Trash it?

[jordan86]

Found this little transformer in a speaker cabinet in a junk pile to be trashed. Was going to what looks like a wirewound pot or rheostat, then into an 8 ohm Quam speaker. For attenuation I presume. I’m still learning but it l like it’s single ended? Very tiny. Maybe even too tiny for a champ. Secondary side just has wires for 8 ohm and common.

Worth hanging onto for a project or just trash?

[jewishjay]

looks like a 70 volt transformer. often used in intercom systems, and large venues where you have to send sound long distances.

[SILVERGUN]

https://adn.harmanpro.com/site_elements/resources/860_1425410780/Guide_to_constant_Voltage_systems_original.pdf

[Williamblake]

Normally these transformers don't have an airgap, but you can add one yourself. Or you leave it without airgap and use it with a 12au7 in pushpull. Maybe as an output tube or as a reverb driver.

[1blueheron]

This might be useful to you in deciding.

[jordan86]

Very cool. So it sounds like it could be used for a single ended build or maybe a small push pull amp, maybe 12AT7 power section. I’m still unclear how the 70v factors in. I did find a pretty bulky matching 70v PT in the junk heap as well. Might be fun to build something out of it. I imagine someone has done this before, so I’ll scour the web for something.

[SILVERGUN]

I’m still unclear how the 70v factors in.

There's a great explanation in that .pdf I linked.

Here's some:
How did the 70V line get its name? The intention was to have 100V peak on the line, which is 70.7V rms. The technically correct value is 70.7V rms, but "70V" is the common term. There are 70 volts on the line as maximum amplifier output with a sine wave signal. The actual voltage depends on the power-amplifier wattage rating and the step-up ratio of the transformer. The audio program voltage in a 70V system might not even reach 70V. Conversely, peaks in the audio program might exceed 70V.

[shooter]

typically Techs N engineers talk in RMS, Peak2Peak is for the sales folk BS'n the potential customer 

[PRR]

In the US, Tom Edison sold power at 100 Volts. Any less was not worth fooling with. Any more was dangerous. (We've had second thoughts on both these points, but the legacy is that 2-digit voltage "is considered safer" than 3-digit juice.) Also early rubber-covered wire was leaky crap.

Sending precious audio power through long lines at just a few volts (say 10 Watts at 3.2Ω, a typical theater level) is wasteful. 500Ω lines were often used. 10 Watts @ 500Ω is 70.7Vrms. When bigger amps arrived and were used to feed more speakers in larger schools and factories, they kept the 70.7Vrms but lowered the amplifier impedance so more power could be taken.

By some other process, most of the non-US/Can world adopted 100V as nominal large-system distribution voltage. Most wires were already plastic covered, less leaky. And larger systems had become common. (The US has a rarely-seen 140V system, though most very large systems have moved to distributed amplifiers rather than run big audio all around the ballpark.)

Meanwhile, 70V is not dead-safe (so to speak). Even 48V has been disparaged. 25V is usually safe and there is a full selection of 24V line transformers. (24V is 1/3rd of 70.7V within rounding error, so the 70.7V is wound as three 24V windings to be connected series or parallel.)

[HotBluePlates]

PRR, should we ask what "School Intercom Purgatory" you spent time in to learn all that?   

[shooter]

 
I'm guessing his wisdom came from experience? over academic pursuits?

[jordan86]

Here's the big PT that I found as well.
http://www.avguy.com/qsc-ot-300a.pdf

I guess it can be used for an OT though as well in the 70V system. I'll need to do some reading to figure out if its even possible to use it for guitar purposes. Good learning process to understand transformers . I understand how all the specs of transformers affect the amp, and I know those specs are just a reflection of winding ratios, but I don't fully understanding the basics just yet, so it will be good for me. This one seems like a big chunk of iron. Similar size to your typical 30-40W guitar amp iron.

[sluckey]

Did you notice this is an auto transformer, ie, only one winding with multiple taps? There are no separate primary and secondary windings and that makes this transformer unsuitable for use as any tube output transformer.

[Williamblake]

70V aka line transformers normally have a secondary. With 15W or 30W you get close to 3k @ 8Ohm. Line transformers dont have to be as big as regular output transformers to have good bass response for their intended use. But its cheap ...

[shooter]

look at the link in reply #11
1 winding, many taps

[sluckey]

70V aka line transformers normally have a secondary. With 15W or 30W you get close to 3k @ 8Ohm. Line transformers dont have to be as big as regular output transformers to have good bass response for their intended use. But its cheap ...

You're thinking of a line transformer that sits at the ***END*** of the line and steps the high voltage line back down to the speaker level. Kinda like the step-down transformer on the pole that feeds your house.

But this transformer is meant to connect to the amplifier output to step up the voltage to send down the usually long line or distribution lines. It needs to be bigger than the little 70v line transformers

[Williamblake]

Yeah, sorry, didn't notice the autotransformer entering into it, was still talking about the stuff in the first posts.

 I dont know if it translates well but here is somebody talking about using these as output transformers (at the ending, go to "100-V-Übertrager",  the main subject is different).

https://www.b-kainka.de/roehren/trafos.htm

[uki]

This may be interesting: Using 70 volt audio transformers for impedance matching tube radios.