Non ADA Gear > Speakers & Cabinets

Watts and Ohms and Cabs and Oh My!

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McLeanAB:
Hey ADA junkies...

 I have a Marshall 1960av and an ISP Technologies Stealth Power Amp and an ISP Theta rack preamp. I've upgraded the old V30's (didn't like 'em) to two 8ohm Greenbacks 25 watts on top and two 16ohm Creamback 65watts on the bottom... it sounds amazing and the Stealth can handle ohms down to 1ohm or so, so there's no danger there...

What I AM curious about is the 'spread' of watts from the power amp (90 watts per side, but currently running mono so 90 watts) 'across' the speakers. Do the Greenbacks 'absorb' a full 25 watts each and the Creambacks a full 65 watts? How does the distribution of watts from the poweramp spread across speakers? Any and all information is greatly appreciated... I'd love to run this cab stereo, but I'm afraid that 90 watts in each (running stereo) side as opposed to 90 in the whole cab (running mono) might blow the Greenies... let me know what you know so my speakers stay happy!

MarshallJMP:
I just looked it up and you need at least 4 ohm on the output, I think if you go down to 1 ohm you will blow it up. So 4 ohms is the minimum !!

Now for the cab, how did you wire it? If I remember correctly those AV cabs have dual inputs and a mono stereo switch.
It's not such a good idea to mix different speaker impedances in one cab. In your case (depending how they are wired) the greenbacks will "absorb" twice the power of the creambacks because the GB's are 8 ohms and the CB's are 16 ohms.

So let me know the hookup and I can tell you what's safe and what's not. You can always rewire them to get in a safe mode.

vansinn:
For mono operation, wire the two 8 Ohm Greenbacks in series for 16 Ohms across those, then this 16 Ohm series circuit in parallel with both 16 Ohm speakers. You'll end up with a 5.3 Ohm load on your amp.

This will result in fairly even load on each speaker; the two 8 Ohm 25 watters would now take 50 watts across, so the other two should be presented with the same, meaning you cannot load the 65 watters to their max, but only to what the two 25'ers can take combined.

Lousy ascii art schematic :)

   +-8--8-+
--+--16--+--
   +--16--+

MarshallJMP:
In case of Van's schematic each CB will have 30W and the 2 GB combined will have 30W, in case of an output of 90W.So this is safe for the all speakers.

McLeanAB:

--- Quote from: MarshallJMP on November 19, 2016, 02:47:11 PM ---In case of Van's schematic each CB will have 30W and the 2 GB combined will have 30W, in case of an output of 90W.So this is safe for the all speakers.

--- End quote ---

Ugh!  So, as of now, with the two Greenies on top, two Creams on bottom, running with the 'traditional' wiring of the 1960av cab (I didn't alter it at all) so the stereo to mono (8ohms stereo with option of 16ohm or 4ohm mono - this is the labeled cab jacks not the speakers) I should run the the 16ohm mono jack (which equals, what... 12 ohms?) and just keep things mono?  Will THAT keep the speakers safe for the moment?

I would totally rewire according to VS diagram, but I've no idea how to do that!!!!  :)

Hmmmm... anyone want to trade a pair of 8ohm Greenbacks for a pair of 16ohm Greenbacks and I can just be done with this??  :)

Thanks for the info fellas!

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