15MF8 — High-Mu Triode and Beam Power Pentode Compactron

A multi-unit compactron vacuum tube containing a high-mu triode and a beam power pentode, designed specifically for vertical-deflection oscillator and vertical-deflection amplifier applications in color television receivers.

Overview

The 15MF8 is a multi-unit vacuum tube of the compactron configuration containing a triode and a beam power pentode (also described as a beam power tube) in a single envelope. The triode section is designed for service as a vertical-deflection oscillator, and the pentode section is designed for service as a vertical-deflection amplifier in color television receivers.

The 15MF8 is electrically identical to the 6MF8 except for its heater characteristics and ratings. It utilizes a 12-pin compactron envelope and requires a standard duodecar 12-contact socket.

Construction, Physical Dimensions, and Basing

The 15MF8 features a Compactron T-12 glass envelope construction with a button 12-pin base (designation E12-74). The physical dimensions of the tube are:

• Maximum Diameter: 1.562 inches
• Maximum Seated Height: 2.750 inches
• Maximum Overall Height: 3.125 inches

Pinout and Basing Connection (12DZ)

The tube utilizes the 12DZ basing arrangement. Viewed from the underside of the base, the pin connections are as follows:

Pins 6 and 7 are connected internally, and Pin 9 connects both the cathode and the beam-deflecting electrodes (Grid No. 3) of the beam power unit.

Heater Characteristics and Ratings

The 15MF8 is designed for series-string heater applications. Its heater is designed to operate with either AC or DC power. The heater characteristics are:

• Heater Voltage (AC or DC): 14.7 V (for a bogey tube at If = 0.6 A)
• Heater Current: 0.6 ± 0.04 A (or 600 mA)
• Average Heater Warm-up Time: 11 seconds

The average heater warm-up time is defined as the time required for the voltage across the heater to reach 80% of its bogey value (11.76 V) when 4 times the bogey heater voltage (58.8 V) is applied to a circuit consisting of the tube heater in series with a resistance equal to 3 times the bogey heater voltage divided by the bogey heater current. This series resistance is calculated as:

$\text{Series Resistance} = 3 \times \frac{14.7\text{ V}}{0.6\text{ A}} = 73.5\ \Omega$

Heater-Cathode Voltage Ratings

The maximum heater-cathode voltage ratings (Design-Maximum Values) are:

• Heater Negative with Respect to Cathode:
  • Total DC and Peak: 200 V
• Heater Positive with Respect to Cathode:
  • DC: 100 V
  • Total DC and Peak: 200 V

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 15MF8 are measured without an external shield. The typical values are:

Triode Section

• Grid to Plate (g to p): 6.0 pF
• Input (g to k + h): 6.5 pF
• Output (p to k + h): 1.6 pF

Pentode (Beam Power) Section

• Grid No. 1 to Plate (g1 to p): 0.54 pF (Maximum)
• Input (g1 to k + g2 + g3 + h): 9.5 pF
• Output (p to k + g2 + g3 + h): 7.0 pF

Inter-Section Coupling Capacitances

• Pentode Grid No. 1 to Triode Plate (g1B to pT): 0.12 pF (Maximum)
• Pentode Plate to Triode Plate (pB to pT): 0.32 pF (Maximum)

Electrical Characteristics and Class A1 Operation

The 15MF8 contains two distinct electrical systems: a high-mu (or medium-mu) triode and a beam power pentode. The typical operating characteristics of each section under Class A1 amplifier conditions are:

Instantaneous Plate Knee Values

For the beam power unit, the instantaneous plate knee values under specific conditions are:

• Plate Voltage (Eb): 60 V
• Grid No. 2 Voltage (Ec2): 250 V
• Grid No. 1 Voltage (Ec1): 0 V
• Plate Current (Ib): 200 mA
• Grid No. 2 Current (Ic2): 20 mA

Vertical-Deflection Service Ratings

The 15MF8 is rated under the Design-Maximum Rating System for operation in a 525-line, 30-frame television system. The maximum ratings and circuit values for both sections are:

Pulse Duration and Circuit Protection

• Pulse Duration: For the peak positive pulse plate voltage rating of 2500 V on the beam power unit, the pulse duration must not exceed 15% of a vertical scanning cycle, which corresponds to 2.5 milliseconds.
• Excitation Protection: In stages operating with grid-leak bias, an adequate bias resistor or other suitable means is required to protect the tube in the absence of excitation.
• Grid Circuit Resistance Limits:
  • For Fixed-Bias Operation: Beam Power Unit (Amplifier) = 1.0 M$\Omega$ (Maximum)
  • For Cathode-Bias Operation: Triode Unit (Oscillator) = 2.2 M$\Omega$ (Maximum); Beam Power Unit (Amplifier) = 2.2 M$\Omega$ (Maximum)

Technical Summary

The 15MF8 is a high-performance Compactron T-12 multi-unit tube containing a high-mu triode and a beam power pentode, specifically optimized for vertical-deflection oscillator and amplifier applications in color television receivers. It features a series-string heater rated at 14.7 V / 0.6 A with a controlled 11-second warm-up time, and utilizes a 12-pin duodecar base with a 12DZ pinout configuration.

The triode section provides an amplification factor of 58 and a transconductance of 4100 µmhos under typical Class A1 conditions, while the beam power section offers a plate dissipation rating of 12 W and can withstand peak positive plate pulses up to 2500 V in vertical deflection service. The 15MF8 is identical to the 6MF8 except for its heater voltage and current ratings.