I replaced the Groove Tubes GT34LS with JJ KT77 tubes on the Manley Snappers. The harsh midrange is now gone. Hallelujah!
I noticed the stickers on the GT tubes had "Performance Rated" numbers on each tube. Looking this up on GT website, I found that the scale is 1-10, 1 being the most distortion and wildest dynamics, designed for rock and blues guitarists wanting the most distortion even at low volumes. They have low headroom. The highest rating of 10 is tubes with high headroom, for jazz and bass guitar amps and hifi amps. Lowest distortion, and "Max Headroom"
The tubes that came with my Snappers were marked 4,4,4,1 and 4,2,2,1 as quads in each amp. So it seems (and sounds) like Manley is voicing the amps toward the rocker side, which has given them some good reviews. Mitch Margolis, the designer of the Snapper is also Chief Engineer with Groove Tubes, and designed this special higher power version of the EL34 for production on the JJ assembly line. So naturally he will specify his own design tubes for his own design amp. I wouldn't be surprised if the Snapper was designed around the new tube. Some hifi tube experts on Asylum have judged the GT E34LS very good, I don't know what performance rating they were using. Good question to ask...
The KT77s are noisy on startup, a loud wind blowing sound and some popping. I hope that goes away during break in. The amps upstairs are much noisier without the dedicated 20A outlet. Not sure if that has to do with the tubes or combination or things. It's bad enough on both Cary and Manley that I will have to bring dedicated power up there.
THe JJ KT77s are the only such tube in production now. Previously they were only made by Genelex years ago. NOS is very precious. So the guy from Eurotubes where I bought mine gave a pair to JJ for reverse engineering to make their KT77 copy. People have compared the original to the new and said they are a good copy, some people even prefer the newer ones. Since they are only $16/tube they might be worth checking out. So far I like the sound of them. They have mellowed out the flamethrowing Snappers and cured the midrange, but also lost some of the intense tight bass and crystalline highs of the GT tube. But they are much more listenable, less of a fireworks show. I'm not sure yet which I prefer. Cool thing about tubes, I can choose and change my mind whenever I want!
I may try to get a set of GT E34LS with the performance rating of 10 on all 8 tubes, to see if that gives me the fireworks without the midrange bite. Also TubeDepot is cryo treating the SED EL34 which is supposed to be the lowest distortion best built EL34 available. That's the latest!
Rich