is Impedance related to ac only ...
Resistance is the opposition to the flow of current. Impedance is the opposition to the flow of alternating current, and includes resistance as well as the frequency-dependent element of reactance.
Said another way: d.c. is like a frequency of 0. Reactance has a formula with frequency of the a.c. applied, so if the frequency is 0, the reactance is also 0. If impedance is resistance and reactance, but reactance is zero because we have d.c., then the only thing left is resistance.
See how resistance matters to both a.c. and d.c., but impedance includes an extra element that only applies to a.c.?
Also is an inducter ... You said "coil of wire"--did you mean wire wound on a coil like a transformer ...
Yes.
Inductor = choke = transformer winding. It is very literally a wire wound into a coil, because the field set up around the wire strengthens and increases the inductance of the resulting coil. Below radio frequency, inductors/chokes/transformers will be wound in coils, and will generally have a metal core of some kind to increase inductance.
You know already that transformer windings have some resistance, but the 4kΩ primary of your OT doesn't measure as 4000Ω when you attach your meter to measure resistance. Also, a choke may look like thousands of ohms at 120Hz, but have a d.c. resistance of only tens of ohms. You should also notice that "
d.c. resistance" is redundant, but has become common because there are other places where we might wrongly say "resistance" when talking about something that only exists with an a.c. signal.
So I really didn't know the transformer blocked dc and passed ac like a capacitor but that should be obvious because the OT has high voltage dc on the primary and ac only on the secondary.
Yes sir!
It's one of the important functions of a transformer that we often don't think much about: keeping d.c. levels separated while coupling a.c.
You could say a cap and a transformer (or more broadly, a choke) do the same types of things, but come at the problem from opposite directions. Which is a better choice in a circuit pretty much comes down to the frequency being handled and cost considerations.
Ok I'm thinking out loud---in the OT primary you have high voltage dc and ac [plate] signal. So the high voltage dc is blocked at the OT primary and the ac [plate] voltage is stepped down and the current is increased by the same equal ratio the ac is steppped down on the secondary? so I'm thinking by tapping this step down process at different points is where we get the multi Ohm taps?
Yes sir!
Power is the same at all the taps. The 16Ω tap has twice the voltage and half the current (for the same power) as the 4Ω tap.
Example:
- Say there is 16vac at the 16Ω tap. 16v / 16Ω = 1A. 16v * 1A = 16w
- The 4Ω tap has half the voltage as the 16Ω tap, or 8vac. 8v / 4Ω = 2A. 8v * 2A = 16w
- See how the voltage and current changed for each of the taps, but power stayed the same?
Also I'm assuming from what you said lowering voltage and increasing current lowers impedance and vice versa higher voltage and lower current raises impedance???
Yes sir!
If you firmly know ohm's law and the equation for power, then pick a number for power (say, 10w). Pick a number to be your voltage (let's try 500v), and calculate your current for that power (10w / 500v = 20mA). Use ohm's law to see what resistance allows the current to flow when the voltage is applied (500v / 0.02A = 25kΩ). Now keep the same power and pick a radically different voltage, and repeat (25v; 10w / 25v = 400mA; 25v / 0.4A = 62.5Ω).
You can see whichever way voltage goes, so does impedance. You might say if the impedance gets bigger, it takes more voltage to push a given amount of power through the impedance.
Also I'm wondering at what level or threshold does a ac signal go from being high impedance to low impedance or vice versa?
You know Einstein? It's all relative...
For a preamp tube plate, 600Ω looks like a low impedance. For an 8Ω speaker, 600Ω looks like a high impedance.
"Low" or "high" only makes sense when you talk about it in reference to some other impedance or circuit point/element.
Topic: Picked up a Silverface Fender Pro Reverb Head in Pieces (Read 24630 times)