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 in tubes amplifiers can be lethal if they are not discharged correctly.
Using Matched Power Tubes
When switching out power tubes, you should always try to use matched tubes - that is, power tubes that are sold in a set together. Even when they are manufactured in the same place using the same process, power tubes 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.
The Signature Sounds of Different Tubes
EL34: A classic tube for rock and gritty blues, the EL34 is characterized by its pronounced, aggressive midrange and tight lows. Four EL34 tubes are used in the power sections of Carvin Audio’s X100B and Steve Vai Legacy all-tube heads to help give them their signature crunch, bite, and definition.
EL84: This tube 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 EL84 power tubes for its thick, rich sound.
6L6GC and close brother the 5881: These tubes are known for their higher perceived headroom and their ability to stay cleaner longer. Not nearly as gritty as the EL34 or EL84, these tubes 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 tube manufacturing with some companies labeling these two tubes interchangeably. A side note: the speakers used in the classic amps noted for using 6L6 tubes 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 tube 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 tube 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 power tube types make sure your amp works with them. Several amplifiers 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 tube 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.