12MD8 — Medium-Mu Triple Triode

The 12MD8 is a Novar-type vacuum tube containing three electrically distinct triode units that share a common cathode within a single T-9 glass envelope. Designed primarily for use in the matrixing circuits of color and black-and-white television receivers, it functions as a chroma matrixing amplifier. The 12MD8 is identical to the 6MD8 except for its heater ratings and heater arrangement.

Construction, Heater, and Common Cathode

The 12MD8 features a Novar T-9 construction with a button 9-pin base (E9-75 or E9-89). Unlike tubes with independent cathodes, the three triode units in the 12MD8 share a single, common cathode. This structural design simplifies wiring in matrixing applications but requires all three sections to operate at the same cathode potential.

The heater is designed for series string operation with a controlled warm-up time.

To ensure reliable operation and prevent insulation breakdown, the voltage difference between the heater and the common cathode must be kept within specified limits:

• Heater negative with respect to cathode: The maximum total DC and peak voltage is 200 V.
• Heater positive with respect to cathode: The maximum DC voltage is 100 V, and the maximum total DC and peak voltage is 200 V (alternatively specified as an average value of 100 V and a peak value of 200 V).

Pinout and Physical Dimensions

The 12MD8 utilizes the 9RQ basing diagram, which distributes the connections for the three plates, three grids, common cathode, and heater across the nine pins of the Novar base:

The physical dimensions of the T-9 envelope are as follows:

• Maximum Diameter: 1.188 inches
• Maximum Seated Height: 2.580 inches
• Maximum Overall Height: 2.960 inches

Electrical Characteristics and Class A1 Operation

Each of the three triode units in the 12MD8 is designed as a medium-mu amplifier. Under typical Class A1 operating conditions, each section exhibits the following electrical characteristics:

• Plate Voltage: 250 V
• Grid Voltage: -10.5 V
• Plate Current: 11.5 mA
• Amplification Factor (μ): 17
• Plate Resistance (rₐ): 5500 ̩ (approximate)
• Transconductance (gₘ): 3100 µmhos

The fundamental triode relationship is demonstrated by this typical calculation:

Calculated Amplification Factor = 0.0031 S x 5500 ohms = 17.05

This calculated value closely matches the specified amplification factor of 17.

The cutoff characteristics of each triode section are also defined:

• At a grid voltage of -14 V, the plate current decreases to 4 mA.
• For a plate current of 50 µA, the required grid voltage is approximately -23 V.

Maximum Ratings and Circuit Values

The 12MD8 is rated under the Design-Maximum Rating System. These limits must not be exceeded under the worst-case operating conditions:

• Maximum Plate Voltage: 330 V
• Maximum Grid Voltage (Positive-bias value): 0 V
• Maximum Plate Dissipation (per section): 3 Watts
• Maximum Grid-Circuit Resistance (for fixed-bias operation): 1 Megohm

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 12MD8 are measured without an external shield. Units 1 and 2 are identical in their capacitive characteristics, while Unit 3 exhibits slight differences due to its physical positioning within the envelope:

Average Plate Characteristics

The average plate characteristics of each triode unit illustrate the relationship between plate voltage, grid bias, and plate current.

At a grid bias of 0 V, the plate current rises rapidly with plate voltage, reaching approximately 30 mA at 140 V and 40 mA at 170 V. As the grid is biased more negatively, the curves shift to the right. For example, at a grid bias of -10.5 V, the plate current is 11.5 mA when the plate voltage is 250 V. At a grid bias of -20 V, conduction begins around 200 V on the plate, and the plate current reaches 10 mA at approximately 370 V.

Technical Summary

The 12MD8 is a Novar-based, medium-mu triple triode featuring a common cathode and a 12.6 V, 0.45 A series heater with an average warm-up time of 11 seconds. It is specifically designed for chroma matrixing amplifier service in color and black-and-white television receivers. Each of its three triode units provides an amplification factor of 17, a transconductance of 3100 µmhos, and a plate resistance of 5500 ̩ under typical Class A1 operating conditions. With a maximum plate voltage rating of 330 V and a plate dissipation limit of 3 Watts per section, the 12MD8 offers a compact and efficient solution for television matrixing circuits requiring three matched medium-mu triode stages sharing a common cathode potential.