8BQ5 — Controlled-Warm-Up Miniature Audio Output Pentode

An eight-volt, 600-milliampere heater version of the 6BQ5/EL84 power pentode, intended for high-fidelity audio output and receiver equipment with series-connected heater circuits.

Overview

The 8BQ5 is a nine-pin miniature audio power output pentode designed for radio and television receivers, home music systems, and medium-power high-fidelity audio amplifiers. Its distinctive feature is an 8.0 V, 0.60 A heater with controlled warm-up characteristics, suited to equipment whose tube heaters are connected in series. Electrically, the manufacturers identify it as corresponding to the 6BQ5/EL84 except for heater ratings.

Westinghouse specifically describes the 8BQ5 as intended for receivers employing series-connected heaters, particularly television receivers in which the picture-tube heater is in series with other valve heaters. Controlled warm-up helps minimize the temporary heater-voltage surges that can occur as different heaters change resistance at different rates during starting.

The 8BQ5 is also a fully documented audio-frequency power output valve, not merely a heater variant with no amplifier data of its own. Tung-Sol provides detailed single-ended Class A, push-pull Class AB, push-pull Class B, and triode-connected amplifier characteristics, including plate-current curves, distortion curves, and power-output plots.

Heater, Cathode, and Warm-Up Behavior

The cathode is coated, unipotential, and indirectly heated. The heater may be supplied with AC or DC.

Westinghouse gives a technical definition for heater warm-up measurement: it is the time required for heater voltage to reach 80% of its rated value in a prescribed test circuit after applying a multiple of rated voltage through a series resistance. The 11-second warm-up specification therefore concerns controlled behavior in the defined test, not a guarantee that full cathode emission begins at exactly 11 seconds in every installation.

The 600 mA heater rating is especially relevant to 600 mA series-string receivers. It must not be confused with the 6BQ5/EL84 heater of 6.3 V at 0.76 A or the 10BQ5 heater of 10.6 V at 0.45 A.

Electrode Construction, Basing, and Mechanical Form

The 8BQ5 is a single power pentode containing a control grid (grid No. 1), screen grid (grid No. 2), suppressor grid (grid No. 3), plate, and indirectly heated cathode. It uses a small-button E9-1 nine-pin miniature base with 9CV basing and a T-6½ glass envelope.

The common family basing provides control grid on pin 2, cathode and suppressor grid on pin 3, heater on pins 4 and 5, plate on pin 7, and screen grid on pin 9. The remaining positions are designated internal connections in historical manufacturer diagrams and should not be appropriated as general-purpose circuit terminals.

Sylvania gives approximate maximum dimensions of 0.875 inch diameter, 2.812 inches seated height, and 3.062 inches overall height. The tube can be mounted in any position.

Design-Center Ratings

The principal design-center ratings for the 8BQ5 are:

Tung-Sol, Westinghouse, RCA, and Sylvania all publish the principal 300 V / 12 W design-center rating set. A special Westinghouse and Sylvania condition reduces plate and screen voltage to 250 V and plate dissipation to 9 W where the supply is obtained from a storage battery using a vibrator.

The plate and screen dissipations are separate limits. Screen current can rise significantly at large signal levels; output-stage design must therefore account for screen-grid heating as well as plate dissipation.

Typical Electrical Characteristics

Tung-Sol specifies the following typical pentode operating point:

The relatively high transconductance contributes to high audio power sensitivity: the output-stage current can be controlled with a comparatively modest audio drive voltage. The pentode's dynamic plate resistance is much higher than the typical effective output-transformer load.

The quoted amplification factor is a grid-to-grid characteristic given in the pentode sheet, not a conventional triode plate-voltage amplification factor.

Single-Ended Class A Pentode Amplification

Tung-Sol provides multiple single-tube Class A conditions at approximately 250 V plate voltage, varying screen voltage, cathode resistance, and output load. Representative operating conditions include:

The data show that a 5.7 W single-ended output figure depends on a particular supply and load. Lower screen voltage or altered bias changes plate current, transconductance, output power, and distortion.

Westinghouse provides additional load choices of 4.5 kΩ and 7 kΩ, along with harmonic-distortion components. Its detailed tables demonstrate how second- and third-harmonic contributions change as output power and load are varied.

Class AB Push-Pull Amplification

A pair of 8BQ5 pentodes can operate in Class AB push-pull through an output transformer. The manufacturer examples specify an 8,000 Ω plate-to-plate load and 130 Ω common cathode resistor.

These are the output figures repeatedly published for the BQ5 audio-output family. They describe a pair of tubes, not a single 8BQ5. The 8,000 Ω load is the full plate-to-plate transformer impedance, and the current values shown are totals for the two valves.

Class B Push-Pull Amplification

Tung-Sol and Westinghouse also document Class B operation of two 8BQ5 tubes. At plate and screen voltages of 250 V or 300 V, typical control-grid bias values are −11.6 V and −14.7 V, respectively. Both cases use an 8,000 Ω plate-to-plate load, with rated outputs of 11 W and 17 W at approximately 3% and 4% total harmonic distortion.

The difference between Class B and Class AB is important even when their maximum output figures match. The operating tables specify different idle currents and bias conditions. They are separate circuit designs rather than interchangeable descriptions of the same bias setting.

Triode-Connected Audio Output

The 8BQ5 may be operated in triode connection, with the screen grid connected to the plate. This changes the effective operating characteristic while retaining the same physical pentode structure.

Tung-Sol documents a single-tube Class A triode-connected example using 250 V plate voltage, 270 Ω cathode resistance, and approximately 3.5 kΩ plate load. It produces approximately 1.95 W with 9% total harmonic distortion under the published conditions.

For two triode-connected 8BQ5 tubes in Class AB push-pull, the sheet specifies a 270 Ω common cathode resistor and 10 kΩ plate-to-plate load. The representative outputs are 3.4 W at 250 V and 5.2 W at 300 V, both at approximately 2.5% distortion.

These triode-connected examples illustrate a substantial reduction in maximum output compared with conventional pentode operation, accompanied by a different impedance and distortion characteristic.

Ultra-Linear Connection

Sylvania also publishes a Class AB1 ultra-linear family example with 300 V plate voltage, 270 Ω cathode resistance per tube, 8 kΩ plate-to-plate load, and approximately 11 W output. This is a distinct output-stage connection, separate from fixed-screen pentode operation and direct screen-to-plate triode connection.

Interelectrode Capacitances

The manufacturer sheets give approximate direct capacitances:

The small grid-to-plate capacitance is relevant to signal feedback and stability. Input and output capacitance measurements include the electrode groups specified in the manufacturer's measurement definitions.

Characteristic Curves and Practical Interpretation

Tung-Sol and Westinghouse provide plate-characteristic and transfer curves, along with output-power, plate-current, screen-current, input-drive, and harmonic-distortion graphs. The sheets include separate graphical treatments for pentode and triode connection and for Class A, Class AB, and Class B operation.

These curves show why screen voltage, grid bias, transformer impedance, and input signal must be considered together. They also document that maximum-signal screen current is considerably higher than the quiescent screen current in the high-output push-pull examples.

Relationship to 6BQ5 / EL84 and 10BQ5

RCA explicitly states that the 8BQ5 and 10BQ5 correspond to the 6BQ5/EL84 except for heater ratings:

The shared audio characteristics explain why these types are grouped in manufacturer output-stage tables, but the different heater requirements prevent unqualified electrical substitution. In particular, the 8BQ5's controlled warm-up characteristic and 600 mA current rating make it especially relevant to series-heater television and receiver designs.

Technical Summary

The 8BQ5 is an 8.0 V / 0.60 A controlled-warm-up, nine-pin miniature audio output pentode, with 9CV basing, 12 W plate dissipation, 2 W continuous screen dissipation, and approximately 11.3 mA/V transconductance at its standard operating point.

It is documented for single-ended Class A audio output, Class AB and Class B push-pull amplification, triode-connected output stages, and ultra-linear operation. Typical manufacturer examples provide 5.7 W from one tube and 17 W from two tubes in push-pull, subject to the specified operating conditions and distortion.

Its combination of high power sensitivity, controlled heater warm-up, and compatibility with 600 mA series-heater arrangements distinguishes the 8BQ5 within the 6BQ5/EL84 family of miniature audio output valves.