Used aboard ships.
Frequency ranges: 400 , 600 and 2800 metres.
Feature; The functions of HF amplification, detection and LF amplification are all accomplished in one valve.
Amplifier / detector tube : Type FE1 (tetrode/screen grid) valve
Made by : Marconi UK
 |
| TYpe 91 photo courtesy History of Science Museum, Oxford UK |
 |
| Type 91 nameplate info |
 |
| .Type 91 schematic. Download image to enlarge. In spite of having a screen grid, the FE1 valve does not function as a screen grid valve in this circuit. |
 |
| An alternate simplified diagram of the type 91 amplifier. Download image to enlarge. |
 |
| Tuning curves for the three frequencies (L-R) of 400 , 600 and 2800 metres. Download image to enlarge. |
TYPE FE 1 VALVE USED IN THIS DETECTOR AMPLIFIER
 |
| Above and below: F.E.1 valve. It was first issued in 1920. The introduction of a screen grid to the triode considerably reduced the interelectrode capacitance between the grid and plate. This kept the valve from breaking out into self oscillation. |
 |
Contributors and Credits:1) Lewis Bodkin <05bodkin555(at)gmail.com>
2) Saga of the Vacuum tube pg. 225
3) Handbook of Technical Instruction for Wireless Telepathists
May 22/19