How to measure output transformer?

[Original Burnedfingers]

Having never done this and not knowing what I have in this unknown output transformer can someone help? I have a 6L6 PP output transformer. I applied a small voltage of 6.91 volts to the tube side primary of the transformer and got .234 out of it from common to 8 ohm terminal. Now, would this be a 29.5 ratio? How do I figure what impedance the transformer is from this?

[pieter t]

Yes, when you kept the secondary unloaded you have well measured the primary to secondary winding ratio.
The winding ratio squared gives the impedance ratio, so for your transformer the primary plate - plate impedance is around 7k referred to the 8 ohms secondary impedance.

[SemperFi]

Ratio squared multiplied with the load, so it's (29.5x29.5)x8ohms=6962ohms.
The results will be a little lower with a loaded secondary (or primary - doesn't matter which), but your probably close enuff.

You could also use and inductance meter to find the inductance, tho this is only important if you want to know the lower frequency response since it creates a high pass filter with the tube's plate impedance. (You can also use a signal generator and a set resistance in series and that way find the lower -3dB frequency and thus inductance using L=R/[2(pi)f], but this aint much importante).

Edit: Pieter beat me to it, sorry for double-posting

[costis_n]

On a similar topic, how do I measure the primary inductance of an output transformer?
My digital LCR meter gives a number that is too low to be believable, and it has been suggested to me that i should measure it under load? I have a pretty good lab, maybe someone enlighten me?

[pieter t]

Originally Posted by costis_n

My digital LCR meter gives a number that is too low to be believable, and it has been suggested to me that i should measure it under load?

What inductance do you measure?
What LCR meter are you using? (batteries checked?)
Don't measure under load (you probably measure leakage inductance that way)

[costis_n]

I use an B&K electronic LCR from the 80's , it generally is OK. It measures something around 1 Henry, with the secondary open, primary connected to nothing else but the LCR.
Further instructions?

[pieter t]

1 Hy is too low for whatever output transformer.
What transformer do you have there?
Did you check correct functioning of the LCR meter with some other (known) inductance?

[costis_n]

I knowwwwww it's low! I was told it measures that low because it is supposed to measure like that with just a LCR. I should use the signal gen and the scope to measure it, but i don't know. Anyone?

[pieter t]

You make me curious: you were told that measuring that low inductance with an LCR meter is normal?
Well it is not.
First of all: inductance measurements (with inductance meters or whatever) are steady state measurements, which means measurements with a fixed AC signal at a fixed frequency. Your B&K meter will specify the measurement conditions.
Normal inductance meters measure with only a couple of volts, mostly with less than a volt; measuring frequency is mostly 100Hz and 1kHz.
Real professional (and expensive!) meters can measure over an extended and continuous range of frequencies, but at the same time not with AC voltages approaching practical circumstances in a tube amplifier.
Inductance depends on frequency, AC voltage and core material.
In a tube amp output stage the primary inductance of the output transformer can approach values of hundreds of Henries because of the high AC voltages at the transformer primary, and the static measurement of the inductance meter gives only a hint.
To give an idea, for a 5k output transformer we wish to have some 25 to 30 Hy measured with the L meter, but it is a static measurement.
Your 1 Hy is too low; therefore I asked what it is.

[Original Burnedfingers]

Thank you for the information.