Raindance,
Cool stuff!
Thanks for detailing your mods.
I have to admit that I've never encountered any issues with the i34 in terms of channel tracking or unusual popping noises. So far, the only operational complaint I have to submit pertains to the 60Hz transformer buzz. Even then, I am not sure if I can blame the i34, as every amp I've had that uses a Toroidal transformer does the same thing.
I'm curious; what version of the i34 you have. As you may already know, Boris makes two versions of the i34. One is built for high gain, the other is built for low gain. I've had both and find myself likening to the low gain version. If you are running the original i34 (which has higher gain), that could partially explain why you're running into a bit of unnatural grain in the top end. I'd also be curious to know what speakers you're running!
With that said, I too have thought about replacing the caps and op amp. Years ago, I took a few budget front ends and performed the same basic mods. The results were very good.
As for the tubes, I've only experimented around with a few power tubes. So far, I've tried the following: The Stock JJ EL34's, Sino/Chinese EL34B's, and the Tung Sol EL34B's. Here are my very basic impressions.
Stock JJ EL34's: Good sounding tube for the money, albeit less linear than the other two. The JJ's have a grittier top end and lack the gravitas of the other two tubes in terms of bass presence and dynamic slam.Their mids are real nice. Believe it or not, I really love these tubes when they are set to uneven bias. In the case of the i34, I run 340 on the left side and 360 on the right. The presentation suddenly becomes downright lovely. Bass presence, super smooth top end, blossomed midrange. Real good stuff.
Sino Chinese/EL34B's: Throwing these into the mix is like giving the i34 nearly twice the HP to draw from. These tubes are super powerful and linear. Tons of bass, crystal clear highs, and exceptional dynamics. The Sino EL34b's make the i34 sound a lot closer to a good solid state amp. The downside is that I think these tubes are just too much of a good thing. I love their power and linearity, but they are a bit too forward in the mids and highs for my taste. I also feel like the polarity of one of the tubes I had may have been off, as the imaging on the i34 became a lot more diffuse with them in place. Still, I can't help but wonder how these particular EL34B's would sound if paired up with a very warm/lush sounding set of ECC832's. Oh yeah, I forgot to mention that these are the quietest tubes of the bunch.
TungSol EL34B's: These are so new that I really can't have much of an opinion on them yet. So far, they appear to hit a good balance between the Chinese tubes and the JJ's. They are very balanced from top to bottom. Extended highs, though not quite as 'gritty' as the JJ's (when biased evenly, that is). Great mids. Articulate and detailed yet not too blossomed out, and good bass. So far, the only complaint I'd have is that they seem to be a bit dynamically constrained. Then again, they are brand spanking new, so I will need to put a bit more time on them to come up with anything conclusive.
Ultimately, I really enjoy the performance of the i34. Even with stock tubes and no mods, I've yet to find another affordable tube integrated in the same price point that does what it can do.
Once again, thanks for taking the time to detail your mods! I look forward to reading about how it all goes as you dig deeper down the rabbit hole!
