Marshall JCM 900 impedance question

[charvel750]

Guys, on the back of my JCM 900 amps the impedance selector works like this:

(4) 16 8 ohms.

But the switch is only 2 way. So if you run it at 4 ohms, the switch is full left, right? But then what if you want to run it at 16 ohms? Still full left? I don't get it, please help...

[VitaminG]

on the heads, it switches between 8 & 16 ohms. The "4" means nothing on the heads & the 4(16) just seems to be there to create confusion.

I haven't checked out a 900 combo to confirm, but I figure Marshall must've used the same rear plates for combos & heads, since the combos switch between 4 & 8. So the left position will be 4 or 16 depending on if you have a combo or head respectively.

[Robert Burns]

on the heads, it switches between 8 & 16 ohms. The "4" means nothing on the heads & the 4(16) just seems to be there to create confusion.

I haven't checked out a 900 combo to confirm, but I figure Marshall must've used the same rear plates for combos & heads, since the combos switch between 4 & 8. So the left position will be 4 or 16 depending on if you have a combo or head respectively.

************
Just make sure the amp impedence setting is LESS than the ohm's of
the spearker. If the speaker is actually 6.4 ohms...set the amp at 4 ohms
if the speaker is 9 ohms...set the amp at 8 ohms..
you just dont want an 8 ohm speaker on the 16 ohm amp setting.

[JerryP]

The heads are labeled 8 and 16 (4) meaning 8 and 16 ohms jacks and it can be rewired for 4 ohms.
The combos are labeled 8 and 4 (16) meaning 4 and 8 ohms jack with the ability to be rewired to 16 ohm. The transformer taps are there, they just wire the combo and head different to fit the most common load configurations for each.
Jerry

[charvel750]

Thanks guys, so to confirm, on the head version, you can only run it at either 16 ohms or at 8 ohms, right? If you want to run the head at 4 ohms you'd need to rewire it, right?

The 1960 4X12 runs at 16 ohms or 4 ohms mono. In stereo it's 8 ohms a side, so all bases seem to be covered.

[VitaminG]

Thanks guys, so to confirm, on the head version, you can only run it at either 16 ohms or at 8 ohms, right? If you want to run the head at 4 ohms you'd need to rewire it, right?

The 1960 4X12 runs at 16 ohms or 4 ohms mono. In stereo it's 8 ohms a side, so all bases seem to be covered.

seems like it, Joe!

nice one, Jerry. I wondered about that, but have never seen a 900 combo to compare. thanks for the info

[JerryP]

Thanks guys, so to confirm, on the head version, you can only run it at either 16 ohms or at 8 ohms, right? If you want to run the head at 4 ohms you'd need to rewire it, right?

The 1960 4X12 runs at 16 ohms or 4 ohms mono. In stereo it's 8 ohms a side, so all bases seem to be covered.

That is correct.
Jerry