Help with Jack plate wiring needed for mono 4 OHM stereo 8 OHM

kipple

I need help from someone who knows what they are doing please.

So if you are any good with this sort of stuff could you have a look at this please and see if you can help me understand WTF is going on with the wiring on this cab 

It is an old Rivera TAC 2x12 black carpet covering with metal grill which has 2 x 8 OHM  Electrovoice EVM12L  speakers.

On the jackplate there are two jacks the one jack is labelled mono 4 OHM and the Other is labelled stereo 8 OHM but both speaker jacks on my multi meter give an ohm reading of  6 ohm.

When I put a multi meter on the stereo Jack I would expect a single 8 ohm speaker to read 6 OHM ish and it does read 6 OHM so that seems normal

But ...When I put the meter on the mono jack that is labelled 4 ohm I would expect the 2 x 8 ohm in parralel to give a reading of say 3 ohms but it is also reading 6 ohm 

why is the Mono jack reading 6 OHM ? is the cab wired incorrectly .

Both the white wires are going to the red terminals on the EVM12L and both the black wires are going to the black terminals on the EVM12L .

I have a Rivera M100 amp that has just started playing up really badly. It redplates as soon as I turn it on and I am wondering if using this cab has damaged it as I have been running the amp at 4OHM into the mono jack and I only just realised the mono jack is actually reading 6 OHM.

Can Anyone help with the wiring of the cab.

http://i233.photobucket.com/albums/ee186/druckpig1/_DSC0443_zpsfntzugg3.jpg

http://i233.photobucket.com/albums/ee186/druckpig1/_DSC0444_zpsa05bnhdi.jpg

ICBM

By "putting the multimeter on the jacks", do you mean on the inside of the jacks without anything plugged into them?

The stereo jack has a switch in it, so if there's no plug in it the speakers are connected in parallel - or at least they should be! So if you didn't put something in to open the switch you may have only one working speaker and you were reading that no matter which jack you tried. Or conversely, it's possible the switch point contact has got corroded so it's no longer connecting the second speaker… that would explain why you get the same reading as for mono if both speakers are OK.

First, test each speaker by putting any plug or cable in the stereo jack, and measure the resistance on the wires to the speakers inside - that will tell you if both speakers are definitely OK. If they are and you're still getting a reading of 6 ohms on the mono jack with nothing in the stereo jack, the stereo jack switch is faulty - you can usually clean them by running a small piece of folded sandpaper through the contact.

It's unlikely that this will have damaged the amp though - even if the switch isn't making contact you'll have had a load on the amp if the speaker that's connected directly to the mono jack isn't blown. Running at the wrong impedance by 'one step' usually won't cause any damage.

If the amp redplates immediately on all four valves it has a bias supply failure of some sort. If it's just two of the valves, it could be one shorted one which is pulling down the bias for its partner as well. It's possible that a failed valve could be caused by running at the wrong impedance, but usually only if the amp was really cranked up.

kipple

Hi . I should of mentioned I placed a speaker cable into each of the jacks and then took a reading from the tip and sleeve.
I will look into the switch tommorow . Thanks for the help.

p.s. I have been running the amp Really hard absolutely caning it a couple of times a week for well over 18 months and only just realised the impedance is wrong.

kipple

@ICBM ;;
I followed your advice and put some sandpaper inbetween the contacts on the switch on the speaker jack socket that is labelled stereo and when I take a reading off a speaker lead inserted into the speaker jack socket that is labelled mono it now reads 3 OHMS. 
That's bloody marvellous thanks very much for your help.

 I just have to get the amp sorted now but I'm going to take it to to an amp tech for that as I do not fancy putting a soldering anywhere near the bias circuit as everything is placed very closely together on the circuit board and I do not want to cock it up.
I have lost count of how many times you have helped me now so thanks once again.
Cheers Ian

ICBM

No problem!

That's a very common fault with that type of switching - to be honest, I don't think it's a good idea to rely on such a small point contact to handle that kind of current - even when it's working, the heat that can be generated there will accelerate oxidation. If you can, it's actually better to run the cab in 'dual mono', if you have two speaker cables and the amp has two speaker jacks - ie connect both speaker jacks on the amp to both speaker jacks on the cab, as if it was two separate 1x12"s.

You're lucky that with two EVs in the cab, even if only one of them was actually connected to the amp there would be no risk of blowing it since they're 200W speakers. Whether or not it's contributed to the amp fault I don't know, but you're sensible to leave that to a pro, they are not the easiest amps to work on.