Marshall Laws
Ohms Law
"The Law of Ohms" or "Ohms Law"…whatever you feel like calling it is pretty straightforward stuff and, seeing as I’m a guitar player, I prefer to keep things simple…real simple! So, let's learn by looking at an example of "Ohms Law" at work. First though, let's remind ourselves of Mr. Ohm's often forgotten formulas. There are two and they enable us to find out the combined impedance of speakers (or cabinets) when joined together in either Series or Parallel.
Let's imagine that R1 is the impedance of our first speaker (or cabinet); R2 in the impedance of the second one and R3 is the resulting impedance when they are hooked-up together. Here are the two formulas:
The Parallel formula is R3 = R1 X R2 & the Series formula is R3 = R1 + R2
R1 + R2
So, let's look at the Parallel law in action. This is the one you need to be most familiar with because although speakers inside cabinets are often wired up in series, when it comes to hooking-up cabinets to combos or heads it is ALWAYS done in parallel (Marshall wise).
The most common question asked in stack world is, in my experience this, "What impedance do I set my tube head to when I'm using two cabs (such as a DSL100 head and a 1960A and 1960B)?" Here's an idiot proof hook-up list:
Make sure both cabinets are switched to Mono operation.
Make sure you have two SPEAKER cables to do the hook-up with (NEVER use guitar cables).
Plug one end of one lead into the 16 Ohm MONO input of the 1960A (NOT the 4 Ohm input…remember, there are two because of the stereo option).
Plug one end of the other cable into the 16 Ohm MONO input of the 1960B.
Now, as we're hooking two 16 Ohm cabinets up to our DSL100 head in parallel, let's plug R1 (16) and R2 (also 16) into our trusty formula:
R3 = 16 x 16 = 256 = 8 Ohms
16 + 16 32
Set the 4 Ohm/8 Ohm switch below the two parallel speaker outputs on the back panel of our DSL100, plug the speaker cable ends into said outputs….
Switch the amp on, plug in, dial in a tone and rock!!
And there you have it. In fact, a simplified offshoot of the Parallel law we've just learned is this:
Whenever you're hooking-up two cabinets of the same exact impedance (and let's call that impedance "X") in parallel, the resulting impedance is X/2.
So, 16 & 16 = 8; 8 & 8 = 4; 4 & 4 = 2, etc…