#1
Hey. I got a Peavey 412MS 4x12 cabinet from a friend for $30 dollars in perfect condish. Only thing is, it is a stereo cabinet (8 ohm stereo, 16 ohm mono).

My amp head puts out 120 watts tube at 8 ohms, and 60 watts at 16 ohms.

Currently, I have the amp switched to 16 ohms and the cabinet is running in mono at 16 ohms, but this is only half the power the amp is capable of producing, which sucks.

I want to get the full 120 watts out of my head for obvious reasons. Can I use the two amp outputs and plug both of them into the cabinet without worry (the cab has left and right channel inputs when being used in stereo)? Has anyone with a stereo cab tried this? I know an 8 ohm cabinet would be ideal, but I got this thing for $30 bucks and it sounds pretty rad. I hope that running the same signal to both channels won't be an issue.
Regards,
PowrSlave
Last edited by PowrSlave at Feb 18, 2007,
#3
more than likely yes. from my experience, that shouldn't damage anything, but damn, thats gonna be loud.
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#4
Before you do anything, what kind of head is it?
It's pretty likely you can rewire the speakers to make 8 ohms in mono. All I need is the ohmage of the individual speakers.
"Everybody, one day will die and be forgotten. Act and behave in a way that will make life interesting and fun. Find a passion, form relationships, don't be afraid to get out there and fuck what everyone else thinks."
#5
The amp is a Crate Blue Voodoo. The speakers in the cabinet are Sheffield 1230s or 1290s and are 16 ohms each from what I recall, but I will crack it open tomorrow and have a looksee. I can't think of a way to wire them up to make 8 ohms mono though.

And yeah it will be loud...gotta be ready for it when/if it's needed and sometimes it is.

I'm just hoping I can direct both amp outputs to both inputs on the cabinet in stereo mode.
Regards,
PowrSlave
#6
16 ohms eh... that makes it tricky. I can't think of a wire to get them down to 8 ohms easily....you could just put the money you saved on the cabinet into some good 8 ohm speakers?
"Everybody, one day will die and be forgotten. Act and behave in a way that will make life interesting and fun. Find a passion, form relationships, don't be afraid to get out there and fuck what everyone else thinks."
#7
Quote by PowrSlave
Hey. I got a Peavey 412MS 4x12 cabinet from a friend for $30 dollars in perfect condish. Only thing is, it is a stereo cabinet (8 ohm stereo, 16 ohm mono).

My amp head puts out 120 watts tube at 8 ohms, and 60 watts at 16 ohms.

Currently, I have the amp switched to 16 ohms and the cabinet is running in mono at 16 ohms, but this is only half the power the amp is capable of producing, which sucks.

I want to get the full 120 watts out of my head for obvious reasons.


Whether running at 8 or 16 ohms you ALREADY are running your tubes at full power with the master volume on 10.

The "power vs resistive load" ratings "120 watts at 8 ohms", and "60 watts at 16 ohms" are indicative to allow you to pick the right speaker cabinet, i.e. the ratings tell you that if you try running your amp cranked in a 60 watt - 8 ohms cab, you'll blow up the speakers or eventually the crossover circuit if you have a multiway cabinet, i.e. bass cabients which often have an additional tweeter or horn for high frequencies.

If an amp has different output power ratings, it generally has a "pentode/triode" switch that allows you, i.e. to change the operating mode of the tubes, as running pentode tubes in triode mode, or eventually shutting down some tubes.
See picture below of the backplane of a Marshall 30th anniversary. On the far right is the pentode/triode switch. (Additional to another Low/High power switch).

Some amps allow you to simply take some power tubes out for reduced power.

Last edited by ColdGin at Feb 18, 2007,
#8
Coldgin, you are obviously quite knowledgeable on this subject, thanks for the info. Have you ever used both speaker outputs on a single stereo 4x12 cabinet? One for the right channel and one for the left? The impedance of my cabinet is 8 ohms ONLY in stereo configuration which leads me to believe that I could set the selector on my amp to 8 ohms and get full power.
Regards,
PowrSlave
#9
Quote by PowrSlave

Have you ever used both speaker outputs on a single stereo 4x12 cabinet?
One for the right channel and one for the left?

I have stereo power amps like the Boogie 50/50 that I feed a to single 4x12 stereo cab.
One channel of the amp going in the left input of the speaker cab, the second channel going in the right.

Keep in mind, your Blue Voodoo BV120H is a MONO amp, NOT stereo.


The impedance of my cabinet is 8 ohms ONLY in stereo configuration which leads me to believe that I could set the selector on my amp to 8 ohms and get full power.


In mono, your cab has 4 speakers wired in series, each speaker rated at 4 ohms, total impedence of the cab in mono = 4 x 4 ohms = 16 ohms.

In stereo, the cab uses 2 speakers wired in series per channel, which suggests you can use the 4-speaker cab, as two 2-speaker cabs, each 2-speaker cab with an impedence of 8 ohms.

In mono, you can daisy chain 2 cabinets together, a top and a bottom cabinet, using only 1 output of your amp to go to top cabinet, and connecting the top cabinet to the bottom cabinet.

When you daisy chain two cabinets together, you connect the cabinets in parallel.
In parallel, the resistive loads of each cabinet don't add up as in series.
In parallel, the resistive loads divide by the number of loads. When daisy-chaining two 16-ohm cabs, the total speaker impedence your amp is going to have to serve is 8 ohms and you must then set the impedence switch of your amp to 8 ohms.

On the back of your amp, you have 2 outputs labelled "Speakers 100 watt RMS", the left output labelled "Main (Use first)", the right output labelled (EXT.).

The manual of the Crate Blue Voodoo BV120H says

Set the selector switch to the 8 or 16 ohm position, depending on the total impedance of your speaker cabinet(s). The chart below can help you determine that impedance based on the following combinations of speakers (by "speakers", here they really mean cabinets) connected in parallel.

Cab impedence - # of cabs - Total impedence

8 ohms - 1 - 8 ohms
16 ohms - 1 - 16 ohms
16 ohms - 2 - 8 ohms
32 ohms - 2 - 16 ohms
32 ohms - 4 - 8 ohms


Now hang tight.
You say you want :
My amp head puts out 120 watts tube at 8 ohms, and 60 watts at 16 ohms....
I want to get the full 120 watts out of my head for obvious reasons.


If you want to run your amp at 120watts at 8 ohms, all you have to do is:
- set the impedence switch of your amp on 8 ohms
- set the cabinet switch on stereo
- connect your main amp output to any channel of the cab
- DO NOT CONNECT the EXT output.

Happy ?
Last edited by ColdGin at Feb 19, 2007,
#10
Quote by ColdGin
In mono, your cab has 4 speakers wired in series, each speaker rated at 4 ohms, total impedence of the cab in mono = 4 x 4 ohms = 16 ohms.


I thought he said his speakers were 16 ohm, meaning two running in parallel?
"Everybody, one day will die and be forgotten. Act and behave in a way that will make life interesting and fun. Find a passion, form relationships, don't be afraid to get out there and fuck what everyone else thinks."
#11
ColdGin, the only problem with setting the amp switch to 8 ohm and running a cable to the cabinet while it is switched as stereo is that sound only comes out of two of the four speakers. I figured I could use the EXT output and send that to the other input of the cabinet while it is still switched set to stereo. This way I would just have the same signal being sent to both "channels" of the cabinet while it is in stereo mode which I assume is not going to be problematic or is it?
Regards,
PowrSlave
#12
Quote by PowrSlave
ColdGin, the only problem with setting the amp switch to 8 ohm and running a cable to the cabinet while it is switched as stereo is that sound only comes out of two of the four speakers. I figured I could use the EXT output and send that to the other input of the cabinet while it is still switched set to stereo. This way I would just have the same signal being sent to both "channels" of the cabinet while it is in stereo mode which I assume is not going to be problematic or is it?


That would be running the 4-speaker 16 ohms cab as two 2-speaker, 8 ohms cabs connected in parallel, for a total speaker impedence of 4 ohms, which YOUR MONO amp can NOT handle, while MY STEREO Boogie 50/50 can.

A stereo amp is a package of 2 mono amps.

What you can do is run a second head like your BV120H in the second input of the cab.

If you still don't get it, here is what the 8 page manual of your BV120H says about running a wrong load of speakers:

26. MAIN SPEAKER JACK: Use this jack to connect the amplifier to your primary speaker cabinet. Always keep the impedance at 8 or 16 ohms, with the impedance selector switch (#27) at the proper setting.


Impedance Selector Switch: Proper impedance matching is essential for optimum performance and life expectancy of a tube amplifier. These amps allow the use of 8 or 16 ohm speaker configurations (page 5).


27. IMPEDANCE SELECTOR: For the best performance and least strain on your amplifier, you MUST properly match the impedance of your amplifier to that of your speaker cabinet(s).


To help prolong the life of your tube amplifier, and to keep it in top performing condition, please observe the following guidelines:
• Always match the impedance of your speaker cabinet(s) to the amplifier’s mpedance selector switch setting.


That's 4 warnings to say: DON'T DO IT, YOUR AMP CAN ***NOT*** HANDLE IT.

Now what you want to do would be possible if the cab was loaded with 8 ohm speakers instead of the original 4 ohm speakers.

That would give you a total mono 4-speaker impedence load of 32 ohms, that your amp can not handle, but also two 2-speaker 16 ohm loads which your amp could handle individually, or like you are wishing, in parallel for a total impedence of 8 ohms which is supported by your amp.
Last edited by ColdGin at Feb 18, 2007,
#13
Quote by ColdGin


If you want to run your amp at 120watts at 8 ohms, all you have to do is:
- set the impedence switch of your amp on 8 ohms
- set the cabinet switch on stereo
- connect your main amp output to any channel of the cab
- DO NOT CONNECT the EXT output.

Happy ?


Riiiight...except in this config sound only comes out of two speakers which is stupid. Unless you know of some way to get sound out of all four speakers.
Regards,
PowrSlave
#14
Quote by PowrSlave
Riiiight...except in this config sound only comes out of two speakers which is stupid. Unless you know of some way to get sound out of all four speakers.


There is no way (that I can think of at the moment anyway) that you can get an 8 ohm load out of four sixteen ohm speakers.
ColdGin is totally missing the point you've said your speakers are 16 ohm, meaning that each of the pairs is MOST PROBABLY wired up into 8 ohm banks. I'm not sure with your amplifier, but if you can use the normal speaker out AND the extension out AT THE SAME TIME at 8 OHMS, then I can't see why you can't do it this way.
I still stand by my idea of getting some newer (and most probably better sounding) speakers and hooking running them all as an 8 ohm load.
"Everybody, one day will die and be forgotten. Act and behave in a way that will make life interesting and fun. Find a passion, form relationships, don't be afraid to get out there and fuck what everyone else thinks."
#15

Originally Posted by the_random_hero
ColdGin is totally missing the point you've said your speakers are 16 ohm,

Admittingly, the Peavey 412MS has 4 12" speakers, each speaker has an impedence of 16 ohms, and capable of delivering 75 watts continuous power.

Let's recalculate the impedence from grounds up.

With a speaker impedence of 16 ohm, for each stereo channel to act as a two-speaker, 8 ohms cab, each pair of speakers are connected together in parallel:


When running in mono, achieving a cabinet impedence of 16 ohms is achieved by connecting, at the flick of the stereo/mono switch, both two-speaker stereo channels in series, as to end up in a parallel/series speaker layout:


What does it change from the amp's perspective ? Nothing.

Because the problem does not lie with the speakers but with the capability of the amp to handle two different 8 ohms loads through the Main and the EXT outputs.


Originally Posted by the_Random_hero
I'm not sure with your amplifier, but if you can use the normal speaker out AND the extension out AT THE SAME TIME at 8 OHMS, then I can't see why you can't do it this way.

The manual of his amp clearly states that if you connect one load on the Main output and another load on the EXT output, both loads get connected to the amp in parallel.


This represents therfor a total impedence of (1/8 + 1/8 = 1/t = 1/4) 4 ohms , which his amp can NOT handle.
Last edited by ColdGin at Feb 19, 2007,
#16
Ahhh, I couldn't see that in what you wrote, but I probably just missed it.
"Everybody, one day will die and be forgotten. Act and behave in a way that will make life interesting and fun. Find a passion, form relationships, don't be afraid to get out there and fuck what everyone else thinks."
#17
Quote by PowrSlave
Riiiight...except in this config sound only comes out of two speakers which is stupid.

It's not stupid, it's physics.
What you don't seem to understand is that, when cranked, your amp is working and pumping exactly the same, the maximum it can possibly deliver within it's range of operation, whether running from the MAIN output at 8 ohms into one 2-speaker channel of the cab set to stereo operation, or when running at 16 ohms into the 4-speaker mono configuration.

Unless you know of some way to get sound out of all four speakers.

Run your cab in mono, and set your amp's output impedence at 16 ohms.

If this doesn't make your day, do you know how to handle a soldering gun ?
Last edited by ColdGin at Feb 19, 2007,