Amplifier Tone Tips: the Mighty KT88 Tube

Hello folks. This week we're going to talk a bit about an oft overlooked power tube, the KT88. The mighty KT88 is a lesser known tube to some that falls somewhere in between an EL34 and a 6550. The cool thing about it is that it is easily interchangeable with most 6550 amplifiers and can be interchanged in most EL34 based amplifiers with quick, easy mods. So the usual disclaimer is in order since we are talking tube amps...please do not attempt any modifications or tube swaps if you aren't sure of the specifications of your amp or you are not at least a technically savvy hobbyist. A bias adjustment at least will have to be done and if you're not comfortable opening and working on your amp, send it to a tech. They will be happy to mod it for you. Remember, there are deadly voltages inside tube amps.

KT88s

Ok, back to the KT88. Back in 1956 GEC introduced the KT88 as a larger variant of the KT66. It was originally produced in the UK. Modern production is limited but they are currently produced in China (Shuguang), Slovakia (JJ Electronic), and Russia (Svetlana and New Sensor). It never really gain much popularity or see large scale production so it is rare to find a guitar amplifier designed around the KT88. 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. There are others but the list is short.

The KT88 is the largest power tube in its class, specifically designed for audio amplification. 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. 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. Run in class AB1 push/pull, a pair of KT88's are capable of producing 100 watts at 2.5% total harmonic distortion. That's twice the output of an average pair of 6L6 tubes.

Tech stuff aside, the KT88 is a gorgeous sounding tube. Since this is all in the ear of the beholder, I'll tell you what I hear from a KT88. Somewhere in between the soft, glassy tone of an EL34 and the raw, edgy tone of a 6550 lies the KT88. I've found them to have a similar glassiness and upper midrange sparkle to an EL34. They tend toward having rich harmonic response as well as a tighter, more defined low end, much akin to the 6550. The strange thing I heard from a KT88 is a big, round low end. 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. It's almost like tightly defined low end that has a subwoofer behind it to me. I fell in love the first time I heard one cranked up. 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. They truly are a great tube choice for rock players.

So the question is, how do I put KT88's in my amplifier? Well, it's not too hard to do. Most 6550 based amplifiers can take a KT88 with just a bias adjustment. 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). 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. Even having done it before, I would double check with the manufacturers first.

EL34 amplifiers are a little trickier. I know a lot of people that believe you can just swap out EL34's with KT88's and re-bias. While I've seen people do this with no problems, I don't recommend it. Most EL34 based amplifiers are going to require a change of the bias feed resistors from 220k to 100k. 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 EL34 circuits). 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. In other words, if you don't change these resistors out, the KT88 will not perform well and may not sound good at all. As with any other tube swap, make sure the rest of the circuit is designed to accommodate them. Your tech or manufacturer would be able to tell you.

Genalex Gold Lion KT88 Vacuum Power Tubes

Please do not try to swap the KT88 out with 6L6 or 6V6 power tubes. If you have an amp with these particular tubes, check into a KT66 instead and as always check with your technician or the manufacturer before swapping any tube. This week’s article is geared toward the more advanced electronic hobbyist or technician so please make sure you know what you're doing before attempting any of this. The mods are simple to me but I have over 14 years of experience in electronic circuits and have done this a few times to say the least.

If you haven't experienced the raw power and tonal bliss that can be had from a KT88, I highly recommend it. It may be perfect for you and it may not be but the mods are fairly simple and easily reversible. So that's it for this week folks, short and sweet. Thanks for reading and we'll see you next time, in The Corner.

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