Tape recorder by Grundig (about 1950th).<br>The transistorized Grundig TK1 was manufactured in 1960-1963, as a portable, battery-powered entry-level tape recorder - alongside with the "serious" all-tube, mains-powered TK25/TK30..etc line. The TK1 weighed merely two kilograms (without batteries). Single speed 9.5 cm/s, around 15 minutes playback from the tiny reels. The TK1 had seven semiconductor devices and one subminiature vacuum tube - the DM71 "magic eye" indicator. The high voltage for the tube was generated via a winding of the bias/erase oscillator (sort of a "free" DC-DC converter). The batteries were strictly low-voltage, 6V for the electronics and 3V for the motor.
<a href="//commons.wikimedia.org/wiki/User:Hgrobe" title="User:Hgrobe">Hannes Grobe</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>