Voltage Pentode - Remote Cutoff (single) ·Pentode
Remote-cutoff RF pentode for gain-controlled RF and IF amplification, providing smooth automatic-gain-control action in receiving equipment.
The 12AC5 is a remote cut-off radio frequency (R.F.) pentode designed with a coated unipotential cathode. It was introduced in October 1955 by N.V. Philips' Gloeilampenfabrieken.
The 12AC5 is constructed in a compact glass envelope utilizing an E8-30 base. It is designed to operate in any mounting position. The physical dimensions of the tube are as follows:
| Dimension | Value |
|---|---|
| Maximum overall length | 2-3/8 inches (60 mm) |
| Maximum seated height | 2-1/16 inches (53 mm) |
| Maximum diameter | 7/8 inches (22 mm) |
The tube utilizes the 8GW basing connection arrangement. The pin assignments, viewed from the bottom of the tube, are:
Pins 3 and 4 are reserved for internal connections and must not be used for external circuit tie points. Pin 7 internally ties the cathode, the suppressor grid (Grid No. 3), and the internal shield together to minimize interelectrode coupling and simplify external wiring.
The heater of the 12AC5 is designed for series or parallel operation with the following electrical specifications:
The maximum operating limits are established under the Design Centre Rating System. Operating the tube beyond these limits may impair its performance and longevity:
| Parameter | Maximum Rating |
|---|---|
| Plate supply voltage | 550 volts |
| Plate voltage | 250 volts |
| Plate dissipation | 2 watts |
| Grid No. 2 supply voltage | 550 volts |
| Grid No. 2 voltage (at plate current < 4 mA) | 250 volts |
| Grid No. 2 voltage (at plate current = 7.2 mA) | 150 volts |
| Grid No. 2 dissipation | 0.3 watts |
| Cathode current | 10 mA |
| Grid No. 1 circuit resistance | 3 megohms |
| Voltage between heater and cathode | 150 volts |
| External resistance between heater and cathode | 20,000 ohms |
Under typical Class A amplifier operating conditions, the 12AC5 exhibits the following electrical characteristics:
The remote cut-off characteristic of the control grid is demonstrated by the bias required to reduce transconductance to a very low level. Specifically, a Grid No. 1 bias of -34 volts reduces the transconductance to 23 µmhos.
The direct interelectrode capacitances of the tube, measured without an external shield, are:
The extremely low plate-to-grid capacitance (0.002 µµF) is achieved through the internal shielding connected to Pin 7, which is critical for stable high-gain R.F. amplification without self-oscillation.
The relationship between control grid bias, screen grid voltage, plate current, and transconductance is illustrated by the tube's characteristic curves across a plate voltage range of 100 to 200 volts.
The transconductance curves show a smooth, logarithmic decline as the negative Grid No. 1 bias increases. At a Grid No. 2 voltage of 200 V, the transconductance remains relatively high over a wider bias range, whereas lower Grid No. 2 voltages (such as 50 V or 75 V) result in a much sharper cut-off.
When operating with a screen-dropping resistor, the transconductance characteristic is modified. For a supply voltage of 170 V with a 40,000-ohm Grid No. 2 series resistor, or a supply voltage of 100 V with a 40,000-ohm series resistor, the transconductance curve transitions dynamically across the constant-voltage curves. This series-resistor configuration provides a more linear control characteristic over a wide range of input signal levels.
The plate current curves mirror the transconductance behavior, showing a gradual reduction in plate current as the Grid No. 1 bias becomes more negative. Under constant Grid No. 2 voltage conditions, higher screen voltages significantly increase the plate current at low bias levels. The addition of a 40,000-ohm series resistor to Grid No. 2 (from either a 170 V or 100 V supply) automatically reduces the screen voltage as plate current rises, preventing excessive plate and screen dissipation at low bias levels while maintaining a smooth remote cut-off characteristic.
Small-button Noval 9-contact
General specifications
Properties common to every published source and operating point.
Electrical assignments shared by every tube using this base designation.
No verified pinout is recorded for this base designation.
1 physical section, each with unique specifications.
Section 1
Source-dependent data
Each block preserves the test conditions and published values from one reference. Values from different sources are intentionally kept separate.
Data set 01
Published operating points and test conditions from this source.
Plate voltage
200 V
Screen voltage
116 V
Plate current
7.2 mA
Screen current
0.7 mA
Grid voltage
-3 V
Transconductance
2300 uS
Plate resistance
1000000 ohm
Maximum plate supply voltage
550 V
Maximum grid circuit resistance
3 Mohm
Grid-to-heater capacitance maximum
0.05 pF