EL34 vs. KT66 output tubes

Trelja,
FYI, I should clarify that I'm using only vintage gears because I prefer the sound from vintage gears much more than the modern gear.
I do agree that the modern EL34 amps sold a lot more than the modern 6L6 variants because no manufacture makes any good modern 6L6 amp these days but it does not imply that 7027/KT66/6L6GC are not as competent.

Also, not all 6L6 tubes are equal. 7027/KT66/6L6GCs are the high output tubes in the 6L6 family. They can produce same amount of power as the EL34 type.

The 6L6 variants were used for many guitar and organ amps.
There are abundant supplies of NOS 6L6 tubes. I'm not sure why manufactures today didn't pursue making the KT66 amps these days. Profit margin or it is hard to design a good KT66 amp these days since there werern't many clone circuits and good transformer available?

As far as EL34 vs KT66 tube goes for the modern home audio equipment, the modern EL34 has much higher demand and longer history than the modern KT66 due to hard to find NOS EL34 mullards and Telefunkens. Therefore, the Eastern European EL34 tubes has come a long way to make something decent when compared to its KT66 line.

I would not buy any modern KT66 amps in the future either.
I would buy the vintage PP KT66 amps.

Our friends here in NY definitely prefer the KT66/7027/6L6GC and EL84 over EL34 because the mids sounded more full bodied. Not sure if that was the reason why they use these tubes for guitar amps. They seemed to match our speakers much better than EL34s.
The EL34 might be more balanced for rock music but not the string instrument, vocal, saxaphone and drums just don't have the authority and tonal quality we like ( again, except for Marantz 9 mono blocks from the 60s.)

Again, just my opinion. I couldn't stress enough about amp design.

S23chang, I don't think we disagree.

Having laid out your case well, along with things that most people often neglect to include in the mix such as type of amplifier and speaker as well as musical preference, it's easy to see where you are coming from. I don't doubt the 6L6 variants are best for you and your associates.

On my own end, I should submit to you that I am using amplifiers from the past decade, with speakers of the same vintage, and tend more to rock, jazz, and popular music. And, to be honest, when all is said and done, so far, I have hitched my wagon to the KT88, though we'll see with a new amp coming whether or not I will revert back to the JJ E34L.

While our means and ends seem to be different, we both are looking for the best possible way to enjoy the music we listen to. In the end I guess I have to repeat my assertion, that's why they make vanilla AND chocolate.

Trela and S23chang.As you both well now, there are terrific examples of each of these tube topologies. Saying one tube is better than the other is like saying "a ferrari is better than a benz". Like Trela says "Some preferr vanilla to chocolate". S23chang alluded to his preference to vintage amplifiers over the more modern designs and I can understand his anology here as many of the better vintage amplifiers of the day employed top shelf output transformers when compared to the standards of today. However....chang also mentioned the marantz model 9. An obsenely expensive amplifier in it's day, the sota amp of the era that would still be competetive with the top shelf stuff being offered today.That said: The marantz 9's were never really sold with el-34's as the output tube per sey, The tube employed in them was actually the genelex kt-77, a beam power tube that would electrically sub for the el-34 [a pentode]. And once the comsumer got a taste of the Kt-77, many of Marantz's customers would also purchase the kt-77 as a more expensive option for the Marantz model 8b as well. The kt-77 is a beam power tube similiar to the kt-88 and possesses a kt-88esque like bass quality combined with the more liquid mids of a good nos el-34 and the air,purity and finese in the treble region of a nos el-84. It mainly comes down to output power and the amplifiers ability to drive a given load. Ever heard the Conrad Johnson premier 8XS with el-34's in it? Obviously not your typical el-34 based amplifier and sounds nothing like your typical el-34 fitted amplifier based on that tube topology regardless of vintage or modern. Some of the more recent amplifiers designed around the el-34 are very very good indeed in that they capture the frequency extremes as well as the endearing qualities of the midrange in a way the older designs failed at.Have a listen to the German made Klimo Kent,a 35 watt el34 based amplifier that been around now for 10 years or more. World class by anyones standards to be sure! or the decade old el34 based amplifier by Beard in England to name a few. The kt-88/6550/6L6 family of amplifiers introduced in that era were really brought to the market primarily because of their higher power offerings. Speaker design was making a radical shift away from the large high efficient speakers to smaller design's that required more power. Given the proper speaker load on the amplifier.... all things being equal here....and personally speaking here.... In most any given push/pull circuit, I would choose the qualities of the el-84/6bq5 family of pentodes over any of the el-34,kt-88,6L6 family of tubes because,like the kt-77.... it possesses and combines the best qualities of each of those tubes. I have heard some mediocre,some good and some great sounding amplifiers with each one of the above mentioned output tubes. All things being equal.... and like s23chang,dollar for dollar I would lean toward the fine vintage amplifiers of yesteryear. That said: Many of the great [read expensive here] modern tube amp designs of the last decade are becoming available on the used market at prices below the very best of the vintage years.

https://photos.app.goo.gl/3jdQU89NNsi3REv5A

I have an interesting KT66 based bi-amp single mono block setup that I want to have converted to a 2 channel setup. It came from an organ. The tubes are all old and not NOS and check out very good. I am very interested in you comments as to what I have here and if it is worth spending $800.00 on it to convert. It is very cool looking with a transformer style that I have never seen before. All be it I have to get 2 matched Opt's for it.