Ohms and stuff on my new amp

[spauldingrules]

Well I pulled the string and got the Epiphone Valve Junior head and cab after spending much time playing all kinds of tube amps.  Sounds fantastic!  But I have a question - the head has outs for 4, 8, and 16 ohm.  The cab has a 16 ohm input (rated to 70 watts).

Can I use any of the outs or will it ruin the cab or amp?  I don't understand the whole ohm thing, and everything I see on the web is too scientific - just give me the lowdown, with tone and damage potential information please.  The instructions offer no guidance.  Oh, and in the store they had the 8 out into the cab.  At home I have only tried the 16 into the cab.

Thanks for any info!

- TS

 

[Sixbender]

General safe rule is to match the impedance (ohms). If your cab is 16 ohm, and your amp has a 16 ohm out, use that.
Impedance is cut in half if you run speakers in parallel. Two 16 ohm cabs will then be an 8 ohm load.
Make sure that the amp is seeing the load that you're using. Don't run the 16ohm out into an 8 ohm load, dont run the 8 ohm out into a 16ohm cab, etc.

 

[spauldingrules]

What is likely to be more harmful, if at all - 8 into 16 or 16 into 8?  I don't understand what ohm measures.

 

[WMHOLP]

What is likely to be more harmful, if at all - 8 into 16 or 16 into 8?  I don't understand what ohm measures.

Much more damage is possible by running a higher impedance (ohms) output on the amp to a speaker cab rated at a lower impedance.  Basically, a lower impedance output means the output transformer is pushing more current.  It will basically fry your output transformer.  So, (in general with tube amps, especially British type) 16 ohm output to an 8 ohm cab is a very bad idea.  The cabinet wants more power than the tap on the transformer can deliver.  The opposite (8 ohm output to a 16 ohm cab), is not quite as bad because you aren't over-working the output transformer.  Different amps will react differently, and handle mis-matched loads differently.  Marshalls are particularly sensitive to mis-matched loads.  The impedance of the cabinet should always be equal to the total output load of the amp, or slightly higher in impedance than the output taps on the transformer to prevent a meltdown of your OT (output transformer).

So, with a 16-ohm output tap on a Marshall, for example, you could run a single 16-ohm cabinet, or run 2 16-ohm cabinets by changing the output tap on the back of the amp to 8 ohms.  (Running 2 cabs in parallel cuts the ohms in half that the amp wants to see).

 

[-CB-]

Let me add to the excellent practical advice already given, and say.....

Fender "got away" with all sorts of mismatches in doing two things on their spec output transformers.  First the iron was more than it needed to be.  Second, the winding gauge was also greater, accompanied by larger coils in the bigger iron.  You might just say, they over spec'd the output transformers and impedance mismatches are not an issue.

You get in REAL trouble if you put a larger load on a tube output amp.  That is - the WORST trouble of all is an open output, or high impedance output, because not only will it tear up the plates of the tubes, and the cathodes, but it will tend to trash the transformer real quick too.  Usually you'll get partial shorts in the primary... then it goes poof.

If you DEAD SHORT a tube amp, nothing happens.  Zip.  Nada.  No damage.  In fact, most well designed tube amps have a switched output that dead shorts itself upon taking out the plug, to avoid an open circuit - which would trash the amp. 

An older Marshall, with "closer to the bleeding edge" spec transformers will saturate more, and sooner (great grind), due to the smaller iron and tigher windings (from using smaller wire).  These suffer dramatically from presenting a larger impedance to the output, and suffer less or not at all from putting a smaller load on them.  Again, dead short - no issues on a tube amp.

KEEP THIS IN MIND -
Your tone suffers from mismatches.  And with a solid state output, the rules are completely reversed.  Open is fine on solid state, dead short kills 'em.