Some power amp questions

[wilsonthemany]

Hi,

I'm considering switching to a rack based setup and was wondering a couple things. First of all, could I connect a stereo power amp (like, say, the 2:50) to BOTH of the stereo inputs of my single rectifier 4x12 cab? For example, connect one side of the power amp to one of the stereo inputs of my cab, and the other side of the power amp to the other stereo input? Which ohm output would I use? The cabinet is rated at 8 ohms, but the stereo inputs are both marked with 4 ohms.

My GUESS is that I'd put one speaker cable from one 4 ohm output of the power amp to a stereo 4 ohm inputs for each side of the power amp. This seems kind of obvious and straight forward, but I'm wondering since I was looking at the manual online for some power amp and it didn't detail this connection in the back, even in the part that talks about ohms and speaker/amp connections.

And if theoretically this worked, would this split the 4x12 cab into two 2x12 cabs in a sense? Would my left signal be coming out of the two speakers on the left side of the cab, and would my right signal be coming out of the speakers on the right?

Also, I'm curious about the volume of different power amps. If I was using a 2:50 or fifty-fifty power amp, using the connection method theorized above, would this be as loud as any 100 watt head? I'm using a nomad 100 right now and can get exactly the volume, tube saturation, and headroom I want. I'm guessing that if I had two 50 watt signals both going into the same cabinet, each signal powering two speakers, I'd get exactly the same volume.

I've also seen a couple of threads on this board about problems with the fuse on the 2:50 and the 50-50. Is this problem only with the 50 watt mesa poweramps, or do bigger (2:90, 2:100) and smaller ones (20-20, etc.) also have fuse problems?

I know these are a lot of questions, and thanks if you took the time to read this!

 

[Satchie]

I have 2:fifty power amp and have never had any problems with it. If power amp blows fuses, anyone should check the transformer in it. You can connect
let's say 2:50 power amp to a 4×12 cab as you have stated, but you need to put a 4 ohm resistor (connected to a jack male input) in both remaining 4ohm sockets on the power amp. 4ohm resistor works as a dummy instead of a 4 ohm speaker. You can't leave other 4ohm socket empty.(I was told so by the local amp guru).

About loudness, 2:50 or 50/50 will definetely sound bigger and louder than any 100w head. For example, 100w Marshall head is only for 3db louder then 50w Marshall head. Loudness isn't measured in watts, it is measured in decibels, and most people confuse that. 100w amp ISN'T two times louder then 50w amp.
I hope this helps you

 

[t0aj15]

You can connect let's say 2:50 power amp to a 4×12 cab as you have stated, but you need to put a 4 ohm resistor (connected to a jack male input) in both remaining 4ohm sockets on the power amp. 4ohm resistor works as a dummy instead of a 4 ohm speaker. You can't leave other 4ohm socket empty.(I was told so by the local amp guru).

False, you do not need any resisters. :roll:
If your connecting 4 ohm speakers use a single 4 ohm output on each side of the stereo amp. If your connecting two 8 ohm speakers on each side use both 4 ohm outputs on each side of the stereo amp, because the 4 ohm outs are wired in parallel. So in a nutshell wilsonthemany you are correct, Satchie is not.

As far as blowing fuses the only time I've had fuses blow on my 50/50 is when a power tube failed, and they're supposed to blow then.

When it comes to volume 50 watts per side is very loud.

 

[Satchie]

Sorry t0aj15 but I don't understand what you're trying to say. The guy that told me that has a world reputation in building high class boutique amps.... Circle must be complete either 2×4ohms per channel or none... I'll trust that guy over any mesa manual...

 

[luigi]

If you have a Stereo cabinet then you can use the Stereo poweramp just fine. Just make sure that you plug one side of the cabinet into one side of the amp and the other side of the cabinet into the other side of the amp.

You don't need resistors or any of that. Actually, if you use resistors in this way, you will probably stress your output transformer and damage your poweramp.

 

[Satchie]

If you switch on a power amp not connected to a cab you might hurt it. On a 50/50 power amp you have 2×4ohm and 1×8hm outputs per side. When you have a 4 ohm mono cab, resistor should be put in the other output. But do what you want with your power amp...

 

[luigi]

You could do that with a mono cab. You would need a 50W (I would go higher for safety) resistor. It is not necessary though......just follow the instructions in the manual......turn the volume all the way down and the Presence all the way up and it will be fine if you leave it that way while no speaker load is connected.

Using resistors on poweramps instead of actual speaker loads usually coincides with using the poweramp as a tone generator for a larger amp. The only reason to do this is if you don't want to hear the sound coming out of it.

<Snip>
The guy that told me that has a world reputation in building high class boutique amps.... Circle must be complete either 2×4ohms per channel or none... I'll trust that guy over any mesa manual...
<Snip>

I know you trust your amp tech and he may be well known.........but the guys at Mesa (also world known.....probably even more so than your tech) built these things and convinced all of us on this board that their gear is worth the high $$ that we pay for it.....SO...I am sure THEY know how it works and write the manuals accordingly.

Just my 2 ¢

 

[t0aj15]

Sorry t0aj15 but I don't understand what you're trying to say. The guy that told me that has a world reputation in building high class boutique amps.... Circle must be complete either 2×4ohms per channel or none... I'll trust that guy over any mesa manual...

I think you may have misunderstood what your "world class boutique amp builder" told you, or he was just plain incorrect. The 8 ohm jacks need one 8 ohm load PER SIDE, the 4 ohm jacks need a TOTAL load of 4 ohms PER SIDE, period. That means each side when running it at 4 ohms needs either one 4 ohm load plugged into ONE of the 4 ohm jacks or two 8 ohm loads, each plugged into a 4 ohm jack, which drops the impedence for that side to 4 ohms. I'm not making this up, it's fact. Furthermore you may have misunderstood the original question. He was asking about connecting a stereo amp to a stereo cab, so neither side of the power amp will be without a load. In this case he would simply connect one 4 ohm side of the stereo cab to one 4 ohm jack on side A of the amp, and connect the other 4 ohm side of the stereo cab to one of the 4 ohm jacks on side B of the stereo amp. It's just that simple.
As for "I'll trust that guy over any mesa manual", that would be a serious mistake on your part, the manual is absolutely correct.

 

[luigi]

Yea........what he said! :lol: