Center tap impedance?

[studioit]

Am I crazy? shouldn't the impedance of the center tap to either plate wire(outside leg) on the primary side of an output trans be roughly half that of the imp across the full primary winding? I have this transformer- 1958 output from an ampex 620 ( a powered speaker with a pp 6v6 output section). The 620 worked fine. I took out the transformer to use in a 5e3 style amp I'm building and the primary(blue & Brown) measures 308 ohms. When I measure from center lead to either blue or brown the meter(fluke 73) reads -0 and the
analog bar on the bottom of the display pulses. What is going on?
Also - I have opened up the bell ends and peeled back the paper carefully and all leads are still connected and I get the same results when touching the meter directly where the windings are soldered to the leads. The
center lead has 2 winding wires con to it where as the blue and brown have 1 ea which would seem to make sense.
Any Ideas? what am I missing?
Thanks
Ian Talcroft

[rocket]

The DC resitance should be roughly the have.

The AC resitance (impedance) is mainly determinened by the inductance and this is a QUATER for half the number of windings. i.e. the impedance is also a quarter, it goes with the square of the number of windings.

What does -0 mean on your meter -overflow? or might there be some voltage e.g. from a cap?

[Enzo]

Your meter is sending pulses of current into it to measure it. The inductance of the coil winding is kicking back at the meter confusing it. Depending on the meter, the transformer and what is connected to it, this may or may not happen in any given instance. SHort the other plate lead to the center tap and see if that stops it. Then short the other way to get the second half reading. Or maybe load or unload the secondary. SHould be some way to dampen the thing.

You cannot measure the impedance of the winding, your meter measures resistance.

The transformer has no impedance of its own. The impedance the tubes see are determined by the load and the turns ratio of the transformer. If I take a transformer intended for 6000 ohms primary to 8 ohms secondary and connect a 4 ohm load to it, all of a sudden the "impedance" of the primary changes substantially.


With a given load, the impedance at the primary is determined by the turns ratio in the tranny. Unless it is wound really poorly, the two halves of the primary would indeed have the same impedance, but may or may not have the same resistance. The resistance of the winding is not part of the impedance really. WHatever it is - somewhere between 15 and 100 ohms each half I'd guess - it represents an insignificane part of the several thousand ohms.

The two halves often measure close, and in a well made double wound transformer we expect it. But many transformers are wound simply, which means half the turns are wound, a center tap is connected, then the rest of the turns are wound. In this case, the first windings are around the small core, while the second half windings are around the first half. This means that each turn of the winding in the later half has to be a lot longer to reach around the greater diameter of the thing. The early windings are tight around teh core and have a smaller diameter. Longer wire means higher resistance, but the turns count being equal mena the impedances are the same.

[rocket]

You cannot measure the impedance of the winding, your meter measures resistance.

The transformer has no impedance of its own. The impedance the tubes see are determined by the load and the turns ratio of the transformer. If I take a transformer intended for 6000 ohms primary to 8 ohms secondary and connect a 4 ohm load to it, all of a sudden the "impedance" of the primary changes substantially.

Yes usually a DMM cannot measure impedance directly. I was thinking of measuring the (stray) inductance of the windings in Henry, which many meters can measure, however it will be beyond the meter's range in an big OT.

For small transformers the stray inductance can be easliy measured and will be in the range of tens of Henry.

Measuring the stray inductance can give a good indication of the relations of the number of turn in each winding, which is proportional to to square root of
the inductance.

[studioit]

Shorting the lead not being tested to the center worked . I was just trying to check to make sure the new leads I had connected were ok not trying to determine the impedance- just using the wrong words as well. Anyway thanks for the help.
Ian