What are these & what can I use it for?

[ronsimpson]

Any way you measure inductance it does not work like this.
If you were tough "mutual inductance"; there is none here.
If you think turns squared; it still does not work like this.
I am certain these are resistance measurements. Resistors add up just like this.

What is the copper strap around the center do?

The copper strap around the transformer some times is called a "humbucking strap". In 60hz power line transformers 60hz noise escapes the transformers and caused noise in other places. (hum in power amplifiers)
Your transformer probably works at 100khz. Magnetic noise escapes and the strap helps shorts out the magnet flux. Note the strap does not take the same path as the turns. (some thing like a shield. it is also a shorted turn. because it surrounds the transformer it does not effect the current flow inside the transformer.)

 

[ronsimpson]

Old tube amplifiers have audio output transformers 5000 ohms connected to B+. How is 5K ohms determined?

Many questions.

Tube amps are built for high voltage and low current. BUT Speakers are built for low voltage and high current.
I am going to work backwards.
8 ohm speaker. (high current low voltage)
Speaker connects to a transformer with a small number of turns. (secondary)
Magnetic flux
Transformer with many turns connects to the amplifier. (high voltage low current) (primary)

The transformer is probably labeled 5000:8 ohms. (20 watts or something)
Often the transformers had secondaries like this.
(1)=ground
(2)=4 ohms
(3)=8 ohms
(4)=16 ohms
The tube amp thinks it is driving a 5000 ohm load. (as long as there is a speaker connected to the right pins)

You know how a transformer can translate voltage. 120VAC to 12VAC. That is a 10:1 transformer. If you have 120V and 120 turns on the primary that makes 1 volt/turn. If you wanted 12V secondary you need 12 turns. (volts/turn) In this case you could power a 12V car light bulb from 120V. 12V 1A 12wat bulb on the secondary will look like a 120V 0.1A 12 watt bulb to the power line.

Transformer also translate impedance (resistance). Your 12V 1A bulb has 12 ohms. (when hot) The power line measure 120V and 0.1A. That looks like 1200 ohms to the power line.

Transformer moves voltage by volt turns. I also measure transformers by amp turns. turn:turns=voltage:voltage
Impedance (resistance) goes by turns squared.

 

[gary350]

Many questions.

Tube amps are built for high voltage and low current. BUT Speakers are built for low voltage and high current.
I am going to work backwards.
8 ohm speaker. (high current low voltage)
Speaker connects to a transformer with a small number of turns. (secondary)
Magnetic flux
Transformer with many turns connects to the amplifier. (high voltage low current) (primary)

The transformer is probably labeled 5000:8 ohms. (20 watts or something)
Often the transformers had secondaries like this.
(1)=ground
(2)=4 ohms
(3)=8 ohms
(4)=16 ohms
The tube amp thinks it is driving a 5000 ohm load. (as long as there is a speaker connected to the right pins)

You know how a transformer can translate voltage. 120VAC to 12VAC. That is a 10:1 transformer. If you have 120V and 120 turns on the primary that makes 1 volt/turn. If you wanted 12V secondary you need 12 turns. (volts/turn) In this case you could power a 12V car light bulb from 120V. 12V 1A 12wat bulb on the secondary will look like a 120V 0.1A 12 watt bulb to the power line.

Transformer also translate impedance (resistance). Your 12V 1A bulb has 12 ohms. (when hot) The power line measure 120V and 0.1A. That looks like 1200 ohms to the power line.

Transformer moves voltage by volt turns. I also measure transformers by amp turns. turn:turns=voltage:voltage
Impedance (resistance) goes by turns squared.

I guess my question was not clear. Each vacuum tube has a rating for the audio transformer like 5000 ohms. 50 years ago I did TV repair and audio repair if a 5K ohm audio transformer was fried it had to be replaced with a 5K ohm transformer. I built several amplifiers from circuits drawings and I had a large stash of used parts often audio transformers were not marked but if I know what tube it was used on then I know what the value of the transformer should be then I marked all the transformers so I knew what they are. In those days I had no way to test an audio transformer to find the value & I'm not sure if there is a way to test an audio transformer now to learn the value. I worked for a transformer manufacturing company once and learned how transformers are built & I have built a lot of my own transformers. I have all the formulas and information to design and build my own power transformers. I have never learned how to design an wind my own audio transformer to get 5000 ohms on the primary and 8 ohms on the secondary there should be formulas for that? Maybe that info is online I have not looked, not likely I will ever need it but might be interesting to learn? That technology is hardly used anymore everything is solid state now.

 

[dr pepper]

I'll probably get burned for this as there is a little approximation.
5k is most likely the impedance due to inductance at the lowest frequency the transformer is intended to be used at.
Tube outputs are often designed to have o/p tube Dc bias current through them, meaning an airgap is often employed to avoid saturation, a tube output transformer is different from most other silicon steel trannies.
Power ferrite probably wont be any good at audio frequencies.

 

[ronsimpson]

I guess my question was not clear.

Another way to think about transformers;
Take a 220V to 24V transformer and connect it to 110V and you get 12V out.
In the same way;
5000:8 ohm transformer will function as 2500:4 ohms.
It is all in the turn ratio, (for V or I) or turns squared if we are talking impedance.

 

[gary350]

When I worked at the transformer factory I talked to one of the engineers he said, primary wire size has to match the tube output current and number of turns has to act like a choke to provide 5000 ohm impedance. Turns ratio primary vs secondary determines 8 ohms on the secondary. EI core size on high quality amplifiers is 5 times larger than needed so pulses do not get cut off from core saturation from power serge's. That all makes sense to me but what is the formula to determine number of turns on the primary coil to get 5000 ohms impedance. Its probably the same formula used to wind EI type chokes. I did some tests putting variable Hz into a audio transformer as Hz goes up less and less comes out up to a certain Hz when nothing comes out. EI transformers do not work at all above a certain HZ.

 

[dr pepper]

Makes sense, the core oversize may not just be to avoid saturation it could be to make room for more copper and therefore less parasitics.
To work out the impedance there are a couple of transformer equations, the Al factor of the core comes into account, as does the volt seconds the core can withstand without sauration, basically you want enough turns to make up the required inductance and enough turns so as not to saturate the core. This is made more complicated when you have an air gap.
Some useful info (not so much for air gapped cores though):
https://www.giangrandi.ch/electronics/trafo/trafo.shtml

 

[unclejed613]

the magnetic fields of the DC idle current through the output tubes should cancel. there will still be some heating in the primary since there are DC currents in the winding. the impedance calculation should be simple, as the winding has inductance, and that inductance can be used to calculate the reactance.
there is a company currently making toroidal output transformers for tube amps, and looking at the specs, they outperform EI core transformers, especially in their frequency response.
https://www.plitron.com/toroidal-audio-tube-output-transformers/

 

[dr pepper]

Yes of course.
My experience of tube outputs is primarily from radio's, the ones I have tend to be single ended, a 2 tube layout could be arranged where the bias cancels.

 

[throbscottle]

This is one of the best authors I have come across on the subject: https://ludens.cl/Electron/Magnet.html and https://ludens.cl/Electron/trafos/trafos.html
Check out the rest of his site too - good reading and photos!

 

[gary350]

I will experiment with 1 of these ferrite transformers & rewind it with #14 copper wire to see what value I get.