EL34, 6CA7 and KT77 are all drop-in replacement of each other. You can use 6CA7 or KT77 in place of EL34. Electrically, they are the same.
Now, with that out of the way, there is a fair amount of difference in how these tubes sound. Yes, EL34, 6CA7 and KT77 are not the same when you hear them, and there is a reason for it.
To understand the difference, let's start with a bit of history.
Mullard EL34 - European Invention
Mullard UK designed the EL34 tube in the 1950s.
EL34 tube has a fairly unique sound. In guitar amps, EL34 is all about mids and upper-mids. You get more of that brown tone with your rhythm work. EL34 gives you smooth note separation with complex harmonics and sparkle.
6CA7 was not a copy of EL34. The Americans adopted the same beam-forming tetrode design as the 6L6 to create 6CA7.
In a beam tetrode, a wing-shaped plate forms a beam of electron. Because of the beam-forming wings, the plate assembly is physically larger. 6CA7 comes in a fatter bottle because of this.
The 6CA7 and EL34 tubes sound distinctively different. On a sonic scale, the 6CA7 tube is a hybrid between EL34 and 6L6 tube. Like the 6L6 and 6550, 6CA7 tube has a more pronounced low end.
In a guitar amp, this makes for a tone that is more substantial. People often describe 6CA7 sound as heavier and darker than the EL34 tube.
Current production Electro-Harmonix 6CA7EH is an excellent reproduction of the original Sylvania 6CA7. Electro-Harmonix 6CA7EH does the 6CA7 tone very well. 6CA7EH is worth considering if you are looking to get a bit more bite and roar from your EL34 guitar amp.
GEC in West London, England, introduced the KT77 tube in the late 1950s as the ultimate medium power audio tube.
Current production Genalex Gold Lion KT77 was introduced in recent years to recreate the original magic. Gold Lion KT77 has a smooth texture and balance, and is worthy of consideration as a premium upgrade for any EL34 amp.
Then they were the same...
As years passed and transistors took over, there was less demand for tubes.
In fact, some tube brands got lazy and labeled everything EL34/6CA7.
KT77 all but disappeared when GEC quietly stopped production in the 1980s, as people danced funny to synth music wearing shoulder pads and dangling earrings.
Back to the Roots
In recent years, the tube manufacturers returned to the roots of these great tubes.
To cater to increasingly demanding tastes of tube amp aficionados today, tube manufacturers are releasing tubes that are true to the original design.
Current production EL34 tubes closely resemble Mullard pentode design. Likewise, current production 6CA7 are beam-forming tetrode like the original Sylvania design. And re-issued KT77 deliver the legendary magical sonics of the original.
There is now a wide range of tonal landscape to choose what fits you best. Please visit our EL34, 6CA7, KT77 tubes page to select the right tubes for your amp!