NEW GUY with 8ohm LOADED Question!

[Alexander P]

New Guy here! Gear head, not tech. Please help?
I have 4 8ohm drivers and I want to run them off my early 70s Blackline SVT head. I know the guy I want to talk to is familiar with it. Standard would be a 4 ohm load, If I understand correctly plugging into the secondary speaker outlet automatically taps me into 2 ohms. Wiring the drivers to my understanding I can achieve 2ohm(parallel) or 8ohm(series parallel) but never 4ohm.
Does this sound correct? Is there any reason I shouldn't parallel wire the cab to 2ohm and plug it in the secondary jack direct? Looking for solid advice over opinions!
Thanks if you read this far! Can someone help?

 

[Wasnex]

New Guy here! Gear head, not tech. Please help?
I have 4 8ohm drivers and I want to run them off my early 70s Blackline SVT head. I know the guy I want to talk to is familiar with it. Standard would be a 4 ohm load, If I understand correctly plugging into the secondary speaker outlet automatically taps me into 2 ohms. Wiring the drivers to my understanding I can achieve 2ohm(parallel) or 8ohm(series parallel) but never 4ohm.
Does this sound correct? Is there any reason I shouldn't parallel wire the cab to 2ohm and plug it in the secondary jack direct? Looking for solid advice over opinions!
Thanks if you read this far! Can someone help?

If your amp has two 1/4 jacks and no impedance switch, I believe this is what is going on:

Main speaker out. This is a shorting jack so you have to plug something into it to lift the short. The short is intended to protect the amp from an open circuit in case you forget to plug in a speaker. When you only use this jack the amp is configured for 4 ohms.

Ext speaker out. This is a switching jack. The tip is in parallel with the Main speaker out. When you insert a plug into this jack the amp switches from 4 ohms to 2 ohms, but you need a plug in both the Ext and Main speaker out for the amp to work

If you load all four 8 ohm drivers into one massive cab and wire them in parallel to one jack, the load will be 2 ohms.

Here is what to do:
First, run a speaker cable from the cab the the Ext speaker out. This will configure the amp for 2 Ohm operation. In order to work around the shorting jack on the Main speaker out, just insert an open 1/4" TS dummy plug.

By "open" I mean there is no connection between tip and sleeve. A brand new, unmolested 1/4" TS plug should be all you need. Often a dummy plug will be of the shorting type. In other words, a jumper is run between tip and sleeve. You do not want a shorting plug.

The amp's out circuit should look like this:

But as I mentioned, there is variation from schematic to schematic, so I can't guarantee this is correct. If you have doubts, have a technician inspect how your amp is wired.

 

[Alexander P]

Thanks Wasnex for your valuable time, sounds like I'm headed in the right direction. I didn't realize I'd need a open dummy. I think you saved me a fuse and some frustration. Thanks TalkBass, This is the place!