Originally the KT88 was made by GEC/Genelex, a british company. The American Tung-Sol 6550 was probably the tube they were trying to compete with, but the KT88 came out later and had better specs in power output, max voltages allowed, and plate dissipation among other things. So back in the day, a KT88 was a superior tube. These days it is hard to say as many modern tube makers don't publish their specs, and often share parts between tubes, but the KT88 is probably still the superior tube for functional aspects. For sound, well that is subjective so who knows. Personally I've always used the JJ KT88 in my Sunns and have had great luck and excellent sound with them. YMMV.
I'd say you're dead on, but I'm not sure if it was so much of a competition with the 6550 as it was a higher powered advancement of the existing KT66.
The KT88 was to the 6550 as the KT66 was to the 6L6, pretty much the British equivalent for the same specification, not necessarily made to compete with the US tubes, but to satisfy design specifications for British amplifiers of the day.
Remember, we're talking about a time when transistors were in their infancy;
everything that amplified electronic signals was tube, unlike today when tube amplified devices are only used in a few audiophile and MI instrument applications. Tubes were a dime a dozen, finite lived items that were intended to be replaced at regular intervals at a minimal expense rather than the high dollar glass bottles we use in our amps today.
Which is better tonally is subjective, however the GEC KT88 is arguably the best performing; power, dynamic response, etc. beam tetrode
ever, and soon replaced the 6550 in a lot of high end hi-fi amps of the day because of its higher power output and lower distortion.
Apparently this superiority is still generally accepted; a
single NOS GEC KT88 can run $500, while a matched
pair of NOS GE 6550 can be had for around $300.
As far as current production tubes, well, I don’t think it makes much difference, however many Sunn-o-philes (myself included) have great success with the JJ KT88 in their Sunn circuits.
It’s my personal experience that the JJ’s are a good balance of dynamic performance, power output, and price.
if i recall correctly 6550's are KT88's, just a different name...er... something.. But i get what your saying. I've heard Kt's are closer to how the T's originally sound (being they were equipped with kt88's upon release). all mine have 6550's so i cant give an honest comparison. Stan?
Nah. Sunn went to 6550's several years before the Model T came out. My T (I am the second owner, and was playing in a band with the first owner when he bought it) came with 6550's. 1975, I think.
As Isaac posts, Sunn switched to 6550’s in 1969, and never equipped an amp with KT88’s after that AFAIK either. They claimed it was because of the “superior performance” of the 6550 in the Sunn circuits, but since the Dynaco MkIII that the amps emulated was actually designed in 1957 around the Genelex KT88 specification, my guess is it was a cost cutting decision and not done as an “improvement”, or to remedy problems with the GEC tubes.
Here’s the letter announcing the switch;
