Impedance settings 'n' tube heads

"What impedance do I set my head to?" This is one of the most common question that customers ask and is also one of the questions that invariably receives a wrong/misleading answer. Getting the answer to this question correct is vital due to the fact that setting a tube head to the wrong impedance can and WILL do damage to its output stage. In a nutshell there are two essential rules that HAVE to be followed when hooking-up a tube head to a cabinet or cabinets:  

1. Always use speaker cables; never use a guitar (a.k.a. "signal" or "instrument") cable.  

2. Always ensure that the impedance of the tube head is set to the EXACT same impedance of the impedance of the cabinet(s) being used.

With regards hooking tube heads 'n' cabs up, this small chart should clear up any confusion that a dealer or customer might have. I know the below is common sense to anyone who remembers elementary electronics from school but as it turns out, most guitar players have zero recall of those days - myself included!  

If the cabinet is:                            Set the head to:

  16 Ohms mono                                16 Ohms

  8 Ohms mono                                  8 Ohms

  4 Ohms* mono                                4 Ohms*  

*IMPORTANT NOTE: Some Marshall tube heads (i.e.: 4100, 4500, 2100SL-X and 2500SL-X) need to have a modification performed on the transformer to run at 4 ohms - and this modification should ONLY be carried out by an authorized service technician. That’s the reason 4 is in parenthesis on the back of the chassis. The newer DSL and TSL models have the ability to run at this impedance without any internal modification as do the Plexi re-issues.

If 2 cabinets are used and they're:             Set the head to:

  16 Ohms (mono) each                              8 Ohms

                       8 Ohms (mono) each                              4 Ohms - Don't do this*

  4 Ohms (mono) each Not Recommended (unless rewired in Series = 8 ohms)

* …...unless you like flames & smoke!! (You would have to rewire your cabs)

"Does the same exact set of rules apply to ValveState amps?" Do I hear you ask? 

The answer, in a word, is NO! ValveState amplifiers have a solid-state power stage that doesn't obey the same laws. As it happens, these amps are self-adjusting so you can run them anywhere between 4 Ohms and 16 Ohms but NEVER below 4 Ohms. As you increase the impedance, the output wattage gradually decreases. Let's take a look at the VS100RH for example. This critically acclaimed brute delivers its full 100 Watts @ 4 Ohms. If you hook the VS100RH up to an 8-Ohm cabinet (using a speaker cable of course!) the head will be capable of delivering approximately 80 Watts of power.