Epiphone Valve Junior Cabinet-which head?

[strat1946]

Today I got an Epiphone Valve Junior Cabinet, with a 1 x 12 inch Eminence Lady Luck Speaker, 70 watts RMS at 16 ohms.

I was hoping to use a solid state head, to get more clean headroom. 

Then I realized, do I have to use a head which has a 16 ohm output socket.  This would limit me to the Ephiphone Valve Junior, or the Blackheart, both of which are 5 watt tube heads.

I am trying to learn all of this, and I don't really understand ohms yet.

Any help would be appreciated!

[armstrom]

with a solid state power amp you don't need to worry too much about matching impedance. There are limits to what you can do, but generally speaking as long as you stay at or above the rated output impedance of the power amp. So you could drive your 16 ohm speaker using pretty much any head. Of course, the amp will only produce its rated output wattage at the specified output impedance. So if you buy a 50W SS head meant to drive an 8 ohm load it would only generate 25W into a 16 ohm load.

[strat1946]

Thank you for that answer.

And does that mean that the 50 watt solid state amp with the 8 ohm ouput would now have more clean headroom than the 5 watt tube amp would have?

[strat1946]

I am going to return this cabinet in the morning.

I will eventually get a solid state amp with a 12 inch speaker, with 60-80 watts at 8 ohms!

[armstrom]

Sounds like a plan. You're probably better off choosing the amp you want and then finding a cabinet/speaker (assuming it's a head, not a combo amp) that best suits the amp and your playing style as the design of the cabinet and speaker have a significant impact on the overall sound you will get.

When you go out to shop for the amp I wouldn't concern yourself with the wattage too much. Just plug in a guitar and try it out in the store. Any good music store will let you test drive the amp and most will let you crank it a bit to see what kind of volume you can get while still keeping things clean. You may be surprised that you can get away with a much lower wattage amp than you think. Be sure to branch out from the more obvious brands (Fender, Marshall, etc...) and try other stuff.. Many Roland amps are well known for producing great cleans (Roland Jazz Chorus, or the cube 60, for example).

I noticed in your first post that you expressed some confusion over the impedance rating (ohm value) of speakers and what they mean.
Hopefully this will help:
http://www.rmcybernetics.com/science/cybernetics/electronics_volts_amps_watts.htm#ohmslaw

In the most basic terms an amplifier is simply a good clean power supply generating voltage and a "valve" to modulate that voltage to the speaker. When the "valve" is all the way open (almost) all of the power supply voltage is being delivered to the speaker when it's closed, none of the voltage is being delivered.

So, using one rearrangement of Ohms law we can see that power (P in watts) is equal to the square of the voltage divided by the resistance (or impedance) in ohms of the load its driving (the speaker). So, lets say an amp has a power supply voltage of 10V and for the sake of argument it can deliver all of that voltage to the speaker when running at full volume. If you have an 8 ohm speaker you would end up with the following P = (10^2)/8  or 100/8 = 12.5 watts.

Now, if all you do is change the speaker from 8 ohms to 16 ohms, then the maximum voltage output stays the same but now you're dividing it by a larger number. so 100/16  or 6.25 watts.

So, the reality is that your amp doesn't "produce" wattage, it produces voltage and its the speaker that actually determines the wattage (or work done) by the total system. Of course, that's not to say you can simply keep lowering the impedance of your speaker to make more wattage. There are limits. Most amps are designed for a particular minimum speaker load that they can safely drive. As the load driven by the amp is reduced more and more current is being drawn through the amp circuits generating more and more waste heat. If the electronics cannot safely dissipate this heat they will burn up.

Note: While the basic concepts hold true for tube amps, the fact that they use an output transformer to drive the speaker makes things more complicated. Tube amps need the speaker impedance to be a specific value in order for the power amp circuit to work properly. The transformer also acts as a sort of resistor in the power amp circuit. However, being a transformer, the "resistance" of the primary coil (the part hooked up to the power tube) will change depending on what impedance speaker it's driving on its secondary coil. Tube amps need the primary impedance to fall within a fairly narrow range in order for the power tube to function as designed. This is why speaker impedance is such a big deal with tube amps and not so much with SS amps.

Hope that helps more than it confuses