The Vegaty ST4 signal tracer is a debug tool for HF and LF circuits. The device includes a LF oscillator to inject a signal into a circuit under test. The signal path is traced with the demodulator probe. The instruments contains a low-noise high-gain input amplifier and a double-stage audio amplifier for auditive feedback. In addition, the rectified audio signal drives an indicator tube (magic eye-ish) to give a visual indication of the received signal level. It also contains a neon bulb for testing resistors and capacitors.
The manual (scanned copy) contains the schematic but no theory of operation. However, it is very easy to understand.
Indicator tube EM80 shows the level of the demodulated signal.
EZ80, double-diode used as a full-wave rectifier in the power supply.
ECL82, triode and power beam tetrode, used as preamp and output stage for the audio amplifier, respectively. The tetrode anode load is the output transformer, transforming the 4 R speaker impedance to a suitable load resistance. No followers whatsoever.
EF86, pentode. Input stage of the signal tracer. Provides a first stage of amplification, used in this location because it was designed for low noise and, being a pentode, has much higher gain than a triode (higher plate resistance).
ECC82, double triode. Active device in the oscillator. Connected as a long-tailed pair in a positive feedback loop.
Double capac... condenser: 2 x 50 uF. Part of the power supply T-filter. Probably high-voltage paper type.
OA85 diode, listed as 'tube' in the manual, which it is not, obviously. It is a germanium point contact diode, used as a half-wave rectifier to create the dc to drive the indicator tube.