11HM7 — Sharp-Cutoff Frame-Grid Pentode

The 11HM7 is a miniature, sharp-cutoff pentode featuring frame-grid construction. It is designed primarily for use as a video output amplifier in color television receivers. The tube is characterized by its exceptionally high transconductance and robust thermal ratings, which are suited to the demanding requirements of wideband video amplification.

Mechanical Construction and Pinout

The 11HM7 is constructed in a JEDEC T6-1/2 glass envelope with a JEDEC E9-1 small-button noval 9-pin base. Its physical dimensions conform to the JEDEC 6-3 dimensional outline, with the following physical specifications:

• Maximum Overall Length: 2.625 inches (approx. 66.7 mm)
• Maximum Seated Length: 2.375 inches (approx. 60.3 mm)
• Maximum Diameter: 0.875 inches (approx. 22.2 mm)
• Operating Position: Any
• Cathode Type: Coated unipotential

The tube utilizes the 9BF terminal arrangement. The pin connections, viewed from the bottom of the base, are assigned as follows:

The internal shield and Grid No. 3 are internally tied and connected to both Pin 3 and Pin 9 to facilitate effective RF grounding and minimize parasitic coupling.

Heater Characteristics and Configurations

The 11HM7 features a tapped heater design, allowing for operation in either series or parallel heater chains. The electrical specifications for each configuration are as follows:

Series Configuration

• Nominal Heater Voltage (AC or DC): 11.0 V
• Heater Voltage Range: 9.9 V (minimum) to 12.1 V (maximum)
• Nominal Heater Current: 0.3 A (300 mA)

Parallel Configuration

• Nominal Heater Voltage (AC or DC): 5.50 V
• Heater Voltage Range: 4.95 V (minimum) to 6.05 V (maximum)
• Nominal Heater Current: 0.6 A (600 mA)

Heater-Cathode Voltage Ratings

To prevent insulation breakdown and minimize hum injection, the voltage differential between the heater and the cathode must be kept within the following design-maximum limits:

• Heater Negative with Respect to Cathode: 200 V maximum (total DC and peak)
• Heater Positive with Respect to Cathode: 100 V maximum DC, 200 V maximum (total DC and peak)
• Average Value (General): 100 V maximum

Electrical Characteristics and Typical Operating Point

Under typical Class A1 amplifier operating conditions, the 11HM7 exhibits high transconductance and moderate plate resistance. The standard operating point is defined by the following parameters:

• Plate Supply Voltage: 200 V
• Grid No. 3 Voltage: 0 V (connected to cathode or ground)
• Grid No. 2 Supply Voltage: 135 V
• Grid No. 1 Supply Voltage: 0 V
• Cathode-Bias Resistor: 47 ̩
• Plate Resistance (Approximate): 40,000 ̩ (40 k̩)
• Transconductance: 30,000
  mhos
• DC Plate Current: 30 mA
• DC Grid No. 2 Current: 5.2 mA
• Cutoff DC Grid No. 1 Voltage (for plate current of 100
  A): -4.5 V

Cathode Bias Calculation

Using the typical operating values, the total cathode current ($I_k$) is the sum of the plate current ($I_b$) and the screen-grid current ($I_{c2}$): $I_k = I_b + I_{c2} = 30\text{ mA} + 5.2\text{ mA} = 35.2\text{ mA}$

The resulting self-bias voltage ($V_k$) developed across the 47 ̩ cathode resistor is calculated as: $V_k = I_k \times R_k = 0.0352\text{ A} \times 47\ \Omega \approx 1.65\text{ V}$

Thus, the effective grid-to-cathode operating bias under these conditions is approximately -1.65 V.

Design-Maximum Ratings and Circuit Limits

The design-maximum ratings define the absolute limits within which the 11HM7 must be operated to ensure reliable performance and prevent premature failure. These ratings apply to Class A1 amplifier service:

Maximum Circuit Values

To maintain bias stability and prevent thermal runaway, the resistance in the grid-No. 1 circuit must not exceed the following limits:

• For Fixed-Bias Operation: 0.1 M̩ maximum
• For Cathode-Bias Operation: 0.25 M̩ maximum

Interelectrode Capacitances

The direct interelectrode capacitances of the 11HM7 are measured without an external shield. The typical values are:

• Grid No. 1 to Plate (Maximum): 0.15 pF (RCA documentation) or 0.16 pF (Sylvania documentation)
• Input ($g_1$ to $k, g_3 + \text{IS}, g_2, h$): 14 pF
• Output ($p$ to $k, g_3 + \text{IS}, g_2, h$): 5.0 pF

The extremely low grid-to-plate capacitance (0.15–0.16 pF maximum) is a direct benefit of the frame-grid construction and internal shielding, which minimizes Miller effect feedback and enables stable high-gain performance at video frequencies.

Analysis of Characteristic Curves

The published characteristic curves for the 11HM7 illustrate its performance as a high-performance pentode:

Plate Characteristics

The plate current ($I_b$) and screen-grid current ($I_{c2}$) curves plotted against plate voltage (at a fixed Grid No. 2 voltage of 135 V) demonstrate a sharp pentode "knee" occurring below 50 V. Above this knee, the plate current curves are highly horizontal, indicating a very high dynamic plate resistance.

• At $E_{[c1](/tubes/C1)} = 0\text{ V}$, the plate current reaches approximately 80–90 mA at typical plate voltages, while the screen current ($I_{c2}$) remains low and stable.
• As the control grid bias is stepped more negative (from 0 V down to -3.0 V), the plate current curves shift downward in regular increments, showing linear control characteristics over a wide dynamic range.

Transfer Characteristics

The transfer curves (plotted at a plate voltage of 200 V and a screen-grid voltage of 135 V) show the relationship of transconductance ($g_m$), plate current ($I_b$), and screen current ($I_{c2}$) against Grid No. 1 voltage:

• Transconductance ($g_m$): The transconductance curve rises steeply as the grid voltage becomes less negative, peaking at approximately 45,000
  mhos near $E_{[c1](/tubes/C1)} = 0\text{ V}$. At the typical operating bias of approximately -1.65 V, the transconductance is 30,000
  mhos.
• Cutoff Behavior: The plate current approaches zero asymptotically, with a sharp cutoff occurring at approximately -4.5 V for a plate current of 100
  A.

Technical Summary

The 11HM7 is a high-performance, frame-grid, sharp-cutoff pentode housed in a 9-pin miniature envelope. Designed specifically for video output amplifier service in color television receivers, it features a dual-voltage heater (11.0 V series / 5.5 V parallel) and a 9BF pinout configuration. With a typical transconductance of 30,000
mhos, a plate dissipation rating of 7 W, and exceptionally low feedback capacitance, the 11HM7 provides the high gain-bandwidth product and power handling required for demanding video applications.