19FX5 — Power Pentode Vacuum Tube

A miniature, high-transconductance power pentode designed for the output stages of audio power amplifiers.

Overview

The 19FX5 is a glass-envelope power pentode designed primarily for use in the audio output stages of electronic receivers and amplifiers. It is characterized by its high transconductance, which allows it to deliver efficient power output with relatively low input signal voltages and modest plate and screen-grid supply potentials. The 19FX5 is electrically identical to the 12FX5 and 60FX5 power pentodes, differing only in its heater ratings and characteristics.

Physical Construction and Pinout

The 19FX5 is constructed in a miniature glass envelope requiring a standard miniature 7-contact socket. The tube utilizes the 7CV basing arrangement, which provides the following pin connections (viewed from the bottom of the base):

In this physical layout, Grid No. 1 is internally connected to both Pin 1 and Pin 5. The cathode and Grid No. 3 (suppressor grid) are internally tied together and brought out to Pin 2.

Heater Characteristics and Ratings

The heater of the 19FX5 is designed for series or parallel AC or DC operation. It features a controlled warm-up time to ensure reliable operation in series-string heater circuits.

• Heater Voltage (AC or DC): 18.9 V
• Heater Current: 0.300 A (with an allowable tolerance of 0.020 A)
• Average Heater Warm-up Time: 11 seconds
• Maximum Heater-Cathode Voltage:
  • Peak: 200 V max
  • Average: 100 V max

Using the nominal heater voltage of 18.9 V and nominal heater current of 0.3 A, the calculated heater power dissipation is:

$\text{Heater Power} = 18.9\text{ V} \times 0.3\text{ A} = 5.67\text{ W}$

Interelectrode Capacitances

The approximate direct interelectrode capacitances of the 19FX5, measured without an external shield, are:

• Grid No. 1 to Plate: 0.65 pF
• Grid No. 1 to Cathode, Heater, Grid No. 2, and Grid No. 3: 17 pF
• Plate to Cathode, Heater, Grid No. 2, and Grid No. 3: 9 pF

Maximum Ratings (Design-Maximum Values)

The maximum ratings of the 19FX5 are established under the Design-Maximum system. Operating the tube beyond these limits may impair its performance or shorten its operating life.

Typical Operation and Electrical Characteristics

In typical Class A1 audio amplifier service, the 19FX5 operates with a relatively low plate supply voltage and exhibits high transconductance. The following parameters represent typical operating conditions:

Curve Observations and Behavior

The electrical behavior of the 19FX5 is illustrated by its characteristic curves. When operating at a Grid-No. 2 voltage of 115 V, the plate current and Grid-No. 2 current curves plotted against plate voltage for Grid-No. 1 voltages ranging from 0 V to -6 V display typical pentode characteristics.

Specifically, the plate current curves show a well-defined pentode "knee" occurring around 20 to 25 V. Below this knee, plate current drops rapidly as plate voltage decreases. Above this knee, the plate current curves flatten out, indicating that plate current is relatively independent of plate voltage in the normal operating region. This flat region corresponds to the high internal plate resistance of approximately 17,500 Γ.

Related Designations

The 19FX5 belongs to a family of identical power pentodes designed for different heater supply configurations. It is identical to the 12FX5 and 60FX5 vacuum tubes in its internal structure, electrode configuration, and electrical characteristics, differing only in its heater ratings. These designations should not be substituted without verifying that the circuit's heater supply matches the specific requirements of the replacement tube.

Technical Summary

The 19FX5 is a miniature 7-pin power pentode optimized for Class A1 audio power output applications. Operating with a nominal heater voltage of 18.9 V at 0.3 A, it features a controlled 11-second warm-up time suitable for series-string designs. With a design-maximum plate voltage rating of 150 V and plate dissipation of 5.5 W, it is well-suited for low-voltage, high-efficiency amplification. Its exceptionally high transconductance of 13,500 Ιumhos allows it to deliver effective power output with low grid-drive requirements, making it a highly efficient choice for compact audio output stages.