So now that I've had this mod done for some time, I think I can write more about it and really compare the Twin Jet with both types of tubes. Let me preface with saying that I am using Sovtek KT88's (stock tubes for this amp) biased at about 38ma, Winged C EL34's biased at 34ma, and all Chinese 9th B in the preamp section. Cabinet is a rear-loaded Bogner Uberkab with standard Celestion V30/75 mix, and guitars are Schecters loaded with Seymour Duncan JB's and Invaders. I will also be receiving
a Schecter Hellraiser Avenger with EMG's, so that should be interesting in the future. Lastly, I'm boosting the amp with a Ibanez TS-9 Tubescreamer.
Anyway, on to the tubes. First I'll talk about the KT88's. These tubes are much more powerful and in your face than the EL34's. They have a scooped midrange and really punchy lows, but with somewhat brittle highs. Turning the depth control to about 3 o'clock really makes the low end massive, yet surprisingly focused. The amp sounds a bit more "buzzy" with the KT88's, especially for leads. I've noticed that the amp is much tighter with these tubes. It's really hard to get them to break up. This amp is LOUD with these tubes, and they stay really defined even at ridiculously high volume (as I was able to try at my parents' house before I moved). Nevertheless, with these Channel 1 can get some surprisingly decent if somewhat sterile cleans, all the way up to JCM 800 range that is surprisingly bright, if a bit fizzy and lacking in a rich midrange. Channel One also has very little headroom with them, and Channel Two gives some really, really BROOTAL tones, which are somewhat dry and less saturated in comparison to EL34's. All in all, these tubes are completely suited to what the Twin Jet was designed for: tight, scooped, brutal metal riffs at really high volumes.
The EL34's are a different story. Putting them in, I noticed immediately that the feel of the amp was really changed. It was a lot looser, but not flubby like my Rev Blue was (probably due to the higher plate voltage). The other major difference was the midrange, which immediately stood out to me. It was rich and swirling as opposed to the KT88's, which were somewhat flat in comparison. The highs were also a lot softer and not quite so jagged. The low end was not as focused or punchy as the KT88's, but boosting the amp made a big difference and brought it up to an acceptable level. The bass response was a lot lower with these tubes, and the amp overall was not as loud. Channel One had some really nice cleans, and on the opposite end the amp really began to come into its own. With these tubes, I'd put it up against a real JCM 800 easily. Really nice rock tones and articulate leads to be had on Channel One with EL34's. Channel Two reminded me of the Rev Blue immediately, but with more articulation and a somewhat darker tonality. I imagine it is pretty close to a Rev 2 in its overall sound and feel (having never played one). This channel also seems a lot more saturated and wet with the EL34's. In summation, the EL34's seem to be a bit more rock-oriented than the KT88's, with somewhat less power and volume, more saturation, a much fuller midrange, sweeter highs, and less massive low end.
So now the final question: which one do I prefer? It really depends on the mood I'm in and the music I'm looking to play (really glad that I got the switch!).
Overall I'd say that the EL34's are probably more versatile, but if you bought the amp for metal, then the KT88's are the way to go. For recording, I'd say that the EL34's are probably also preferable. For live applications you might want to try the KT88's, except if the room makes them sound too brittle and glassy (a common complaint about KT88's).
So the next question is how KT77's and 6550's do in the amp. That will be answered at a later date!
