Combo and Cab newbie qustion !!

[MM-S MIKE]

Ok, this is probably  a really basic question but I was feeling curious.

I currently have a Randall RG50 tube combo, ad that is probably the only amp I need now, plenty of versatility, plenty of volume for use in a detached suburban house  :twisted: , and plenty of lovely valves !!!

If I wanted to use it for gigs though presumably I'd need to invest in a bit more power than the 50w celestion 70 eighty. I know lots of bands wire their fender reverbs to 2x12 cabs but does this equate to having a 50w head and 2x12 combo? (i.e. combo speakers bypassed)

I was also reading the last thread on volume and wattage so if my amp is 50w with a 50w speaker presumably it would risk blowing the speaker at more than half volume.  :?

 Also my combo has a 50w RMS output through 8ohms and 4 ohms, what's the difference? Is one for a 2x12 speaker load and one for a 4x12, and would 50w output mean that no matter the cab's power handling it would never exceed 50 watts across all speakers or is that 50 watts + per speaker (so 200 - 300w power when the amp peaks)

Anyone who can help explain the science of plugging combos into speaker cabinets, I would be grateful just for the knowledge (not something I need to do just yet) for those of us who are happy without spending nearly a grand on a decent tube head.

Thanks, Mike

[Roobubba]

I can't answer all of those questions just yet, but here is a bit of the basics which apply to both heads and combos:

The maximum power output (and quality!) is obtained when you match the total impedance of your speakers to the imedance of the amplifier. Some amplifiers, such as yours, have switches which allow you to change the output impedance. In these cases, you should only change the amp's output impedance when you physically change the impedance of all the speakers the amp sees:

So let's say your 70-80 speaker is 8 ohms. If you put another 8 ohm speaker with that in series (look up speaker wiring pages to understand what's going on here, but the basics are this - 2 speakers in series doubles the impedance, 2 speakers in parallel halves the impedance), then you should change your amp's output to 16 ohms. If you have two output sockets on the amp, then they're almost certainly wired in parallel, and so your amp should be set to 4 ohms for the two 8 ohm speakers.

Why does it always take me several huge paragraphs to explain this stuff and everyone else explains it really succinctly?!

Now the other complicating factor with combos is this:
Some combos *disconnect* the inbuilt speaker if you plug in an external speaker. In these cases, the amp output impedance should be the same as the external speaker's impedance.
Some run both together, and usually these should be in parallel. The manual is the best place to start looking for this info, though, as it's completely amp-dependent!

Well, that's a bit to keep you going for now. Sorry I didn't get a chance to address any of the other questions, but hey look how long it took me to write about this one issue!!

Roo

[Will]

The seventy 80 is 80w

http://professional.celestion.com/guitar/products/originals/spec.asp?ID=15

Roo's explanation was pretty good there. just long :P

[hamfist]

I found a bit of info on the RG50TC (I assume your's is this model).  Apparently, the internal speaker is disconnected  by plugging into one of the "external speaker out" jacks. So you simply have to plug a cab of the correct overall ohm rating into the appropriate "ext speaker out" socket(choose whichever ohm rating is the sane as your cab).
  Whether you use the 4ohm output to a 4 ohm cab, or the 8ohm output to an 8ohm cab, you will still get the same power output from the amp.
  The two different ohm outputs are there merely to give you more flexibility about which cabs you can use.
  Please use only ONE of the ext speaker outputs at one time. Do not be tempted to try and use both - burnt electrical smells will occur.

Also, I would try and make sure that the speakers in the cab you are using can handle up to about 80 Watts of power between them (for a 50W amp). Remember that the number of speakers in a cab will share the power from the amp equally between them. So two 25 Watt greenbacks would only have a combined total power handling of 50 W (25W each). Also if you mix speakers and use, say, a Vintage 30 (60W) and a greenback (25W) in a cab, the power will still be split equally between them. The Vintage 30 would not take more of the amp's power than the greenback. So in that example, a V30 and a greenback, when used with a dimed 50W amp might well fry the greenback (and your OT (again !!))

  Please remember to use a proper speaker cable for connecting an amp to a speaker cab. If you try and use a normal guitar cable you will probably fry both the cable and your output transformer.

have fun !

Alan