13GC8 — Triode-Beam Tetrode Vacuum Tube

The 13GC8 is a multi-section vacuum tube containing a triode and an output beam tetrode within a single envelope. Each section is constructed with its own independent cathode, allowing separate biasing and circuit configurations. The tube is designed primarily for use in the audio amplifier or frame time-base (vertical deflection) stages of television receivers operating with series-connected heater chains on AC/DC mains power supplies.

Construction, Heater, and Physical Specifications

The 13GC8 is housed in a clear T6 1/2 glass bulb with a standard 9-pin Noval (B9A, E9-1) base. The physical layout is compact, designed to fit standard television chassis layouts while providing adequate heat dissipation.

The heater is designed for series-string operation, drawing a constant current. The heater and physical dimensions are characterized by the following parameters:

Pin Connections (9KZ)

The 13GC8 uses the 9KZ pinout configuration, viewed from the free end of the pins:

Electrical Characteristics and Maximum Ratings

The maximum ratings of the 13GC8 must be strictly observed to ensure reliable operation and prevent premature failure. The tube features a shared maximum anode dissipation limit that restricts the simultaneous operation of both sections at their individual peak limits.

* Note on Anode Dissipation: The total combined anode dissipation of both sections must not exceed 6.0 W. ** Note on Heater-to-Cathode Voltage: Measured with respect to the higher potential heater pin.

Section Characteristics

At typical operating points, the individual sections exhibit the following electrical characteristics:

• Triode Section (measured at $V_a = 200\text{ V}$, $I_a = 10\text{ mA}$):

  • Amplification Factor ($\mu$): 18
  • Anode Impedance ($r_a$): 5,300 $\Omega$
  • Mutual Conductance ($g_m$): 3.4 mA/V
  • Calculation: The relationship $\mu \approx g_m \times r_a$ is verified by $0.0034\text{ S} \times 5,300\ \Omega = 18.02$.

• Tetrode Section (measured at $V_a = 170\text{ V}$, $V_{[g2](/tubes/G2)} = 180\text{ V}$, $I_a = 32\text{ mA}$):

  • Mutual Conductance ($g_m$): 6.5 mA/V

Typical Operating Conditions

Class A Power Output (Tetrode Section)

The tetrode section is optimized for audio power output or vertical deflection drive. Typical Class A operating conditions are specified with fixed DC potentials:

Circuit Constraint: When operating the tetrode section at its maximum wattage rating, the grid-to-cathode circuit resistance must not exceed 0.5 M$\Omega$ when using cathode self-bias.

Frame Time-Base Operation

In vertical deflection (frame time-base) applications, the output stage must be designed to accommodate component tolerances, valve-to-valve variations, and performance degradation over the tube's operating life. The peak anode current capabilities of the tetrode section under specific test conditions ($V_a = 50\text{ V}$, $V_{[g2](/tubes/G2)} = 180\text{ V}$, $V_{g1} = -1\text{ V}$) are:

• Average New Valve: 96 mA peak anode current.
• Assumed End-of-Life Condition: 62 mA peak anode current.

Designing the deflection circuit around the end-of-life peak current ensures stable vertical height and linearity throughout the service life of the tube.

Inter-electrode Capacitances

The inter-electrode capacitances of the 13GC8 are documented under two distinct conditions: with the holder capacitance balanced out ($\dagger$), and including a standard ceramic B9A unskirted holder without a radial shield (Carr Fastener holder type 77/076) ($\ddagger$). In both cases, "Earth" refers to the electrodes of the other section and all remaining earthy potential electrodes of the section under measurement, with the heater and shields connected to the cathode.

Related Designations and Curves

The triode section of the 13GC8 shares identical electrical characteristics with the triode section of the 30PL1 and the 6/30L2 triode.

The characteristic curves of the tetrode section illustrate the relationship between the control grid voltage ($V_{g1}$), screen grid voltage ($V_{[g2](/tubes/G2)}$), and the resulting anode and screen currents. The transfer curves demonstrate that at a screen voltage of 180 V, the anode current falls smoothly as the grid bias is made more negative, reaching cutoff near -35 V. The plate curves show the typical knee characteristic of a beam tetrode, where anode current stabilizes rapidly above an anode voltage of approximately 50 V, minimizing distortion in audio and deflection applications.