19AU4 — Half-Wave Vacuum Rectifier and Damper Diode

The 19AU4 is a single, indirectly heated half-wave vacuum rectifier designed specifically for television horizontal frequency damper service. It is engineered to withstand high-voltage pulses of line frequency between its cathode and both its heater and plate elements, making it highly suitable for "direct drive" circuits. The tube is commonly utilized as a damper diode in the horizontal-deflection circuits of black-and-white television receivers operating with series-heater strings.

A closely related variant, the 19AU4GTA, shares identical electrical and physical characteristics with the standard 19AU4, except for its higher maximum DC output and peak plate current ratings.

Mechanical Specifications and Pinout

The 19AU4 is constructed with a T-9 glass bulb envelope and can be mounted in any position. It features a short intermediate-shell octal base with either 5 or 6 pins, incorporating external barriers (JETEC designations B5-85 or B6-60).

Physical Dimensions

• Maximum Overall Length: 3-3/8 to 3-13/16 inches
• Maximum Seated Length: 3-1/4 inches
• Maximum Diameter: 1-9/32 inches

Pin Connections

The tube utilizes the 4CG bottom-view pinout configuration. The electrical connections are assigned as follows:

To prevent high-voltage arc-over and minimize electrical leakage, socket terminals #1, #2, #4, and #6 must not be used as tie points. It is recommended that socket clips for these unused pins be completely removed from the tube socket.

Heater Ratings and Controlled Warm-Up

The 19AU4 features a coated unipotential cathode heated by a filament designed for series-string applications.

• Heater Voltage (AC or DC): 18.9 V
• Heater Current: 0.6 A (600 mA)

Controlled Warm-Up Time

The heater is designed with a controlled warm-up time of approximately 11.0 seconds to ensure reliable operation in series-heater strings. This warm-up characteristic is defined by a specific test circuit where a supply voltage of four times the rated heater voltage is applied in series with a resistance equal to three times the nominal heater operating resistance. The warm-up time is the duration required for the voltage across the heater to reach 80% of its rated value (15.1 V).

• Calculated Nominal Heater Resistance: $R_{\text{heater}} = \frac{V_{\text{heater}}}{I_{\text{heater}}} = \frac{18.9\text{ V}}{0.6\text{ A}} = 31.5\ \Omega$
• Calculated Test Circuit Supply Voltage: $V_{\text{supply}} = 4 \times 18.9\text{ V} = 75.6\text{ V}$
• Calculated Test Circuit Series Resistance: $R_{\text{series}} = 3 \times 31.5\ \Omega = 94.5\ \Omega$

When operating the tube in series-heater strings, the use of a series resistor—preferably one with a negative temperature coefficient—is recommended to minimize initial voltage surges during startup.

Electrical Ratings and Damper Service Limits

The ratings of the 19AU4 and 19AU4GTA are defined under the Design-Center System, except where noted as Absolute Maximum values. These limits are specified for operation in a standard 525-line, 30-frame television system (FCC standards), where the duty cycle of the horizontal voltage pulse does not exceed 15% of the scanning cycle (or a maximum duration of 10 Ιs).

Operating Limits

Heater-Cathode Voltage Ratings

Because the damper diode cathode is subjected to high-voltage pulses during horizontal retrace, the heater-cathode insulation is heavily reinforced:

• Heater Negative with Respect to Cathode:
  • DC Component: 900 V maximum
  • Total DC and Peak: 4500 V (Absolute Maximum)
• Heater Positive with Respect to Cathode:
  • DC Component: 100 V maximum
  • Total DC and Peak: 300 V (Design-Center)

Equipment designs must ensure that these Design-Center ratings are not exceeded under normal line-voltage variations (typically 117 V line-center ±10%). Absolute Maximum ratings must never be exceeded under any conditions of supply-voltage, load, or manufacturing variations.

Typical Characteristics and Plate Curves

Under typical operating conditions, the 19AU4 exhibits a low internal voltage drop, which maximizes efficiency in horizontal deflection circuits.

• Average Tube Voltage Drop: 25 V (measured with the tube conducting a forward current of 350 mA).

Direct Interelectrode Capacitances

Measured without an external shield, the approximate capacitances are:

• Heater to Cathode ($H$ to $K$): 4.0 pF
• Plate to Cathode and Heater ($P$ to $[H+K]$): 8.5 pF
• Cathode to Plate and Heater ($K$ to $[P+H]$): 11.5 pF

Plate Characteristics

The average plate characteristics at a heater voltage of 18.9 V follow a typical diode space-charge curve. The conduction begins smoothly at 0 V:

• At 10 V plate voltage, the plate current is approximately 75 mA.
• At 20 V plate voltage, the plate current rises to a range of 200 to 250 mA.
• At 30 V plate voltage, the plate current is approximately 450 mA.
• At 40 V plate voltage, the plate current reaches approximately 600 to 700 mA.
• At 50 V plate voltage, the plate current reaches approximately 1000 mA (represented as a dashed curve above 250 mA in some characterizations).
• At approximately 60 V plate voltage, the plate current reaches its maximum rated peak value of 1050 mA.

Technical Summary

The 19AU4 and its high-current counterpart, the 19AU4GTA, are specialized half-wave vacuum rectifiers housed in T-9 glass envelopes with octal bases. Designed specifically for television damper service, they feature an 18.9 V, 0.6 A heater with a controlled 11-second warm-up time optimized for series-string operation. The tube is characterized by its ability to withstand peak inverse voltages up to 4500 V and peak heater-cathode voltages up to 4500 V (heater negative). With a low typical voltage drop of 25 V at 350 mA, the 19AU4 provides efficient damping performance in television horizontal-deflection circuits.