17AX3 — Half-Wave Vacuum Rectifier

The 17AX3 is a half-wave vacuum rectifier designed primarily for service as a damping diode (damper tube) in the horizontal-deflection circuits of television receivers. It is constructed as a Compactron (Duodecar) type tube with a coated unipotential cathode. The 17AX3 is electrically identical to the 6AX3 and 12AX3 vacuum tubes, differing only in its heater ratings and characteristics.

Construction, Physical Dimensions, and Installation

The 17AX3 features a Compactron construction utilizing a T-9 glass bulb outline (JEDEC 9-59) and a button 12-pin base (E12-70). This design provides a compact and robust envelope capable of withstanding the high-voltage pulses typical of television deflection systems.

The physical dimensions of the tube are as follows:

• Maximum Overall Length: 2.625 inches (or approximately 2.596 inches depending on the specific manufacturing outline)
• Maximum Seated Height: 2.250 inches
• Maximum Diameter: 1.188 inches

The tube can be mounted in any position. Because of the thermal dissipation associated with damper service, the equipment layout must provide adequate ventilation around the tube envelope to prevent overheating.

Heater Characteristics and Warm-up

The 17AX3 heater is designed for series-string or parallel operation, with controlled warm-up characteristics to minimize voltage surges in series-connected heater strings.

The average heater warm-up time is defined under specific test conditions: it is the time required for the voltage across the heater to reach 80% of its nominal (bogey) value (16.8 V) when four times this bogey voltage is applied to a circuit consisting of the tube heater in series with an external resistance. This series resistance is calculated as three times the bogey heater voltage divided by the bogey heater current:

$\text{Series Resistance} = 3 \times \frac{16.8\text{ V}}{0.45\text{ A}} = 112\ \Omega$

Equipment designers must ensure that the nominal operating heater current is centered at the 0.45 A bogey value, and supply variations must be restricted to keep the heater current within the specified ±30 mA tolerance.

Pinout and Basing Configurations

The 17AX3 utilizes a 12-pin button base. Depending on the manufacturer and production run, two distinct basing diagrams—12BL and 12FX—are documented for this tube.

Basing Diagram 12BL

In the 12BL configuration, the connections are as follows:

• Pin 1: Heater (H)
• Pin 2: No Connection (NC)
• Pin 3: No Connection (NC)
• Pin 4: Plate (P) or Internal Connection (IC)
• Pin 5: No Connection (NC) or Internal Connection (IC) — Do not use as a tie point
• Pin 6: No Connection (NC) or Internal Connection (IC) — Do not use as a tie point
• Pin 7: Cathode (K)
• Pin 8: No Connection (NC) or Internal Connection (IC) — Do not use as a tie point
• Pin 9: No Connection (NC) or Internal Connection (IC) — Do not use as a tie point
• Pin 10: Plate (P)
• Pin 11: No Connection (NC)
• Pin 12: Heater (H)

Note on 12BL: Pins 5, 6, 8, and 9 must not be used as circuit tie points, even if they are designated as No Connection or Internal Connection, to prevent voltage breakdown or parasitic coupling.

Basing Diagram 12FX

In the alternative 12FX configuration, the connections are mapped differently:

• Pin 1: Internal Connection (IC)
• Pin 2: Plate (P)
• Pin 3: No Connection (NC)
• Pin 4: Heater (H)
• Pin 5: Heater (H)
• Pin 6: No Connection (NC)
• Pin 7: Cathode (K)
• Pin 8: Internal Connection (IC)
• Pin 9: Internal Connection (IC)
• Pin 10: Plate (P)
• Pin 11: No Connection (NC)
• Pin 12: No Connection (NC)

Electrical Characteristics and Capacitances

The 17AX3 is characterized by low internal resistance and high heater-cathode insulation, which are essential for its role as a damper diode.

Direct Interelectrode Capacitances

The capacitances of the tube, measured without an external shield, are:

• Plate to Cathode and Heater [P to (K+H)]: 5.5 pF
• Cathode to Plate and Heater [K to (P+H)]: 7.5 pF
• Heater to Cathode [H to K]: 2.8 pF

Typical Operating Characteristics

Under typical operating conditions, the tube exhibits a low forward voltage drop:

• Tube Voltage Drop: 32 V (measured at a direct plate current of 250 mA)

Plate Characteristic Curves

The plate current ($I_b$) versus plate voltage ($E_b$) relationship at the nominal heater voltage of 16.8 V displays a smooth, non-linear, exponential-like curve. This behavior is characteristic of space-charge-limited diode conduction:

• At $0\text{ V}$ plate potential, the plate current is $0\text{ mA}$.
• At approximately $18\text{ V}$, the current rises to $100\text{ mA}$.
• At approximately $36\text{ V}$, the current reaches $300\text{ mA}$.
• At approximately $50\text{ V}$, the current reaches $500\text{ mA}$.
• At approximately $67\text{ V}$, the current reaches $750\text{ mA}$.

This low forward impedance ensures high efficiency and minimal power loss during the conduction phase of the horizontal deflection cycle.

Maximum Ratings and Damper Service

The ratings of the 17AX3 are defined under the Design-Maximum Rating System (EIA Standard RS-239) for television damper service in a 525-line, 30-frame scanning system. These limits must not be exceeded under the worst-case operating conditions.

Pulse Duty Cycle Constraints

The peak inverse plate voltage rating of 5000 V is a pulsed rating. The duty cycle of this voltage pulse must not exceed 15% of one horizontal scanning cycle (which corresponds to a maximum pulse duration of 10 microseconds in a standard television receiver).

Heater-Cathode Voltage Limits

Because the cathode of a damper diode is subjected to high-voltage pulses from the deflection yoke, the 17AX3 is designed with robust heater-cathode insulation. The maximum allowable voltages between the heater and the cathode are:

• Heater Negative with Respect to Cathode:
  • DC Component: 900 V
  • Total DC and Peak Value: 5000 V
• Heater Positive with Respect to Cathode:
  • DC Component: 100 V
  • Total DC and Peak Value: 300 V

These ratings allow the heater to be operated near ground potential even when the cathode experiences severe high-voltage transients during horizontal retrace.

Technical Summary

The 17AX3 is a Compactron-type, half-wave vacuum rectifier designed specifically for television horizontal damper service. Operating with a 16.8 V, 0.45 A heater, it features controlled warm-up characteristics and high heater-cathode insulation capable of withstanding peak negative pulses up to 5000 V. With a low forward voltage drop of 32 V at 250 mA and a maximum plate dissipation of 5.3 W, the 17AX3 provides efficient energy recovery and damping in television deflection systems. It is physically housed in a T-9 glass envelope with a 12-pin button base, supporting either the 12BL or 12FX basing configurations.