Before I got into the kind of audio gear that consumers use, I worked at a music store with guitar amps and other items that began to show up early in the last century. I have one guitar amp that, as closely as I can tell, was made around 1937. It has a 6L6G output tube, which became available in the middle of that year and all of the other tubes were already in use. It has a field coil speaker and after installing a grounded power cord, different rectifier tube and new electrolytic caps, it still works great. It had a full set of KenRad tubes and there are no EIA codes on anything, so that means it was made before 1941, when those codes were implemented. It's a potent little amp- 12"w x 12"h x 8"d.