EL34, 6CA7, and KT77 are all drop-in replacements for 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. EL34, 6CA7, and KT77 are not the same when you hear them.
How EL34 Sounds
The magic of the EL34 tube is all in the mids. EL34 gives you smooth upper mids with complex harmonics and subtle sparkles for lead work in guitar amps. For rhythm work, EL34 produces a pleasant crunchy brown tone. EL34 sounds a bit more scooped and doesn't have the kick and sizzle of 6L6 or 6550. Instead, EL34 accentuates the upper mids that break up smoothly when pushed.
In audio amps, the smooth velvet mids of EL34 take center stage. With the right source, the smooth textures of vocals and wind instruments reproduced by EL34 are life-like. If you prefer classical or jazz music with horns and vocals, the EL34 tube is the perfect fit.
How do 6CA7 and KT77 Sound
6CA7 and KT77 tubes are drop-in replacements for EL34, but internally 6CA7/KT77 are different. The 6CA7/KT77 internals are closer to 6L6 or 6550 than EL34. Consequently, the sound of 6CA7/KT77 takes on characteristics from the 6L6 and 6550 tubes.
6CA7/KT77 does not have the same smooth textures and breakups as the EL34. Instead, the 6CA7/KT77 packs more punch in the bass and sizzle in the high end.
In guitar amps, 6CA7/KT77 has a bit more headroom than EL34 tubes, though not as much as 6L6. The tone is not as scooped as EL34, with a good amount of bass and smoothed-out highs. You may find that 6CA7/KT77 is heavier and darker sounding than EL34.
6CA7/KT77 has a good amount of kick and sizzle in audio amps. You won't get the velvet smooth mids of the EL34, but instead, 6CA7/KT77 provides dynamics that EL34 doesn't quite have. The low end is fuller and less tight than EL34. The top end is more extended but controlled. Spitty sibilance, which can be emphasized with 6L6 or 6550, is absent with 6CA7/KT77. Compared to EL34, 6CA7/KT77 has a much easier time handling contemporary recordings with extended range and accentuated low end.
6CA7/KT77 vs EL34
While 6CA7 and KT77 are drop-in replacements for EL34, authentic 6CA7/KT77 differs from EL34 tubes internally. You may, however, come across vintage 6CA7 tubes that are EL34 and vice versa. This has created some confusion as to whether 6CA7/KT77 is truly different from EL34.
To understand this, we need to understand the history of EL34, 6CA7, and KT77.
Mullard EL34 - European Invention
EL34 tube was invented by Mullard UK in the 1950s.
Mullard designed the EL34 tube for compact, high-power audio applications. In a push-pull configuration, EL34 can produce 55W of output. This is the same as 6L6, but the difference was that EL34 came in a skinny glass tube. This allowed for more compact amp designs. The smaller size and high power made the EL34 tube very popular in audio and guitar tube amps.
Today, there are many current production EL34 tubes, such as Mullard EL34 and TungSol EL34B. These current production EL34 tubes follow the original design but with modern manufacturing and quality control. They provide the classic rich mid-tones of the EL34 with consistency and reliability. Current production EL34 tubes such as Mullard EL34 and TungSol EL34B are the go-to tubes for upgrading OEM tubes shipped with the amp.
6CA7 was designed as a drop-in replacement for EL34, but 6CA7 was not a direct copy of EL34. Instead, Sylvania adopted the same beam-forming tetrode design as the 6L6 to create 6CA7.
6CA7 comes in a fatter bottle than an EL34. In a beam tetrode like 6CA7, a wing-shaped plate forms a beam of electrons. Because of these beam-forming wings, the plate box assembly is physically larger. 6CA7 comes in a fatter bottle for this reason.
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 want a bit more bite and roar from your EL34 guitar amp.
KT77 has a unique history. GEC in England introduced the KT77 tube in the late 1950s as the ultimate medium-power audio power tube.
KT77 tube was designed as a drop-in replacement for EL34, just like EL34. KT77 employed an internal construction resembling 6CA7. GEC called it "Kinkless Tetrode," which is what the KT prefix stands for. Earlier power tubes had "kink" in their power curve, resulting in non-linearity.
GEC's marketing claim was that their design eliminated this non-linearity, thus optimizing the KT77 tube for high-fidelity audio applications. The KT77 was known as the ultimate EL34 for audiophile use. In reality, however, KT77 is a beam tetrode very much like 6CA7.
The current production Genalex Gold Lion KT77 was introduced in recent years to recreate the original magic of KT77. Gold Lion KT77 has a smooth texture and wonderful balance and is worthy of consideration as a premium upgrade for any EL34 amp.
EL34 vs 6CA7 Confusion
You may come across vintage EL34 that is marked as 6CA7 and vice versa. This has created confusion over the years, which resulted in some insisting that 6CA7 and EL34 are no different.
Such "mislabeled" EL34/6CA7 are often from the 1970s to 1980s. Back then, as tube demand dwindled, manufacturers consolidated factories and products worldwide. In the US, tube production wound down and eventually ceased. Tubes were imported from Europe to fill the demand that remained.
Since EL34 was a drop-in replacement for 6CA7, imported EL34s were simply re-labeled and sold as 6CA7. Many EL34 tubes from this era were marked as 6CA7 and vice versa. In fact, some tube brands got lazy and labeled everything "EL34/6CA7".
As for KT77, it all but disappeared when GEC quietly stopped production in the 1980s.
Back to the Roots
Tube manufacturers have recently started manufacturing EL34, 6CA7, and KT77. They are not re-branding EL34 as 6CA7 as was done in the past, but they are reproducing the construction that's true to the origin of these tubes.
For example, current production EL34 tubes follow the original Mullard EL34 design and their European variation such as Siemens EL34.
Likewise, current production 6CA7 are beam-forming tetrode like the original Sylvania design.
With current production EL34, 6CA7, and KT77, the difference in sonic is apparent. These tubes are designed differently to offer distinctively different tones. There is now a wide range of tonal landscapes to choose what fits you best. Please visit our EL34, 6CA7, KT77 tubes page to browse and select the right tubes for your amp.