Older table-top AC-DC radios used a classic line-up of tubes. Think 12SA7, 12SK7,12SQ7, 35Z5GT, and 50L6GT. As I grew into my teens, I got interested in how these radios worked and soon discovered that their vacuum tubes could get very hot, especially the last two, the half-wave rectifier (35Z5GT) and the beam power tetrode audio output stage (50L6GT).

One day, I carelessly allowed a window curtain to brush against a hot 50L6GT, and the fabric of that curtain actually melted. Mom was not thrilled.

With that history still fresh in mind, I later came across another vacuum tube called the 117L7/M7GT whose data sheet looked much like this:

Figure 1 A datasheet for the 117L7/M7GT with the two hottest tube functions from previously studied radios in a single unit. 

This thing was scary!

The two hottest tube functions from the radios I’d been studying were combined into one device. Both functions were placed within a single glass envelope vacuum tube.

Take a look at these guys:

Figure 2 Two 117L7/M7GT tubes combining the heat of the beam power tube and the rectifier tube within a single glass envelope.

Imagine the combined heat of the beam power tube and the rectifier tube within a single glass envelope. If the one tube that damaged Mom’s window curtain was thermally dangerous, I cringe to think how hot these tubes could get and what damage they might be capable of causing.

I still shudder at the thought.

John Dunn is an electronics consultant and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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