How to measure output transformer?

[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]

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.

 

[Burnedfingers]

Thank you for the information.

 

[lcsaszar]

Connect the 230V AC to the primary (with due caution!) and measure the current. Then meausure the DC resistance of the primary. From these values you can calculate the inductive reactance and the primary inductance. This is more real-life measurement than using an LCR meter.

 

[kia2094]

I have a same problem , mine is 1.7k ohm primary with inductance measurement of 1.7 henry. I got a Victor LCR meter , measured it with known inductor and results were very accurate.

Any help will be appreciated.

 

[hpeter]

i use 20hz Lprimary ; 20khz for Lpara and shorting secondary
SVETELEKTRO ; Audio trafa, toroidy.pl ?

 

[trobbins]

Connect the 230V AC to the primary (with due caution!) and measure the current.

Whoa, take your time on that one!!

There is no need to be applying high mains VAC to an output transformer. If the OT is not rated for that level VAC at 50/60Hz, you will saturate the core and probably damage the OT winding from over-current stress.

The standard test is to use 5VAC, or perhaps up to 20VAC across the PP terminals, and either feed the VAC supply through a mA meter, or put a series sense resistor in the circuit and measure its voltage to determine the AC current. A PP OT would normally present above 10H at 50/60Hz, with the better hi-fi OT's showing above 100H. The inductance will be more at 20VAC excitation than 5VAC. All other terminals on the OT are left open.

When measuring the turns ratio, then I recommend applying something like 5-20VAC to one half of the primary (ie. plate to B+ CT), then measure the VAC on the other half of the primary PP winding, and use that unloaded voltage for calculating primary to secondary voltage (turns) ratio. Then swap over the primary half-windings being used for signal injection and voltage measurement and confirm that all the measurements are effectively the same.

Ciao, Tim

 

[kia2094]

I obsereved few things that may explain why inductance of my OT is low .

1) I have tested the LCR meter with know inductor , and results very pretty accurate(+-10%) , which gets me thinking may be the OT henry's vary by voltage or frequency.

2) the lower inductnace of transformer means it will distort the signal at low frequencies( not saurate , but distort). Now I have seen in many forums people looking for vintage distorting sound , which made me assume the part of the guitar sound may has been from the ot distorting at the lower frequencies. So ot with low inductance may have been part of the design to distort the lower frequency.

 

[korpberget]

There are a few great tutorials on Youtube on this subject. Just make a search for it.

 

[kia2094]

the question is not how to measure at this point , the question is why off the shelf OT have low inductance . Perhaps if we can get more measurements of guitar amp OTs we can make a conclusion.

 

[gingertube]

Some hammond data:

PART NUMBER PRIMARY VOLTAGE PRI INDUCTANCE
1605 224V 63.66 MIN.
1608 283V 63.66 MIN.
1609 316.5V 106H MIN.
1615 274V 106H MIN.
1618 447.5V 106H MIN.
1620 363.4V 106H MIN.
1626 445V 106H MIN.
1627SE 250V 21H MIN.
1628SE 353.6V 79.6H MIN.
1629SE 403.2V 159H MIN.
1635 387.5V 106H MIN.
1637 445V 106H MIN.
1640SE 177V 25H MIN.
1642SE 612V 53H MIN.
1645 387.5V 75.78H MIN.
1650E 346V 63.66H MIN.
1650F 436V 63.66H MIN.
1650G 480V 68.45H MIN.
1650H 514V 106H MIN.
1650K 412.5V 63.66H MIN.
1650N 508V 63.66H MIN.
1650P 630V 63.66H MIN.
1650R 707.1V 63.66H MIN.
1650T 477.5V 57.9H MIN.
1650W 730V 63.66H MIN.

Cheers,
Ian

 

[jazbo8]

the question is not how to measure at this point , the question is why off the shelf OT have low inductance.

It appears that you still do not know how to measure the OPT's, please see gingertube's post above, do you see what you are missing?

 

[gingertube]

See post#4 for method
http://www.diyaudio.com/forums/tube...roper-way-measure-transformer-inductance.html

The Hammond data was not measured in this way but was provided to me in a .xls spreadsheet by Hammond.
Cheers,
Ian