Lprimary variation in pair old opts

[Klimon]

Hi,

I measured a pair of old, presumably high quality opts and was surprised how much the primary inductance varies: 5,5H and 6,7H. Primary resistances measure identical for both trafo's. Can the variation in inductance be ascribed to one of the cores losing some of it's magnetism over time?

Am I right to suppose that the opt with lowest inductance will have less extended LF? Is remagnetizing a theoretical option?

Cheers -- Simon

[atarashi]

Hi Simon,

no, this can't be the reason, as the core is not magnetized! (not permanetly, only by the signal)

but, and now i assume, that we are talking about se opts,
it could be, that the laminations are bent,
or, more likely, that the airgap is out off shape.

or are the laminations rusty?

and yes, you are right, the core with the lower Lprim will have a higher lf cutoff.
but the difference is only 21%, so it should not make that much off a difference.

hope that helps!

brgds
Michael

[Dave Cigna]

I'd guess that the mostly cause is a difference in the air gap.

But, I really have to question your measurements. 6H is very low, even for an SE output transformer. Not sure what the intended application is, but I'd expect at least 3 times that, probably more.

-- Dave

[dave slagle]

i'm with dave,

i'd guess measurement error might be part of the explanation.

how did you measure them?

dave

[poobah]

Dave's "guess" is dead on the money...

It is just variation in gap that is causing the difference... relax.

[dave slagle]

I'll bet the airgaps are matched and the difference in measured inductance has to do with capacitance.

dave

[Klimon]

Hello,

The pair I measured was push-pull built for el84 parallel PP, from one end to the other.

I measured two other pairs of old trafo's yesterday and they varied even more

Se el84: about 8 and 12H (in pentode-mode with global NFB bass is very extended and tight with these opts)

PP ecl82: about 6 and 9H

All are in good shape (not deformed in any way), there's some surface rust on the first pair but that's no problem or so I read.

The battery of my multimeter is low but a new 10H choke still measures 8.6H so it won't be very far off. As comparison I measured three new opts of my generic budget type; all 9.5H (built for se el84 and similar, 5.2k, 60mA max., 30hz -3db)

I'll measure capacitance to see what that says.

Thanks for your opinions;

Simon

[Svein_B]

What is the proper procedure to measure the OPT primary inductance?
- with the secondary open, shorted, loaded with 8 ohm, or does not matter?

I tried to measure some transformeers some time ago with a professional LCR instrument, but had difficulty geting the expected results.

SveinB

[Circlotron]

Apply 30-50 volts AC sinewave at a known voltage (not too much or you'll saturate it) and measure the current. This will tell you the reactance. Use an ohmmeter and measure the resistance. Resistance is the bottom of the triangle, reactance is the hypotenuse. Inductance is the vertical side of the triangle.

[Circlotron]

Originally posted by Circlotron
Inductance is the vertical side of the triangle.

Whoops! Should say "Impedance is the vertical side of the triangle". From the impedance and the known test frequency you work out the inductance.