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The KT88, originating in the U.K., is interchangeable with EL34s and 6550s with only minor adjustments to the bias circuit. To my ears, the KT88 sits in between the smooth, compressed, glassy tone and soft lows of EL34s and the harder, edgier 6550s. Listeners generally equate classic rock with EL34s and 6550s with metal. KT88s fit nicely in the middle of this tonal spectrum, delivering the smoother tone of the EL34 but offering a much tighter low-end, similar to 6L6s – very rich harmonics and considerably more output and clean headroom.
Rumor has it that Jimmy Page had his 100-watt Marshall heads modified to run Genelec/GEC-branded KT88s back in the day; give a listen to Zep’s The Song Remains the Same: Soundtrack from the Led Zeppelin Film for some sonic samples. A recent check of vintage tube dealers reveals NOS GEC KT88s are currently fetching up to $800 for a matched pair.
My latest encounter with KT88s came via a vintage ‘72 Park 75 head I picked up a few months back. Originally offered in Lead, Bass and P.A. models, my recent acquisition is the latter. Circuit-wise, it’s pretty much an exact Plexi-era Marshall model 1986 Bass 50 circuit, but with KT88s in place of the EL34s, hence the 75 notation since the KT88s offer up more clean output, affording the higher wattage rating. This amp sounds so good it’s ridiculous – think Ziggy Stardust and the Spiders from Mars -era Mick Ronson. This is the coolest Marshall amp I’ve ever owned (Park was a Marshall brand and was made in the same factory).
My Park 75 came with its original GEC KT88s. While they still sounded really cool, there was no denying that they remained 35-year-old tubes and as such, were fairly well spent. For replacements I ordered a matched pair or New Sensor/Electro- Harmonix Gold Lion KT88 reissues. To my very pleasant surprise, I actually preferred the tone of these tubes over the original GECs. Granted they were 35 years old, but I was able to get the gist of their original tone despite their advanced age and condition. The new Gold Lions have a beautiful, smooth clarity and are able to produce raging distortion when cranked while retaining an extremely focused low-end. Plus, the tube breaks no sweat whatsoever when pushed hard, so they should last a good, long while – a nice thing as a matched pair of these tubes aren’t cheap to replace.
New Sensor also offers the KT88EH tube under the Electro-Harmonix brand, reported to sound excellent at almost half the price of the Gold Lion reissue. Additionally, Slovakian tube manufacturer JJ Electronic offers KT88s, which I’m sure are worth a listen. The earlier Chinese manufactured KT88s I’ve used in the past work, but pale sonically in comparison to the tubes I have previously mentioned. Groove Tubes offers matched pairs of KT88s on their rating system – it’s a more expensive option, but no brain power is required and they come with a great warranty.
One last word to the wise: if you’re swapping out EL34s or 6550s for KT88s, take your amp to a qualified tech to have it biased properly to make sure the transition is as seamless as possible. But with that in mind, make sure to check ‘em out!
is co-founder of 65amps