As with any other tube swap, make sure the rest of the circuit is designed to accommodate them. In other words, if you don't change these resistors out, the KT88 will not perform well and may not sound good at all. Not changing it will both shorten the life of the tube (probably dramatically) and also not allow the KT88 to be run close to their design ratings. ![]() The KT88 (and 6550 for that matter) can't quite deal with the higher grid resistance from the 220k resistor (these are typically the rating found in E元4 circuits). Most E元4 based amplifiers are going to require a change of the bias feed resistors from 220k to 100k. While I've seen people do this with no problems, I don't recommend it. ![]() I know a lot of people that believe you can just swap out E元4's with KT88's and re-bias. Even having done it before, I would double check with the manufacturers first.Į元4 amplifiers are a little trickier. I've had no problems with older 6550 based Marshalls and even some Ampeg SVT's (old ones) but I always check to make sure the rest of the amp is up to the task. Always check with the manufacturer or a tech you trust first though, as you want to make sure it has the properly rated supporting circuitry in the amp (transformers primarily). Most 6550 based amplifiers can take a KT88 with just a bias adjustment. So the question is, how do I put KT88's in my amplifier? Well, it's not too hard to do. They truly are a great tube choice for rock players. The clean tones are gorgeous and sparkly and they tend to stay clean at louder volumes due to the ratings but when they are lit up, boy they light up! Overdriven tones range from mild, bluesy cut (think Plexi on steroids) to insane saturation that has better definition than any other tube I've heard. I fell in love the first time I heard one cranked up. It's almost like tightly defined low end that has a subwoofer behind it to me. The reason I thought this was strange is that usually that means the low end isn't tight and defined but the KT88s I've heard have both characteristics. The strange thing I heard from a KT88 is a big, round low end. They tend toward having rich harmonic response as well as a tighter, more defined low end, much akin to the 6550. I've found them to have a similar glassiness and upper midrange sparkle to an E元4. Somewhere in between the soft, glassy tone of an E元4 and the raw, edgy tone of a 6550 lies the KT88. Since this is all in the ear of the beholder, I'll tell you what I hear from a KT88. Tech stuff aside, the KT88 is a gorgeous sounding tube. That's twice the output of an average pair of 6L6 tubes. ![]() Run in class AB1 push/pull, a pair of KT88's are capable of producing 100 watts at 2.5% total harmonic distortion. A KT88 is capable of a maximum dissipation of 42 watts from a single tube which is pretty hefty considering the beefy 6550 dissipation is only 35 watts and we know how loud those guys are! The KT88 also can handle larger plate voltages, up to 800 volts. It's high output and low distortion characteristics made it popular in the audiophile world as well as with guitarists looking for more clean headroom. The KT88 is the largest power tube in its class, specifically designed for audio amplification. Some popular amplifiers that came stock with these are the Orange Thunderverb, Hiwatt Custom 200 and 400 bass amplifiers, the 200 watt Marshall Major, and the Marshall 2203KK Kerry King Signature. It never really gain much popularity or see large scale production so it is rare to find a guitar amplifier designed around the KT88. Modern production is limited but they are currently produced in China (Shuguang), Slovakia (JJ Electronic), and Russia (Svetlana and New Sensor). Back in 1956 GEC introduced the KT88 as a larger variant of the KT66.
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