tubes with 117 volt heaters

[Jack1962]

I was just reading a old electronics book from the 30's I think and it was talking about tubes with 117 volt heaters , anyone here now anything about these and are there any of these that would be good for building amps ?

[Tiny_Daddy]

They were used in battery-operated portable radios. Where AC was available the 117V tube could be powered and drive the speaker with more power than was available from the battery-operated tubes.

[Jack1962]

Ahh, I see very interesting , thanks

[PRR]

Hi-volt heaters are handy for wall-power sealed-cabinet devices like radios and intercoms. You can lose the power transformer.

If you can TOUCH the electronics (even your guitar cord), you do NOT want direct line-heated schemes. We called them death-traps. They were pretty-much made illegal in the early 1960s.

Nearly all the high-volt heaters have 6.3V versions. 50L6 is pretty near the same as 6Y6 except the heater voltage.

The only literal 117V heater tube I recall is a combination rectifier and pentode used for "wireless phonographs". It can make its own DC, it can take a weak signal from a phono needle and make a weak radio signal which plays on a radio in the same room. Assuming the tonearm wiring is enclosed, it is safe enough. But pretty poor fidelity.

[Jack1962]

Thanks PRR very informative as always

[jhadhar65]

I still use some of those for AM transmitter toys I play with, in particular the 117L7 and 117M7.  I'm pretty sure PRR turned me onto them.  On some the early ones I built, I used an iso transformer for everything but the heaters - mainly because of the transformers I bought.  I didn't worry too much about it in that application, though, since a transmitter isn't something you want to be sticking your tongue to anyway if you can help it.

The 117-types I play with don't have a lot of horsepower for audio amp work and there are better/safer tubes available for low power applications.  Still, if you happen to have one or two, it would make a nice test project.  Be sure and use a PT that will allow you to isolate everything... or at least test it with a 117V isolated mains source if it turns out to be bench-play only.

If you do try one out, pay close attention to the internal structure of the tube in question.  The two tubes I mention above that I use also have two more siblings in the series with significantly different internal connections, but nearly the same numbers (i.e. 117P7).  You can't isolate individual source voltages with those two and they might not be interchangeable, depending on the circuit you use.  FWIW...

[Tiny_Daddy]

I used one to make a morse code practice oscillator.

[PRR]

> Where AC was available the 117V tube could be powered and drive the speaker with more power than was available from the battery-operated tubes.

You are correct (I was wrong).

A battery radio takes 45V or 67V and makes 0.1W-0.2W.

By adding a rectifier/pentode, you can take 117VAc when available, make 110VDC, drop 60V to the battery-type tubes, and output 0.8W using the big pentode and 110VDC.

One more socket, slightly more OT, and you have a mild beach-radio which roars in the house.

[Tiny_Daddy]

Don't miss the 117Z6 which has 2 separate rectifiers so you can rig up a voltage doubler.