A typical 1930s public address amplifier - adapted from the 1937 Thordarson brochure (the original had 4, not 2 output tubes)
<a href="//commons.wikimedia.org/w/index.php?title=User:Retired_electrician&amp;action=edit&amp;redlink=1" class="new" title="User:Retired electrician (page does not exist)">Retired electrician</a> adapted from Thordarson 1937 brochure <a rel="nofollow" class="external text" href="http://www.tubebooks.org/Books/Atwood/ThordAmp.pdf">page 6</a>
<span class="int-own-work" lang="en">Own work</span>