of course, there are tonal & visual differences...even with 2 different branded 6L6's...there are always tonal & visual differences...
of course, this is true. sorry i didn't elaborate further. it was very late when i posted.
allow me to explain. let's use 6L6 as the baseline and say a 6L6 is a regular 6L6 as we know it. 5881's are built to be sturdier and to fit smaller spaces than regular 6L6's. on the other hand, KT66's are the european version of the 6L6. in other words, each type was constructed with a specific purpose in mind (audio is only one among many purposes). furthermore, during the heyday of vacuum tubes, tubes were made with little regard to their tonality since they were used for different applications anyway (e.g. military, industrial, electronic, consumer, etc.). couple this with the rated plate voltage the tubes can handle, not all of these tubes can used interchangeably without biasing the amp (or not at all, if the amp has very high plate voltage and the amp has no bias control). from this, it can be gathered that the aforementioned tubes are not the same. the risk in assuming that they are one and the same is the potential of shortening the tube's life, frying the tube, and much worse, damaging the amp.
as for the physical appearance, a 6L6 is generally long and cylindrical, a 5881 is shorter than a 6L6 and stubbier, a KT66 is sort of "coke bottle" shaped.
tonally, 6L6's sound clearer and sparklier, while the 5881's sound warmer and smoother. if i were to make a comparison, 6L6's sound more "blackface fender", while 5881's sound more "tweed fender". KT66's, on the other hand, are more desirable for their smooth grind. a benchmark reference for this is john mayall's bluesbreakers's "beano" album, where clapton used a KT66 loaded marshall. purists will argue that no amp can get this tone without being loaded with KT66's.
to sum it up, the names (e.g. "6L6", "5881", "KT66") are more than just differences in how people call the same kind of tube. of course, this distinction isn't as evident since the vacuum tube's most significant purpose right now is for audio, and
for example...the ECC83...another name for the 12ax7...but some would argue that ECC83's are more british sounding...therefore, different...thing is...that's what the british called their 12ax7's...so now, the name ECC83 name is associated with british/european tubes...
unlike the 6L6 and its variants, the terms "12ax7" and "ECC83" refers to the same tube, and differs only in how americans and the british call it. in this case, it is appropriate to say both tubes are the same, simply because they are. a fairer comparison for these types of tubes would be, say, between a 12ax7 and a 12at7 or 12au7 (interchangeable, but with different gain).
so, same thing...same family...different tonalities...
there you said it. they are from the same family. but let me qualify this by saying they are not the same thing. i suppose the best term to use for this is that they are "equivalent" to each other. to say these different names all refer to the same banana is an oversimplification.
hopefully, i was able to clarify my earlier post.
cheers.
