14C7 — Sharp-Cutoff Pentode

The 14C7 is a sharp-cutoff, triple-grid general-purpose pentode designed primarily for detector and amplifier service. It is particularly suited for use in AC-DC receivers employing 150 mA series-string heater tubes. Housed in a T-9 glass bulb with an 8-pin lock-in base, the 14C7 features a coated unipotential cathode and can be operated in any mounting position.

With the exception of its heater ratings, the electrical characteristics of the 14C7 are identical to those of the 7C7 pentode.

Construction, Heater, and Base Pinout

The 14C7 is constructed with a T-9 glass bulb. The physical dimensions of the tube include a maximum overall length of 2-25/32 inches, a maximum seated height of 2-1/4 inches, and a maximum diameter of 1-3/16 inches.

Heater Specifications

The 14C7 is designed for AC or DC heater operation. The technical documentation lists two distinct sets of nominal heater specifications:

• One set of specifications defines the heater at 12.6 V with a current of 0.15 A (150 mA).
• The absolute maximum ratings table lists a nominal heater voltage of 14.0 V and a nominal heater current of 0.16 A (160 mA).

Base and Pin Connections

The tube utilizes an 8-pin lock-in base (designated 8V). When viewed from the bottom of the tube, the pin connections are assigned as follows:

• Pin 1: Heater (H)
• Pin 2: Plate (P)
• Pin 3: Grid #2 / Screen Grid (G2)
• Pin 4: Grid #3 / Suppressor Grid (G3)
• Pin 5: Internal Shield (Si)
• Pin 6: Grid #1 / Control Grid (G1)
• Pin 7: Cathode (K)
• Pin 8: Heater (H)

For typical operation, the suppressor grid (Pin 4) and the internal shield (Pin 5) are connected directly to the cathode (Pin 7) at the socket.

Electrical Ratings

The maximum ratings for the 14C7, interpreted according to the RMA Standard M8-210, are established as follows:

Typical Operating Conditions and Characteristics

The 14C7 is typically operated as a Class A1 amplifier. The published operating points demonstrate its performance under low-voltage and high-voltage plate conditions:

Low-Voltage Operating Point

• Plate Voltage: 100 V
• Screen (Grid #2) Voltage: 100 V
• Control Grid (Grid #1) Voltage: -1 V
• Suppressor Grid (Grid #3) Voltage: 0 V (Pins 4 and 5 connected to Pin 7)
• Self-Bias Resistor: 130 ohms
• Plate Current: 5.7 mA
• Screen Current: 1.8 mA
• Approximate Plate Resistance: 0.325 Megohm to 0.4 Megohm
• Transconductance: 2275 umhos
• Grid Voltage for Cathode Current Cutoff: -7 V

High-Voltage Operating Point

• Plate Voltage: 250 V
• Screen (Grid #2) Voltage: 100 V
• Control Grid (Grid #1) Voltage: -3 V
• Suppressor Grid (Grid #3) Voltage: 0 V (Pins 4 and 5 connected to Pin 7)
• Self-Bias Resistor: 2000 ohms
• Plate Current: 2.2 mA
• Screen Current: 0.7 mA
• Approximate Plate Resistance: 1.0 Megohm
• Transconductance: 1575 umhos
• Grid Voltage for Cathode Current Cutoff: -7 V

Direct Interelectrode Capacitances

When measured with an external shield connected to the cathode, the capacitances are:

• Control Grid to Plate (G1 to P): 0.007 uuf (maximum)
• Input (G1 to H+K+G2+G3+IS): 6.0 uuf
• Output (P to H+K+G2+G3+IS): 6.5 uuf

Characteristic Curves and Interpretation

The published characteristic curves for the 14C7 (and the electrically identical 7C7) provide a detailed view of the tube's behavior across a range of voltages:

Plate Current vs. Plate Voltage

The plate characteristics show that at a fixed screen voltage (Ec2) of 100 V and suppressor voltage (Ec3) of 0 V, the plate current (Ib) rises very sharply as plate voltage increases from 0 V to approximately 50 V. Above 50 V, the curves flatten out, which is characteristic of a high plate resistance pentode. For example, with a control grid voltage (Ec1) of -1.5 V, the plate current remains nearly constant at approximately 4.0 mA across a plate voltage range of 100 V to 500 V.

Screen Current vs. Plate Voltage

The screen current (Ic2) curves show that screen current is highest at very low plate voltages (where the plate is less effective at collecting electrons) and drops to a stable, low level once the plate voltage exceeds approximately 75 V to 100 V. At Ec1 = 0 V, the screen current drops from over 2.0 mA at low plate voltages to a steady state of about 1.7 mA at higher plate voltages.

Transfer Characteristics

The transfer curves plot transconductance (gm), plate current (Ib), screen current (Ic2), and plate resistance (rp) against control grid voltage (Ec1):

• At 250 V Plate Voltage: As the control grid bias is made more negative, the transconductance drops from 2275 umhos at -1 V to 1575 umhos at -3 V, eventually reaching cutoff at -7 V. The plate resistance (rp) rises from approximately 0.3 Megohm at -1 V bias to 1.0 Megohm at -3 V bias, and continues to rise rapidly toward several megohms as the grid bias approaches cutoff.
• At 100 V Plate Voltage: The curves exhibit a similar trend, with the transconductance and plate current falling smoothly to zero at the -7 V cutoff point, while the plate resistance rises sharply.

Technical Summary

The 14C7 is an 8-pin lock-in, sharp-cutoff pentode designed for general-purpose detector and amplifier service. It features a unipotential cathode and a heater rated at either 12.6 V / 0.15 A or 14.0 V / 0.16 A depending on the rating standard applied. Electrically identical to the 7C7 except for its heater requirements, the 14C7 is optimized for series-string AC-DC receiver designs. Under typical Class A1 operating conditions with a 250 V plate supply and a 100 V screen supply, it provides a transconductance of 1575 umhos and a plate resistance of 1.0 Megohm, while drawing a plate current of 2.2 mA and a screen current of 0.7 mA.