I've now spent over a week comparing my JJ EL34IIs to my JJ 6CA7s in both my Rogue Cronus Magnum II and my ST-70 modern build amp.
My conclusion is that the JJ versions of these two tubes sound very similar, but the EL34II wins in the ST-70 / Devore Super Nines system and the 6CA7 wins in the CMII / Klipsch Heresy II system. I find that the EL34II is a bit more expressive - it is more detailed and forward from the midrange up to the treble, and has tighter, "pluckier" bass. On the other hand, the 6CA7 strikes me as a bit more linear from top to bottom, with deeper bass and an overall rounder, more laid back sound. Also, the 6CA7 *looks* better in the CMII because it's closer to the width of a 6550 so it doesn't completely vanish in the recessed sockets like the EL34II does. Since the ST-70 doesn't have recessed sockets, the spelt EL34 profile looks less tiny and more elegant. If that matters, lol.
You might want to check out the link below from the JJ site. It's pretty fun in that they describe the technical aspects as well as the sound profiles of their various EL34 types narratively.
I've put their descriptions together below so you need not click on each tube type to see them:
"EL34 is an octal power tube with a maximum plate power of 25W. Very robust and reliable due to its internal construction."
"E34L is a more powerful EL34 with more headroom and deeper lows. It is a little more aggressive than the EL34"
"EL34 II [which I have] has fuller low mids and low end but does not lose headroom in the high end. Stronger than regular EL34 but not as aggressive as the E34L."
"6CA7 [which I also have] is a beam power pentode with octal base. It can be used in most EL34 amplifiers. Even more powerful than KT77, and very similar to 6L6 sound. The 6CA7 provides a typical American sound."
and
KT77 is a beam power pentode with octal base. It can be used in most EL34 amplifiers. It has more power, fatter bass and more headroom than the EL34 and E34L while providing very detailed tones.
I was surprised to see that they describe the 6C7A as more powerful than the KT77. YMMV, naturally, as may similar tube types from other manufactures, but I found the descriptions pretty interest, though I suspect they might be more oriented towards guitar amp users?
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