Grateful Dead Music Forum

A place to talk about the music of the Grateful Dead 

 #92576  by redbeard
 Sat Jan 22, 2011 12:05 am
So I've been dealing with this old 1972 AIMS 120w Tube amp for the last while, and I've tried to sell it on my way to the SMS + power amp + speakers route, and I've come up with an offer just to buy the head, which has led me to wonder what I need to figure out to determine if the 6x10 speaker cab which is originally for the AIMS would work with a power amp, and more importantly what sort of rating is safe. Now, the output on the amp says 120w RMS at 5 ohm, and there is also an external speaker jack. Now, I am assuming then that the cabinet is designed to at least handle 120w @ 5 ohm. Looking at power amps,l for example, the mosvalve mv962 says in the manual to use with speaker cabinets capable of at least 80 watts RMS Continuous power, but also never to load the speakers outputs with less than 4 ohm loads, with 120 watts @ 5 ohm would be, right?

I really think I may never understand this power rating thing. It also helps when you have gear that doesn't say its ratings anywhere and has speakers without a lick of writing denoting anything. Probably time to get out of the super old tube amp game, I reckon. Thanks for any help,
 #92606  by williamsaut
 Sat Jan 22, 2011 5:15 pm
Most Amps are designed for rated power at a 4ohm load. BUT a Fender Super Rev. for example has a 2ohm power transformer for driving 4x10s in parallel and of course, the McIntosh amps that have some multi-tapped output transformers and can handle 2ohms. Most multi-tapped amps don't go below 4ohms. It's my understanding and experience from blowing brand-new speakers ( darn it! ), that if you load it LOWER than rated impedance with a speaker or combination of speakers with a total of a number less than say 4ohms, like 2ohms or something, this equates to being closer to a dead short and voltage will go up trying to achieve the rated power ( wattage ). This will over-heat the speaker voice coil and fry it or deform it so the coil starts rubbing. Go the other way, say 16ohms or more with a different speaker or combination of speakers and this is closer to being an 'OPEN CONNECTION' and the current would try to go up trying to get to the rated power output but it can't because most amps are 'current limiting' by design to protect the output transformers ( but never run a tube amp without a speaker! ). This can damage the output transformer of the amp.

NEVER run LESS than 4 OHMS unless the output transformer is marked as such on the back of the amp.
Going MORE than 8 OHMS and you'll experience less power and the amp will run harder and 'break-up' sooner but probably not a good idea.

So IMHO, It's unlikely that the cabinet is really 5 ohms. Actually impedance of 8ohms will read 5ohms DC resistance on an Ohm Meter. Not a big enough difference to damage speakers or Amp.
Hope someone else chimes in since I'm not really a Amp Tech guy. Just a Guitarist who's messed with and fried a lot of stuff.

 #92864  by redbeard
 Thu Jan 27, 2011 4:02 pm
I sold the AIMS for full asking price! I was amazed, now for the Blues Jr. I can almost smell the SMS!

 #92865  by redbeard
 Thu Jan 27, 2011 4:16 pm
Okay, I'll try another one and see if I can make any sense of it.

Looking at the Mosvalve MV962, it says on the manual that the output power is 160 watts RMS at 4ohms( 80 watts per channel, both channels driven)

Looking at a Soundtech cx-x212 2x12 speaker cabinet, the impedance is listed as 8 ohms, RMS 100 watts, continuous 200w.

Can anyone tell if this would work? I can get two of the 2x12 soundtech's for $160, was thinking only one is necessary, but can you tell if the Mosvalve would work well with the Soundtech cab power wise?

I promise I'll stop asking questions soon, as soon as I can make any sense of this.
 #92875  by SarnoMusicSolutions
 Thu Jan 27, 2011 9:27 pm
You may really like the sound of the Mosvalve amp. It's a pretty musical sounding MOSFET power amp and clips in a fairly cool way and should take some abuse nicely.

 #92893  by redbeard
 Fri Jan 28, 2011 9:19 am
The Blues JR I bought in 2007 from original owner, it has a modified bigger reverb tank, thats the only mod. Sounds great, great shape, probably selling for $300 this morning, unless I find a better offer.