Tone Tips: The Mighty KT88
As featured in “Andy’s Corner” on ProGuitarShop.com
Guest Contributor: Rusty Wiseman
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.
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 K T88. 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 K T88 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.
Ok, back to the K T88. 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 K T88. 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, spe-
cifically designed for audio amplification. It’s high
output and low distortion characteristics made it
So the question is, how do I put KT88’s in my ampli-
fier? 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 (trans-
formers 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.
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