OT mounted to speaker

[cmoore]

I usually see this on much older amps..... the OT mounted right on the speaker frame.
Why was this done, or what was the advantage.?
Thank You

[cmoore]

Huh..... no takers on the Speaker/OT mystery.

Did i mention a 1000 USD contest involved with this.?

[martin manning]

It was very common on radios, which usually had a low-power single-ended audio output stage. My best guess is that it was done to save chassis space for other things. Did I win?

[cmoore]

It was very common on radios, which usually had a low-power single-ended audio output stage. My best guess is that it was done to save chassis space for other things. Did I win?

You have, very much, won my appreciation if that is any consolation.

[cdemike]

I remember seeing or reading somewhere that in some cases they were used to energize coils on speakers. Not sure if that was the OT or choke, though. I apologize for the vague answer, but I do know in some cases transformers mounted to speaker frames played a functional electromagnetic role in addition to some likely space savings.

[maxkracht]

Not sure if that was the OT or choke, though.

Field coil often doubled as the choke.


I'm guessing safety regulations are a part of why no one does this anymore. Cloth-covered wires without extra sheathing in the back of a cabinet probably didn't seem so crazy in the field coil days when you expect B+ around the speaker. At least the average person was probably a bit more cautious when poking around in the '40s.

[R.G.]

Some --very-- early speakers used electromagnets for the speakers. The bias currents for the output stage, or other DC current source ran through the speaker magnet windings to energize the speakers.

If I remember right - that was even before my time.

[Stevem]

When you see 4 wires going to the speaker then two of those wires are as RG posted, high voltage DC to make a magnet.
This was done before the times of permanent magnets like Alnico became affordable.

[sluckey]

Here are some common field coil circuits I am familiar with. One circuit is often found with the OT mounted on the speaker frame.

[katopan]

I have an old reel to reel tape player, fully self contained / portable unit. Had a single ended 6AQ5 output stage with the standard single ended OT (not field coil) riveted to the speaker.

The chassis was cramped as was the rest of the internals, so it appeared to be for saving space.