Dual control Pentodes
Nearly all pentodes were designed such that an applied voltage to the suppressor grid G3 has little or no effect on plate or screen grid G2 currents.
The primary purpose of the suppressor grid in a pentode is to return any secondarily emitted electrons by the plate back to the plate. When the plate voltage drops below the screen voltage, secondarily emitted plate electrons would be attracted by the screen grid G2 if the Suppressor grid G3 were not at a much lower potential than the plate, so that it effectively hides the attraction of the screen grid G2.
The secondary emission of the plate that ends up at the screen creates a negative conductance that may cancel out the positive conductance slope of first hit electrons at the plate, to the point that the net slope is negative over a range of plate voltages below the screen voltage, but usually above 0V. This is what gives the tetrode it's negative conductance kink.
More subtle effects over secondary emission are possible, as can be seen in these curves for a DF97 filamentary pentode. All the curves at this link cover the effect of the Suppressor grid G3 at 0V and positive voltages.
A dual control pentode is designed such that the plate current can be controlled by the Suppressor grid G3 voltage as well as by the usual Control grid voltage G1. A dual control pentode differs from other pentodes in that the Suppressor grid G3 controls the plate current for any voltage below 0V.
Nearly all pentodes will exhibit Suppressor grid G3 control under one or more of these three conditions:
1-Suppressor grid G3 is brought sufficiently negative, perhaps by -100V or more.
2-The plate voltage is dropped well below the Screen voltage G2.
3-The screen voltage is set much lower than the recommended nominal value.
None of these conditions are needed for Suppressor grid G3 control of plate current in a Dual Control Pentode.
When the Plate is under Suppressor grid G3 control, the Plate resistance drops dramatically and looks like the characteristic of a triode. This behaviour occurs for all pentodes, once they are biased for Suppressor grid control.
Plate current vs. screen current
One important characteristic of Suppressor grid G3 control over Plate current is that a nearly equal, but opposite, effect is exerted over the Screen grid G2 current. In other words, the sum of the Plate and Screen currents, which is to say the Cathode current, stays nearly constant. See plate and screen I/V sweeps for the 6AS6 dual control pentode.
Non-inverting gain to Screen grid G2
The opposite behaviour of the screen current with respect to plate current under the control of the Suppressor grid G3 means that there is a negative transconductance from Suppressor grid G3 to Screen grid G2. This negative transconductance means that the voltage gain from Suppressor to Screen is positive, or non-inverting.
Intuitive triode model
All the functions of the suppressor grid in it's control region behave like differential triode pair with another triode as the current source.
Applications for Suppressor Grid G3 control
1-Mixing. Perhaps the first application for Suppressor grid control was as an injection terminal for the local oscillator in a super heterodyne radio. The form of mixing that is achieved this way is Multiplicative, as opposed to additive, where the RF signal to be mixed and the local oscillator voltage are both applied to the control grid, cathode of any tube, or even the plate of a triode. The low plate resistance of pentodes under Suppressor grid control gives this type of mixer a lower conversion gain than tubes with more grids that aim to restore the high plate resistance. The required LO voltage at the suppressor grid was also very high so that the pentode was either in normal pentode operation with G3=0V or under G3 cutoff, spending little time in the intervening low plate resistance voltage range.
2-Gain Control. Another early application of Suppressor grid control was to control tube gain from the Control grid G1 to the Plate. The transconductance from the Control grid G1 to the plate drops as the Suppressor grid is brought more negative in it's control range of voltages. An additional effect is the great reduction in plate resistance which also drops the gain of a stage that is loaded by the high impedance of a tank circuit at resonance. The usually undesired side effect is a loss of selectivity by this tank circuit due to plate resistance loading. Hexodes, such as the RENS1234, added a 4th grid to bring the plate resistance back to a high value for high gain with a tank circuit load.
3-FM IF detection in TV sets. See adjacent schematic. Much later, in the 1950's, the Suppressor grid G3 in the dual control pentodes listed below, was induced to capture a FM signal synchronously, with loose internal capacitive coupling from the space charge into a grid-leak (R5,C6) biased parallel resonant tank circuit (L1,C5). Other parasitic feedback capacitances were harnessed to sustain a continuous oscillation at this tank circuit. Hence the name "Locked oscillator FM detector". When an unmodulated FM carrier is present at resonance, the internal loose coupling capacitance and the equivalent parallel resistance of the tank circuit creates a synchronous carrier that is 90o ahead of the incoming carrier. This gives the Suppressor grid it's name of "Quadrature grid" in this application. As the incoming carrier frequency varies under FM modulation, the phase of the loosely coupled voltage at the suppressor grid varies according to the phase curve of the tank circuit. The additional phase shift that is caused by a resonant circuit is an essential characteristic of other FM detectors like Discriminators and Radio Detectors. The detected signal is available at the plate as the product of the incoming FM modulated signal and the loosely coupled signal into the suppressor grid G3 that has additional phase modulation from the tank circuit phase curve. This product results in a frequency dependent duty cycle. The insensitivity of the oscillations at G3 to incoming FM signal amplitude variations accomplishes a limiting function. The equivalent detection operation in a Discriminator or Ratio detector, is an additive detection with diodes, of the sum of a direct FM signal with a signal that has additional phase modulation by a resonant tank circuit. The locked oscillator detector with dual control pentodes was very popular in American TV sets and I know of no cases where it was used in a European TV or FM sets. GE even had a line of 12-pin Compactrons that included a dual control pentode for FM detection and an audio power amplifier to drive the speaker. These Compactrons are listed below.
4-Pulse control and Pulse generating circuits. Perhaps the best known circuit of this type is the Transitron oscillator. This class of circuits takes advantage of the non-inverting voltage gain from the Suppressor grid G3 to the Screen grid G2. In the simplest RC type of relaxation oscillator, a capacitor couples G2 and G3, with a load resistor at G2 to a positive supply, and a resistor to ground at G3. Other reactive elements can replace the the capacitor of the relaxation oscillator to create LC Transitron oscillators and crystal Transitron oscillators with the appropriate blocking capacitors and bias levels in place.
5-NAND gate. 1950's digital computers also used dual control pentodes as two input NAND gates.
Either grid could cut off the tube. The Sylvania 6888 was developed for this specific purpose. The 8W max plate dissipation of this tube also made it suitable to drive magnetic ferrite core memories.
6-TV receiver pulse circuits. A dual pentode version, the 6BU8 and it's heater voltage variants, were used in a number of American TV sets in the deflection synchronization circuits.
7-Modulator. A variant of the mixer application is as a modulator in signal generation circuits.
8-Synchronous detector. Another variant of the mixer application is as synchronous detector in TV color circuits. One such example os the 6BV11 Compactron which contains two identical pentodes.
9-Non-inverting amplifier. E. W. Herold from RCA, published a paper in Volume 23 No10 of IRE Proceedings October 1935, where he discussed the application of a 57 pentode as a non-inverting amplifier. He described it's operation as "The use of a 57 as a retarding field negative transconductance tube". The title of this paper was "Negative resistance and devices for obtaining it". Herold biased his 57 with G1=0V, G2=100V, G3=-10V, P=22.5V. The non-inverting gain from G3 to G2 produces a negative input capacitance by Miller multiplication, when resistively loaded.
A list of pentodes designed for dual control
I leave out the various heater voltage variants that a number of these tubes have.
6F33 is the only dual control European pentode I know of. Jacob Roschy shared this info.
6BU8 two pentodes share a control grid G1 and Screen G2, but have separate suppressor grids G3 and Plates.
PF86 for Transitron oscillator applications
EF50 designed as a conventional RF pentode, but can be biased for dual control.
The following tubes are all G2-Compactron types with 12 pins. Each tube includes a dual control pentode for FM detection and an audio power amplifier for service as the audio section in American TV sets.
The next three tubes include a gated beam beam pentode that is similar to the 6BN6.
These two tubes were used in American TV sets for sweep synchronization and synchronous color demodulation.
6BA11 gm1=1.8mS twin-plate pentode + triode
6BV11 gm1=3.7mS dual pentode gm3=2m7S
Please let me know, or post any info that you may have about other dual control pentodes not listed here.
A collection of suppressor grid measurements has been posted and helps to illustrate the behaviour outlined here.