1. Introduction and History
The AK2 is a variable-mu radio frequency (RF) pentode vacuum tube developed in the early 1930s, primarily manufactured by Mullard and Philips in the Netherlands. This valve was part of the 'A' series of tubes designed for battery-operated radio receivers when portable and home radio sets were becoming increasingly popular. The 'K' in its designation typically indicated its function as an RF amplifier tube, while the '2' represented its generation or variant in the series.
Mullard, a significant manufacturer of the AK2, was founded in 1920 and later became part of the Philips group in 1927. The AK2 tube was produced primarily in Mullard's facilities in Holland, which explains the "Mullard Holland" designation often found on these tubes. The AK2 gained popularity in Europe during the 1930s and early 1940s, particularly in battery-powered radio receivers where its efficient design and low power consumption were valued attributes.
2. Technical Specifications and Design
The AK2 is a 4-volt indirectly heated pentode with the following specifications:
- Filament/Heater: 4.0 volts at approximately 0.65 amperes
- Plate (Anode) Voltage: Typical operating range 100-250 volts
- Screen Grid Voltage: Typically 60-100 volts
- Control Grid Bias: Variable, typically -1 to -10 volts (variable-mu characteristic)
- Plate Resistance: Approximately 1-2 megohms
- Transconductance: 1.5-2.0 mA/V (variable, depending on grid bias)
- Amplification Factor: Approximately 800-1000
- Base Type: 5-pin side contact base (European style)
The AK2's physical design featured a glass envelope with a characteristic bulbous shape typical of European tubes of that era. As a variable-mu (remote cutoff) pentode, it was designed with a non-uniform control grid wire spacing, allowing for a gradual change in amplification as the grid bias was varied. This characteristic made it particularly suitable for RF amplifier stages in receivers with automatic gain control (AGC).
The internal structure consisted of the standard pentode arrangement with five electrodes: cathode, control grid, screen grid, suppressor grid, and anode (plate). The suppressor grid, connected internally to the cathode, prevented secondary emission electrons from the plate from reaching the screen grid, improving efficiency and reducing distortion.
3. Applications and Usage
The AK2 was primarily employed in the following applications:
- RF Amplification: The AK2 excelled as a radio frequency amplifier in TRF (Tuned Radio Frequency) and superheterodyne receivers. Its variable-mu characteristic made it ideal for handling signals of varying strengths without distortion.
- Battery-Operated Radios: The tube's modest power requirements made it particularly suitable for portable and battery-powered radio receivers, where efficiency was paramount.
- Intermediate Frequency (IF) Amplifiers: In superheterodyne receivers, the AK2 could be employed in IF amplifier stages, typically operating at frequencies of 455-470 kHz.
- Early Television Receivers: Some early television designs utilized the AK2 in RF sections, though this was less common than its radio applications.
In typical circuit configurations, the AK2 would be coupled to tuned circuits at both its input and output, with the AGC voltage applied to its control grid to regulate gain. The tube's variable-mu characteristic allowed it to handle strong signals without cross-modulation distortion, a significant advantage in receivers that needed to process both weak and strong signals.
4. Equivalent or Substitute Types
Several tubes can serve as potential substitutes for the AK2, though often with some circuit modifications:
- VP2: Another Mullard/Philips variable-mu pentode with similar characteristics
- SP2: A sharp-cutoff pentode from the same family (lacks the variable-mu characteristic but can work in some applications)
- 6K7: An American equivalent, though with a different heater voltage (6.3V) and base configuration
- EF39: A later European pentode that can substitute in some applications with appropriate adaptations
- 78: An American variable-mu pentode that shares some electrical characteristics
- 6D6: Another American variable-mu pentode that could be adapted as a replacement
When substituting, considerations must be made for differences in heater voltage, pin configuration, and gain characteristics. Direct substitution without circuit modification is rarely possible, and adaptations to socket wiring, bias voltages, and sometimes coupling components are typically necessary.
5. Notable Characteristics
The AK2 possesses several distinctive features and characteristics:
- Variable-Mu Design: The non-uniform grid spacing provided excellent handling of signals with varying strengths, reducing cross-modulation distortion.
- Low Microphonics: Compared to some contemporaries, the AK2 exhibited relatively low microphonic noise, making it suitable for portable equipment subject to vibration.
- Efficient Operation: The tube could provide good gain with modest power consumption, an important consideration for battery-operated equipment.
- Historical Significance: The AK2 represents an important stage in vacuum tube development, bridging early triode designs and later more sophisticated pentodes.
- Collectibility: Today, the AK2 is sought after by vintage radio enthusiasts and collectors, particularly examples with clear Mullard or Philips branding and original packaging.
The AK2's variable-mu characteristic was achieved through its special grid construction, where the spacing between grid wires varied along the length of the grid. This allowed the tube to handle a wide dynamic range of input signals without overloading or creating intermodulation distortion, a significant advancement for radio design of that era.
Testing of AK2 tubes is typically performed on specialized tube testers like the AVO Valve Tester, which can accurately measure the tube's emission, transconductance, and other parameters. When evaluating an AK2 for use or collection, the condition of the glass envelope, pins, and internal structure should be assessed, along with electrical testing if possible.
