This Cenco audio amplifier/ speaker was found at an antique radio swapmeet. With its silicon diode half-wave rectifier, the amp contains just two tubes, a 6AV6 used as preamp and a 6EH5 as audio output. The amp was in very good cosmetic condition, just needing removal of a dymo label and some paint marks. It was sold by Cenco, a supplier of equipment for high-school and college science programs. Cenco currently sells a solid state amp for amplifying audio for experiments such as demonstration of a sonometer (a one-string instrument for demonstrating the physics of frequency, beats, and harmonics of string vibration as related to tension, density, etc.) Cenco's sonometer has a pick up (as in "electric guitar") which plugs into the amp. I assume this tube-type amp was used in similar fashion.
Repairing a One-note amp
When powered, the little amp could only hum one tune at only one note, 60 Hz.
Yes, you guessed the problem correctly. It needed replacement electrolytics. The original electrolytic was a dual section 60 MFD at 150 VDC in a cardboard sleeve. I removed the cap and sliced the cardboard sleeve lengthwise. That made it easy to remove the cap internals. I then re-used the cardboard sleeve and the original chassis clamp to hold a smaller replacement cap. That solved the hum problem. A scratchy volume control was given a proper cleaning with deoxit.
After repairs, the amp sang tunes from my MP3 player as shown in the picture above. It is a decent little amp acting as a vacuum-tube-based powered speaker that will see lots of use amplifying line-level or headphone-level audio.
A Signal Corps BC-348Q receiver was the previous item on the bench.