The Pentode

Part of Vacuum Tubes

The pentode is a five-electrode vacuum tube that combines the tetrode’s high gain capability with a suppressor grid that eliminates secondary emission distortion — the dominant tube type in radio receivers and transmitters from the 1930s through the 1960s.

Why This Matters

The pentode is the workhorse of radio equipment. Walk into any radio station from 1935 to 1970, and you will find pentodes in the IF amplifiers, the audio preamplifiers, the driver stages, and the power output stages. The 6BA6, 6SK7, EL34, EL84, KT88, and dozens of other widely available types are all pentodes or pentode variants.

In salvage, pentodes outnumber triodes because they were cheaper to manufacture and offered better performance for most radio applications. Understanding the pentode — its operation, circuit configurations, biasing requirements, and key applications — is essential for building and maintaining radio receivers and transmitters.

The pentode’s high plate resistance (compared to the triode) makes it behave as a nearly ideal current source, which simplifies amplifier design: the gain is simply transconductance times load impedance, without the parallel plate-resistance complication that limits triode gain.

Construction and Electrode Functions

The pentode has five electrodes: cathode, control grid (G1), screen grid (G2), suppressor grid (G3), and plate — counting the heater separately.

Cathode: identical in construction and function to the cathode in any tube. Emits electrons when heated to operating temperature. The oxide-coated cathode surrounded by the heater inside the cathode sleeve is the standard construction for receiving pentodes.

Control grid (G1): the signal input electrode. Fine wire helix positioned close to the cathode. Maintained at negative potential during normal operation. Controls plate current by varying the electric field seen by emitted electrons.

Screen grid (G2): a coarser helix positioned between G1 and the plate. Maintained at a positive DC voltage (50-75% of plate voltage). The screen accelerates electrons past the control grid and into the drift space between screen and plate. The bypass capacitor from screen to cathode maintains the screen at constant AC potential, shielding G1 from the plate and raising the plate resistance dramatically.

Suppressor grid (G3): a widely-spaced helix between the screen and the plate. Connected to cathode potential (negative relative to plate and screen). Creates a potential barrier that prevents secondary electrons ejected from the plate from reaching the positive screen grid. Secondary electrons are decelerated and returned to the plate.

Plate: the final electron collector. In a pentode operating correctly, the plate collects essentially all the electrons passing through the suppressor grid, minus those collected by the screen and suppressor.

Key Parameters and How to Use Them

The pentode is characterized by transconductance (gm), plate resistance (rp), and screen voltage requirements, rather than the mu (amplification factor) used for triodes. This is because the pentode’s behavior at the plate is dominated by the constant-current-source character (high rp), making voltage gain calculation simpler.

Transconductance (gm): the change in plate current per unit change in control grid voltage, at constant plate and screen voltage. Measured in milliamperes per volt (mA/V) or microsiemens (µS). Typical values for small-signal pentodes: 1-5 mA/V. Power pentodes: 5-15 mA/V.

The voltage gain of a pentode stage is approximately:

Av = gm × Rload

where Rload is the effective load (plate resistor in parallel with following stage impedance). Unlike the triode formula, there is no parallel rp term to worry about as long as rp >> Rload (which is almost always true for pentodes). A 6BA6 with gm = 4.0 mA/V and a 47kΩ plate resistor provides:

Av = 4.0 × 47 = 188

This gain of 188 from a single stage drives the design of superheterodyne IF amplifiers, where cascaded pentode stages achieve total voltage gains of 10,000 to 100,000.

Screen voltage: the screen must be held at a stable positive voltage, typically 100-150V for common receiving pentodes. The screen dissipates power proportional to screen current times screen voltage. Screen current is typically 10-20% of cathode current. A 6BA6 with 8mA plate current and 2mA screen current at 100V screen voltage dissipates 0.2W at the screen — within the 0.5W screen dissipation rating.

Variable-Mu Pentodes for AGC

Superheterodyne receivers encounter signals ranging from microvolts (distant stations) to millivolts (nearby stations). The IF amplifier must handle this 1,000:1 range without overloading on strong signals or being too insensitive for weak ones. Automatic gain control (AGC) varies the IF amplifier gain to compensate.

Fixed-mu pentodes have an abrupt control grid cutoff characteristic — the plate current falls sharply as the grid becomes more negative, then cuts off completely over a narrow voltage range. Varying the bias to reduce gain with a fixed-mu tube causes abrupt cutoff and distortion.

Variable-mu pentodes (also called remote cutoff or super-control pentodes) have a non-uniform control grid winding — the wire spacing varies along the length of the grid. Some sections have fine pitch (high mu, cut off at small negative voltages) and some have coarse pitch (low mu, require much more negative voltage to cut off). The result is a gradual cutoff characteristic spanning 20-50 volts of control grid swing, allowing smooth gain reduction over a 60-80 dB range.

Common variable-mu pentodes: 6SK7, 6BA6, EF86. The gain is controlled by the AGC voltage applied to the control grid, which may range from 0V (maximum sensitivity) to −20V or more (minimum gain for strong signals). The AGC voltage is developed by a detector circuit that measures average signal level and feeds it back.

Power Pentodes

Power pentodes deliver audio or RF power to loads: speakers, output transformers, antenna systems. The EL34, EL84, and 6CA4 are audio power pentodes. The 807 is a classic RF power pentode used in transmitter PA stages.

Power pentodes operate with higher plate and screen voltages (300-600V) and higher plate currents (50-150mA) than small-signal types. The output power in Class A single-ended operation is typically 3-8W per tube. In Class AB push-pull, 15-40W per matched pair.

The 807 deserves special mention for community transmitters. It is a beam power tube (variant of the pentode principle) rated for 65W plate dissipation in Class C RF service. With a 600V plate supply, it can deliver 50-75W of RF output power — enough for a community radio station covering 20-50 km radius on HF or short wave. Hundreds of thousands were produced for military and civilian radio service and are commonly found in salvage.

Connecting a power pentode in “triode mode” (suppressor grid tied to plate, or screen grid tied to plate) produces triode-like characteristics with lower gain but often lower distortion for audio applications. Some audiophiles prefer triode-mode operation for audio power amplifiers despite the lower power output. This flexibility makes the power pentode a versatile tube that can serve in either role.