If you’re a gigging guitarist, chances are you’ve turned up at a show with your amp head and the hopes of using someone else’s cabs. And chances are you’ve then had problems trying to understand the equipment’s various impedance levels, when really you should’ve been focusing on playing a great show instead. Here, we break the topic down into layman terms so you can rock on stage without having to worry about that troublesome little Ω symbol ever again!
Most guitarists would probably not consider themselves experts when it comes to the subject of natural sciences. Although you may have a vague idea of the different scientific phenomena that hide behind every note you play, your main focus as a player – quite naturally – is usually on the music itself, and the instrument. Boring science is just something that would get in the way.
And yet, consider this: the everyday electric guitar player probably namechecks a bunch of famous physicists regularly, even though they might not be overly au fait with the scientific laws associated with these long-dead genii in white coats. Hertz, Ohm, Volt, Ampere – these are all familiar names that belong in the basic vocabulary of any slightly technically-minded guitarist.
These things duly considered, it can be highly insightful for you to get to know the stories – and the theories – behind those names in a bit more detail. One particularly important physicist for guitar players is the German Georg Simon Ohm (1789-1854), who gave the world of electricity the eponymous Ohm’s law.
Every guitarist who has stood in front of a tube amp wondering which speaker output it’s best to hook his or her cab (or cabs) up to has inevitably come face to face with Ohms. And it’s that universally recognised – but not usually understood – Greek letter, Ω, that causes confusion. Referred to in English as Omega, Ω is the symbol of a unit of electrical resistance, and when you see it written on your all-tube amp, you’ll probably start asking questions like these ones:
Why does a “load” – that is, a loudspeaker – have to be connected?
Can I use a 4 ohm speaker with a 16 ohm output?
Can I run two cabs with different impedances in parallel?
These are just some of the questions we often get asked about tube amps, so let’s try and unravel some of the mysteries surrounding the murky world of impedance. The physics behind all this are pretty darn complex at times, so we’ll try and stay as practical and understandable as possible here! Now, lab coats on, and let’s get started.
A guitar amp’s tube power amp works on the principle of matching impedance. In practice, this means using corresponding speakers and amps. For example, if you connect an 8 ohm speaker to the 8 ohm output on a tube amp, the internal resistance of the source (that is, the tube output stage) is exactly that of the load, or speaker. In this way, and only in this way, can the amp unleash its full power on your unsuspecting audience. This is also the only way you will get your amp to sound exactly like the designers intended it to sound.
Of course, you can still experiment and try out new things if you like – and if you stick to a few basic rules, you won’t be in danger of breaking anything. In some cases, you might not be able to match your impedance – for example, you might be playing a stage where only an 8 ohm output is available, but the cab has an impedance of 16 ohms – and in this case, you’ll have no option but to rock a mismatched combination.
But what are the consequences of doing this: can it damage your equipment, or can it have a negative influence on your all-important tone?
First of you all, you should commit this critical ground rule to memory: never – really, NEVER! – operate your tube amp without a load/speaker(s). If you do, the result will be a high level of induction voltage that could damage – or even destroy – your amp’s output transformer and tubes. As long as you remember this, you should be on the safe side, even with a mismatched setup.
But what happens exactly when you use the various impedance combinations? Here are a few examples:
Case 1: running a 16 ohm speaker with an 8 ohm amp output
With this combination, the voltage at the speaker output will rise, while the current will almost halve. The power will drop, although you probably won’t notice it too much, as this combination will likely increase the mids in your tone. So long as you don’t overdo it – make it a rule of thumb not to connect a speaker with more than double the output impedance of that of the amp – this method can be quite useful, as you can effectively boost the mids in a cab that might otherwise be lacking in this department.
Case 2: running a 4 ohm speaker with a 16 ohm amp output
Here, the complete opposite to Case 1 occurs: while the speaker output voltage significantly falls, the flowing current only marginally increases. This will alter your tone too, but in the exact opposite way from before – you’ll get a drop in your mids! If you’ve got a cab that is way too mids-heavy, then, this can be the perfect method to better balance your overall tone. Because this combination can help lessen the self-induction effect that might occur in your tubes, it is generally safer to use than the example in Case 1. In terms of the amount of mismatching you can get away with here, we’d go with the 25% rule: so, for instance, don’t connect a speaker with less than a 4 ohm output to a 16 ohm amp output!
Case 3: the exotic combination of 16 and 8 ohms together, and how to hook them up together…
Almost every gigging guitarist will have experienced this before: you’ve got one cab with 8 ohms, and one with 16 – and you want to connect them both to a speaker output at the same time. Is that even possible?
To put it simply, yes. You just need to know the total impedance you’ll have if you’re running the two speakers in parallel. This can be surprisingly easy to calculate. Another ground rule here: if you’re connecting resistors in parallel, then the total resistance is always going to be smaller than the smallest partial resistance. In this case, that would mean we will end up with a total resistance somewhere south of 8 ohms. To be more exact, we can calculate the precise figure using the following formula (which, by the way, only works in cases where you have two parallel resistors!):
Or, in our case, that’s (8×16) / (8+16) = 128/24 = 5.333 Ω
Simple, right? Well, you’ll get used to it!
Based on these variables and peculiarities, you can start to think about whether you would prefer to use this combination with a tube amp output of 4 or 8 ohms. It’s certainly technically feasible, and the difference in volume between the two speakers will also be less than most people would think. And by the way, you should avoid combining 4 ohm cabs if you can, because you’ll end up in a red zone of under four ohms in total (for example if you go for the following ohm combinations: 4+4, 4+8, 4+16). Most amps aren’t designed to go this low, and you could cause serious damage.
As a final point, and just like with another of our recent posts – When could, would and should you replace tubes? – don’t be afraid of what’s written above! If you pay attention to those few ground rules, chances are you won’t blow your amp up by accident. In return, checking out the various options you have with your amps and cabs could give you way more options to improve your tone.
And by the way, we’ve put all the combinations mentioned above to the live test many times – sometimes for years on end! To date, we’ve never managed to even bring a tube amp out of its comfort zone, let alone leave one a smoking wreck!
What about you?
First published: June 20 2014. Most recent update: October 16 2015.