Swapping out a set of power amp tubes for another can change up your tone in a subtle but significant way. This article will highlight the basic tonal characteristics of common power tubes and help you find the right one for you. Please note that swapping out tubes is a process that is best left to an experienced amplifier technician- the amount of voltage present can be lethal if they are not discharged correctly.
When switching out power tubes, you should always try to use a matched set - that is, power tubes that are sold in a set together. Even when they are manufactured in the same place using the same process, they have natural variations in their current draw. For this reason, manufacturers test the current draw of each tube and match them as closely as possible with others. And always get your amp biased when changing the output tubes.
EL34: A classic choice for rock and gritty blues, the EL34 is characterized by its pronounced, aggressive midrange and tight lows. Four EL34s are used in the power sections of Carvin Audio’s X100B and Steve Vai Legacy heads to help give them their signature crunch, bite, and definition.
EL84: This is the lower power little brother to the EL34 and breaks up more noticeably, producing a warmer tone at lower volumes. It also has a more prominent, punchy high end with more clarity and shimmer in the treble frequencies. The Carvin Audio Belair 2X12 is equipped with four EL84s for its thick, rich sound.
6L6GC and close brother the 5881: These are known for their higher perceived headroom and their ability to stay cleaner longer. Not nearly as gritty as the EL34 or EL84, these are great for a squeaky-clean sound that’s fairly neutral across the frequency spectrum. The speaker used may make a more significant tone difference. The “GC”, sometimes dropped or shortened when referring to this tube, indicates the glass container or top. The original 6L6 had an all metal top instead of glass, like many original tubes had. The 5881 was the 25 watt version of the 30 watt 6L6GC, but this difference has been blurred in modern manufacturing with some companies labeling these two interchangeably. A side note: the speakers used in the classic amps noted for using 6L6s are very different, so don’t get caught thinking your British rock tone will change to American tone with just a power tube change.
6550: The 6550 is a different kind of beast and much higher wattage. This one doesn’t see much use in guitar amps today, but its warm, full, and powerful output places it right at home in bass amplifiers with over 100 watts.
Note: Not all amplifiers can change to different types of tubes, and many types are not possible to be interchanged without a circuit change. Just because the socket fits does not mean the amp is designed to work with that tube. So, before jamming in different types make sure your amp works with them. Several amps have switches for EL34 and 6L6/5881 tubes. Be sure the switch is in the correct position. The wrong switch setting could damage your new tubes and possibly your amp.
Of course, your amplifier design is just as important as which power tubes you choose. Usually the amp is designed around a specific output tube type, so it may not be exploiting all the benefits of another type. If you want to experiment with your tone and switch up your sound without switching amps altogether, trying different power tubes is a great place to start.
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Even if you’ve matched your bass head and cab properly impedance wise and set your amp for clean sound, sending simply too much power to your bass cab can result in blown speakers. This often happens when you are using a rig you are unfamiliar with, as we tend to know the limitations of our own equipment and have chosen that setup for a reason. Borrowing another bassist’s amp or using a backline rig only to blow it up is definitely not a great feeling.