the electrode groupings specified in the manufacturer sheets and should not be confused with isolated two-electrode measurements.

Plate Curves, Distortion, and Output-Stage Design

The extended Tung-Sol sheets include characteristic curves for plate current versus plate voltage, control-grid bias, screen-voltage conditions, triode-connected operation, and the relationships among input drive, plate current, screen current, output power, and harmonic distortion.

These curves complement the fixed operating tables. They show why output power cannot be separated from supply voltage, grid drive, effective transformer load, and the rise in screen current at large signal amplitudes.

Tung-Sol provides separate plots for conventional single-ended Class A, Class B push-pull, Class AB push-pull, and triode-connected circuits. They are useful for interpreting how different operating choices change current demand and harmonic behavior. The numerical values quoted here are taken from the manufacturers' printed tables rather than estimated from imperfect OCR of the curves.

Relationship to 6BQ5 / EL84 and 8BQ5

RCA explicitly identifies the 10BQ5, 8BQ5, and 6BQ5/EL84 as corresponding power pentodes with different heater ratings:

The different heater ratings are essential when selecting a tube for a particular amplifier power supply or series-heater arrangement. Similarity in audio operating characteristics does not permit substitution without ensuring that the heater supply is appropriate.

The 10BQ5's 11-second heater warm-up characteristic is specifically noted in the Tung-Sol and RCA data. This reinforces the importance of heater behavior when comparing the family members.

Technical Summary

The 10BQ5 is a miniature nine-pin, indirectly heated audio power pentode with a 10.6 V / 0.45 A heater, 9CV basing, 12 W maximum plate dissipation, and 2 W maximum screen dissipation under its published design-center rating system. It is documented for single-ended Class A audio output, two-tube Class AB and Class B push-pull output, triode-connected amplification, and ultra-linear operation.

Its representative audio performance ranges from 5.7 W single-ended pentode output to 17 W Class AB push-pull output, while triode-connected examples produce lower output power at their respective stated conditions. The tube is closely related electrically to the 6BQ5/EL84 and 8BQ5, but differs in heater voltage, current, and warm-up requirements.

The manufacturer's published ratings, characteristic curves, output-transformer load requirements, control-grid bias arrangements, screen-current behavior, and harmonic-distortion data collectively define the 10BQ5 as a versatile medium-power high-fidelity audio output valve.