Output Transformer Impedance Verification

[mokomon]

Can you determine the output impedance of an output transformer on an amp by watching the output waveform on a scope or watching a watt meter , while you switch the output load between 4,8 &16 ohm ? Or do you just have to inject a signal into the secondary of the output transformer and measure the voltage on the primary and do the math ?

[Enzo]

The second one.

[pdf64]

'Or do you just have to inject a signal into the secondary of the output transformer and measure the voltage on the primary and do the math ?'
Alternatively inject a signal to the primary and measure the secondary (it may be a bit easier on the signal generator).

A tube amp may deliver similar power levels over a wide range of loads, especially if the power supply is saggy.
Pete

[g1]

Theoretically, you will get the most power output measured when the speaker impedance matches the specified OT secondary tap impedance. This is called maximum power transfer.
Practically, it does not always work out this way. Power supplies can factor in like pdf64 mentioned. Also, the OT primary impedance is not always matched up to the power tubes, often by wide margins.

[Bruce / Mission Amps]

That question was just a bit nebulous so I'll just ask...
do you have an amp with an unknown OT and speaker combo where you are trying to figure out what speaker load to work with for the best results?
Or, are you asking a technical question so you can see if there is an alternate way to determine what actual turns, voltage or impedance ratio of a given output transformer?

[mokomon]

Well the question arises from an amp I am working on . The only schematic I could find says it requires a 16 ohm speaker. There was an 8 ohm speaker in it when I got it.So after I repaired the amp I was trying to figure out if indeed it is supposed to have a 16 ohm load. So I was watching the output wave forms and my watt meter to see what effects different loads gave me . A load of 4 ohms didn't give me much output , and 8 & 16 ohm loads were much higher outputs and both about the same.So my question was ... can you determine the load requirement of any output transformer by looking at wave forms and the output wattage . But it sounds like the only way to really know is to figure out the transformer turns and impedance ratio and see if it matches the output tube requirements.

[g1]

A load of 4 ohms didn't give me much output , and 8 & 16 ohm loads were much higher outputs and both about the same.

This further suggests that the amp is designed for a 16 ohm load. With a 4 ohm load you are 2 steps removed from the proper impedance so the output is quite noticeably reduced.
If the amp were designed for an 8 ohm load, you would probably get less noticeable power reduction with either 4 or 16 ohm loads, as in either case you would only be 1 step removed from the proper match.