17CU5 — Beam Power Pentode

The 17CU5 (also designated as 17CU5/17C5) is a miniature 7-pin beam power pentode designed primarily for audio power amplifier service. It is widely utilized in the audio output stages of television receivers (including color television sets) as well as AM and FM radio receivers. The tube is engineered to deliver high power sensitivity and high plate efficiency while operating at relatively low plate and screen-grid voltages.

Overview and Related Designations

The 17CU5 is electrically identical to the 6CU5 and 12CU5/12C5 vacuum tubes, differing only in its heater ratings and characteristics. It features a compact physical design and is optimized for series-connected heater strings, incorporating controlled warm-up characteristics to minimize voltage surges during receiver start-up.

Mechanical Construction, Base, and Pinout

The 17CU5 is constructed in a T-5 1/2 glass envelope with a miniature Button 7-Pin base (JETEC E7-1, outline 5-3). The tube can be mounted and operated in any position.

Physical Dimensions

• Maximum Diameter: 0.750 inches
• Maximum Seated Height: 2.375 inches
• Maximum Overall Height: 2.625 inches

Pin Connections (7CV Basing)

The 7CV basing arrangement provides the following pin connections, viewed from the bottom of the tube:

Note: Grid No. 1 is internally connected to both Pin 1 and Pin 5.

Heater and Cathode Characteristics

The tube utilizes a coated unipotential cathode. The heater is designed for series string operation with a controlled warm-up time.

• Heater Voltage (AC or DC): 16.8 V
• Heater Current: 0.45 A (with a tolerance of ±6% or ±0.030 A)
• Heater Warm-up Time (Average): 11 seconds
• Heater Power (Calculated): Using the nominal voltage of 16.8 V and nominal current of 0.45 A, the calculated heater power consumption is 16.8 V * 0.45 A = 7.56 W.

The heater warm-up time is defined as the time required for the heater voltage to reach 80% of its rated value when four times the rated heater voltage (67.2 V) is applied to the heater in series with an external resistance equal to three times the rated voltage divided by the rated current.

Heater-Cathode Voltage Limits

To prevent insulation breakdown, the voltage difference between the heater and the cathode must be kept within the following limits:

• Heater Negative with Respect to Cathode: 200 V maximum (Total DC + Peak)
• Heater Positive with Respect to Cathode:
  • DC Component: 100 V maximum
  • Total DC + Peak: 200 V maximum

Direct Interelectrode Capacitances

The approximate direct interelectrode capacitances of the 17CU5, measured without an external shield, are:

• Grid No. 1 to Plate: 0.6 pF
• Input (Grid No. 1 to Cathode, Heater, Grid No. 2, and Grid No. 3): 13 pF
• Output (Plate to Cathode, Heater, Grid No. 2, and Grid No. 3): 8.5 pF

Maximum Ratings and Circuit Values

The maximum ratings of the 17CU5 are published under both the Design-Maximum and Design-Center rating systems. These values represent limits that should not be exceeded under the worst-case operating conditions.

Operating Environment and Thermal Considerations

At the maximum bulb temperature of 220 °C, high ambient temperatures or restrictive physical shielding may require a reduction in operating dissipation to prevent thermal runaway or envelope damage. If shields are used, they should be ventilated and painted dull black on both the interior and exterior surfaces to maximize heat radiation.

Maximum Grid No. 1 Circuit Resistance

To maintain bias stability, the resistance in the control grid circuit must not exceed the following limits:

• For Fixed-Bias Operation: 0.1 MΩ
• For Cathode-Bias Operation: 0.5 MΩ

Typical Operation and Electrical Characteristics

In typical Class A1 audio frequency power amplifier service, the 17CU5 operates under the following nominal conditions:

• Plate Voltage: 120 V
• Grid No. 2 Voltage: 110 V
• Grid No. 1 Voltage: -8.0 V
• Peak AF Grid No. 1 Voltage: 8.0 V
• Zero-Signal Plate Current: 49 mA
• Maximum-Signal Plate Current: 50 mA
• Zero-Signal Grid No. 2 Current: 4.0 mA
• Maximum-Signal Grid No. 2 Current: 8.5 mA
• Plate Resistance (Approximate): 10,000 Ω
• Transconductance: 7,500 µmhos
• Load Resistance: 2,500 Ω
• Total Harmonic Distortion (Approximate): 10%
• Maximum-Signal Power Output: 2.3 W

Curve Observations and Alternative Operating Points

Average Plate Characteristics

When operating with a screen-grid (Grid No. 2) voltage of 110 V, the plate current curves display a typical pentode knee occurring between 20 V and 40 V. At a control grid bias (Ec1) of 0 V, the plate current peaks near 135 mA. Under this same 0 V bias condition, the screen-grid current (Ic2) drops significantly as plate voltage increases, falling from approximately 50 mA at low plate potentials to about 15 mA at a plate voltage of 250 V.

Alternative Operating Conditions

Under an alternative operating point with the plate and screen-grid both set to 110 V, a control grid bias of -7.5 V, and an RMS signal input of 5.3 V, the performance varies as follows:

• Power Output: Peaks at approximately 1.9 W when using a load resistance of 2,200 Ω.
• Total Harmonic Distortion: Reaches a minimum of approximately 4.5% at a 2,200 Ω load, but rises sharply if the load resistance is increased or decreased from this value.
• Current Variations: As the load resistance is increased, the plate current decreases while the screen-grid current increases.

Technical Summary

The 17CU5 is a high-sensitivity, high-efficiency miniature beam power pentode designed for low-voltage audio output stages in television and radio receivers. Operating with a 16.8 V, 450 mA heater, it is well-suited for series-string heater configurations. With a maximum plate dissipation of 7.0 W (Design-Maximum) and a typical Class A1 power output of 2.3 W at 120 V plate voltage, the 17CU5 provides robust audio performance in a compact 7-pin package.