# Re: Speaker/OHM question for two 2x12" ext cabs

`ScoFF wrote:`

```I searched the group for similar topics but I got tired after 30

`I have a mono Marshall JCM 800 2204 Head.`

`I have two 2x12" extension cabinets.`

`One is a Marshall 1936 cab, the other is a Randall 212CS`

```They both are 8 ohm.  I've been switching back and forth trying to
decide which one I liked better by just using the 8OHM tap on the amp
to the 8OHM Mono input on one cabinet at a time.
I've decided to try running both at the same time using both taps on

```***
If I used both taps for both separate cabs, what would the selector be
set at?  4?
```

Yup! 1/R(t) = 1/R(1)+1/R(2)+1/R(3)...
`***`

From my understanding, parallel divides the impedance by the number of
```speakers:  2x16 ohm in parallel gives 8, 4x16 gives you 4.   1x16 gives
16.
```

Yes, if all impedances are the same. If you have different values, use the equation above (which also works for same impedance value, BTW).

What is series? Adding the impedance together?
`Yes.`

```2x8ohm speakers gives
16 in series?  If you had 2x16 in series it'd be 32. I never heard of a
cab offering 32.```

```What's the difference in parellel and series.  If a speaker blows in
one or the other, does one offer any protection?
```

If a series wired cab blows a speaker, you get instant open and big problemo for the output transformer. If one speaker in a series/parallel configuration blows, you're safe.

I'm assuming my factory configured cabs are in the safest scenerio.

Not always. Why? Because 16 ohms is often the preferred impedance, so a two speaker cab is often two 8's in series.

```Also, is there anything wrong with running your amp selector at 4 or 16
assuming your cabs are wanting that?
```

Always best to match. Many prefer using highest practical value, because you use more of the secondary winding (although a properly designed OPT obviously should be safe at 4 ohms at full power).
```I think 4x16 cabs are in series/parallel configuration at 16 ohms.
`Thanks.`
`.`