Watts and Ohms and Cabs and Oh My!

[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]

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.

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!

[rnolan]

So if the GBs are 8 ohm and CBs are 16 ohm, you can wire one GB and one CB per side in parallel (+ve to +ve, -ve to -ve) which will give you ~ 11 ohms per side. The amp will output less into the higher load (90 w into 4 ohms, maybe 55 w into 11 ohms ?) and run in stereo like that. And mono for the cab will be 5.3 ohms so ok for the poweramp for one channel (80ish watts into 5.3 ohms ?).  However some cab stereo/mono cab switching set ups also include some other circuitry in the switch for impedance matching.

[vansinn]

Yep to all above wiring considerations.
As Nolan stated, dunno if there might be additional components to consider.
You could make us a pic from the inside of the jack/switching/wiring area..

[MarshallJMP]

Ok in your case MC ,this would mean;

Mono 16 ohm input: 10.6 ohms
Mono 4 ohm input: 2.66 ohms ==> DON'T USE THIS !!! your amp won't like that.
Stereo each input:5.33 ohms

Now this how a Marshall cab is usually wired.So to make sure plug in a cable and use and ohm meter to measure the resistance of each possibility.The dc resistance will always a bit lower.
If you can post your measurements here so we can check them.

Because of the mismatch in impedances the GB will take 2/3 of the power and the CB 1/3 of the power.

[McLeanAB]

Hey there MJMP!

Here's what I'm doing... I'm going from only one channel output on my Stealth power amp (I'm assuming 4ohms) to the 16ohm input on the back of the Marshall input jack.  MONO.

So everything is safe for the moment, yes?

Here's what I think I'm gonna do... trade out the 8ohm Greenies for two 16ohm Greenies... OR, just go up and get the 75 watt Creambacks G12H's at 16ohms each and use the cab in stereo.

Thanks again for all the info!

[McLeanAB]

Okay, gentlemen...

It seems I might land some 16ohm 25 watt Greenbacks to compliment my 16ohm Creambacks in my Marshall 1960 cab...

Since the ohms will all match, running mono from my Stealth at 90 watts and seemingly 4 ohms, can I run stereo from the Stealth at 90 watts per side and not blow the Greenies?  When the cab is in stereo, it's 8ohms per side (even though the Stealth is 4ohms which is okay).

So back to my first question:  how are watts 'distributed' across the speakers?  If it's 90 watts per side, with a 90 watt total between the GB and CB, will it be safe and distribute only 25 to the GB and the other 65 to the CB?

Let me know, as I'd love to have a stereo cab!

[MarshallJMP]

According to do specs the amps puts out 45w/side at 8 ohms,so if all speakers are 16 ohms each one will use half the power, so the GB will get 22.5w and the CB also. Should be safe for both but it's close for the GB.

[GuitarBuilder]

Check out the attached article - it has every conceivable 4x12 wiring combination.

[Harley Hexxe]

According to do specs the amps puts out 45w/side at 8 ohms,so if all speakers are 16 ohms each one will use half the power, so the GB will get 22.5w and the CB also. Should be safe for both but it's close for the GB.

  That could be an interesting sounding stereo cab. With one speaker almost driving to full power and one coming through cleaner, that could be an interesting tone.

    Harley 8)

[McLeanAB]

According to do specs the amps puts out 45w/side at 8 ohms,so if all speakers are 16 ohms each one will use half the power, so the GB will get 22.5w and the CB also. Should be safe for both but it's close for the GB.

So, I'm guessing stereo (90 watts per side 8ohms each side) will still overload the Greenies?

[rnolan]

Possibly, e.g. 2 x 25 what 16ohm speakers in parallel will be 50 w 8 ohm.  If the amp delvers 90 w into 4 ohms it will deliver less (usually) into 8 ohms ~60 w.  Unless you play full on thrash that should ok as the output wont be full the whole time.  Overpowering speakers is a valid concept for headroom but you have to be careful or you will blow cones.  Much safer to match the maximums...