• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

"Impedance" of the output transformer

What I'm telling here is very little known: all dealers output transformers for tube amplifiers exhibit the characteristics impedance input, eg 8000 ohms and 4-8-16 ohm output.

A transformer is only one ... impedance transformer, that is to say there is no question of the proper impedance of the windings, the impedance presented by the primary is equal to the connected load on the secondary multiplied by the square of ratio.

An example of a transformer with a transformation ratio of 20, its input impedance will be:

•Zp = 20 000 ohms pour Zs de 50 ohms Zp = 20 000 ohms 50 ohms Zs
•Zp = 10 000 ohms pour Zs de 25 ohms Zp = 10 000 ohms 25 ohms Zs
•Zp = 3 200 ohms pour Zs de 8 ohms Zp = 3200 ohms 8 ohms Zs
•Zp = 1 000 ohms pour Zs de 2,5 ohms, etc Zp = Zs 1000 ohms 2.5 ohms, etc.

Obviously, it is extremely important to respect the speaker connections to the proper terminals, otherwise it is completely the tube output stage door-to-fake!
 
I don't know if I understand your statement correctly, but, yes, transformers are impedence matching devices.
But, the ratings of the transformer (example: 4K primary to 4-8-16 ohm secondary) is where the transformer in question works at it's best. IE. Best frequency response, lowest loss, etc...
Daniel
 
I'm not clear exactly what you are claiming as your insight that is "very little known" by others. The idea that, to a first approximation, a transformer merely transforms impedances is very widely known yet that is what you appear to say. Have I misunderstood? Or have you just discovered this for yourself and feel we might need to discover it too?
 
Let me float an idea. OPTs are often described as, say, matching 8k to 8 ohms. The OP has just discovered that there is no such thing as an "8k" transformer, it would do 6k to 6 ohms almost as well. As it is a revelation to him, he wishes to share this with us.

If I can give some advice, such things are better shared in the form of a question; to check that the new understanding is correct, and to see if it is already widely known.
 
You'll have to pardon my countryman :)

Of course, and I think most of us here know this already: many design factors do indeed depend on the absolute impedances for which the transformer is designed. Daniel already said it. Just think of the number of windings, for example.

The transformer will sort of work at other impedances, but performance will not be optimal.

Kenneth
 
I said a transformer hasn't its own impedance, the impedance on the primary depends of the impedance connected to the secondary.
For example, for à 8k/16-8-4 if there is 8 ohms on the secondary, the impedance is 8000 K on the primary. The ratio is +- 32. I then we connect a load of 6 ohm on the secondary instead of 8 ohms, the impedance of the primary becomes +- 6000 ohms. The impedance of the primary changes and the tubes are badly loaded.
Many people believe that the impedance always stays 8000 ohms because it's a caracteristic of the transformer and it's false.
 
Actually you have reflected impedance in some frequency band. It is the basic function of transformers. Out of this band inductances and capacitances play more and more their role. Also, on lower frequencies, the lower they are, the more distortions you get from core non-linearity. Also, the lower impedances you transform, the closer they are to resistances of own transformer's wires. That means more of thermal losses in the transformer.
Anyway, the transformer is a device that is not ideal, and designing them and their usage you always have to think about equilibrium, in order to achieve optimal results.
 
Why many output transformers are sleeping in the bottom of drawers ? Because the indications are erased or technical data lost.
Just inject known ac voltage to one of the windings, f.e. 1 volt; if on the other you find f.e. 20 volts, the ration is 20. That's to say if the secondary is loaded with a 4 ohms loudspeaker, the primary will be +- 1,6 Kohms
Zp = Zs x k²
If k = 20,
Zp = 20 000 ohms for Zs = 50 ohms
Zp = 1000 ohms for Zs = 2,5 ohms

But do not forget in this last example that the transformer will be better in very low bass response in the second way than in the first.
 

Gilgy

Member
2009-10-29 6:38 am
The point about output transformers being simple impedance matching devices is worth remembering for the musicians among us who use 100 Watt power amps with four EL34's. By pulling out two output tubes and using an 8 ohm speaker on the 4 ohm tap you get a 50 Watt amp without blowing up the transformer.
 

regal

Member
2004-01-04 8:41 pm
MD
I said a transformer hasn't its own impedance, the impedance on the primary depends of the impedance connected to the secondary.
For example, for à 8k/16-8-4 if there is 8 ohms on the secondary, the impedance is 8000 K on the primary. The ratio is +- 32. I then we connect a load of 6 ohm on the secondary instead of 8 ohms, the impedance of the primary becomes +- 6000 ohms. The impedance of the primary changes and the tubes are badly loaded.
Many people believe that the impedance always stays 8000 ohms because it's a caracteristic of the transformer and it's false.

You do realize that transformer billed as 8k/16-8-4 will have three separate secondary windings? So if we connect 4 ohms load to the 4 ohm tap we will still have 8k on the primary ?
 
You do realize that transformer billed as 8k/16-8-4 will have three separate secondary windings? So if we connect 4 ohms load to the 4 ohm tap we will still have 8k on the primary ?
It's an evidence

If you go to the shop and wanna buy an output transformer, the seller will ask you "what power" "what impedance". He had to ask "for wath impedance" and that's different.
 
Many people believe that the impedance always stays 8000 ohms because it's a caracteristic of the transformer and it's false.
I hope it is not "many people". Some newbies may believe this, and others who have played with audio for many years without learning anything, but I would have thought that at most this would be "some people". Anyone who knows even just the basics of how a transformer works would not believe this. Of course, we will never know because people who do believe this are unlikely to admit it in public!

As others have said, a transformer will work best at its design impedance because this gives the best balance of bandwidth but it is a broad balance not a tight constraint.
 

regal

Member
2004-01-04 8:41 pm
MD
It's an evidence

If you go to the shop and wanna buy an output transformer, the seller will ask you "what power" "what impedance". He had to ask "for wath impedance" and that's different.

You Europeans can actually walk into a shop and buy an output transformer to fit your needs?

Here we write down are requirments and email them to Olenics or Electraprint and the build them just as we need. I can't imagine driving to a shop, that would be 3 day drive for me :)
 
In the UK we have Maplin shops in most major towns. They carry a limited range of medium-quality OPTs (made by Danbury?), but are often out of stock so you have to mail order anyway. We also have mail order to VVT and Sowter, depending on how big your wallet is.

Maplin used to be a good component supplier, but now they concentrate on selling electronic toys to the masses.