measuring OT's primary impedance

[Robert M. Martinelli]

I compare it to a set of gears. The gears always have a speed ratio between them, but they have no inherent speed of their own.

Enzo,
That's a very effective and easy-to-understand way to explain it, as usual you manage to make things easier to understand to everyone than I do,
( only, I' d talk about a gearbox' s inertia ratio rather than speed in your example, since inertia is similar to impedance, and it's proportional to the gearbox' s ratio squared, just like impedance is proportional to the OT ratio squared, while voltage and current in an OT can be seen as speed and torque in a gearbox respectively )...OOps, sorry.....memories from my past as an engineering student, a long, long, long, long time ago....( My elder son actually refers to that as "the dinosaur age". )

Bob

[rocket]

Actually the inductance methode described in the original postl gives the same result as the voltage methode described later.

That is quite reasonable as a transformer actually is 2 or more coupled coils and each of those has an inductance that can be measured and is mainly o function of the number of windings (besides of course of the core's µ and the shape, which however are the same for all windings)

[lowell]

Hey guys,
Informative thread.... how does primary Z relate to measuring primary resistance w/ an ohmmeter? Or should I ask if they are completely different. Impedance is still kind of tough for me to completely grasp. I know it is a frequency dependent measurement but that's about it. Just curious because I'm reading this and other threads finding that primary impedance in a Deluxe Reverb is around 6.5k, but when I measure primary resistance of the one in my shop it is 400ohms end-end.

[Enzo]

Reread this thread. The impedance is a reflection of one side to the other. Transformers have impedance ratios only. They do not have inherent impedance. That 6k/8 ohm transformer ONLY shows 6k to the tube plate if there is indeed 8 ohms across the secondary.

The measured resistance of the winding wire has ZERO to do with the impedance

[lowell]

ahhhh ha... ok thanks. A bit clearer now.

[Steve Conner]

I'd just like to add a little to the concept that an OT has "no inherent impedance of its own". While this may be true, there's certainly a range of impedances that it's designed to work well with.

If you try to run a lower impedance than the OT is designed for, you'll lose a lot of your power as heat in the windings. If you try to run a higher one, you'll lose bass because the OT's magnetizing inductance shunts it away from the speaker.

Generally a good OT is usable about a factor of 2 either side of what it was designed for, so you could get away with using a 3k OT anywhere between 1.5k and 6k. With the common trick of pulling one pair of tubes from a 100W amp, and moving the impedance selector down one tap to compensate, this is basically what you're doing: using a 3k OT as a ghetto 6k unit.

If you try to push it much further, to use the mechanical analogy, it's like using a scooter gearbox on a tractor or vice versa. The gear ratio might be right for the job, but the gears are either so light they'll strip, or so heavy that the engine can't even spin them. For a graphic demonstration of this you could try two paralleled Deluxe OTs in a 400PS, after all the impedance is right

[Enzo]

Well, OK, but that is outside the scope of the point. Obviously if I tried to draw 100 watts out of an impedentially correct (is that a real word?) 2 watt transformer it would burn it up. So sure they are designed for a purpose.

But I would also say that given a transformer with no markings, we would be hard pressed to determine what the spec impedance is for it. We might infer from its size certain things. That 400S OT we could safely predict was not intended for a pair of EL84s or a CHamp circuit. And we could decide approximately its power capability and guess what tube combinations and speaker loads are likely, but that too is outside the point.


Lowell's question was about measuring resistance of a winding and equating that to impedance.

[R.G.]

I'd just like to add a little to the concept that an OT has "no inherent impedance of its own". While this may be true, there's certainly a range of impedances that it's designed to work well with.

If you try to run a lower impedance than the OT is designed for, you'll lose a lot of your power as heat in the windings. If you try to run a higher one, you'll lose bass because the OT's magnetizing inductance shunts it away from the speaker.

You're right, this is a better way to look at it. However, it's one that the neophyte can't appreciate. I find I get better teaching results by whacking them in the head with "it has no impedance of its own", which is correct by the letter of the statement, if incomplete, and more useful to them for what they're trapped in at the moment. Later as they study more, if they do, they find that there are ranges of OK impedances, if not one specific one, which is the fuller story. Telling the full story early has impeded a number of newbies of mine that I later had to un- and re-teach.