The KT77 is a power output valve used in some guitar amplifiers. Marshall have used these in several of their designs in the past.
Normally you will find the amp can use either a KT77 valve or an EL34, fairly interchangeably.
First let’s look at what the ‘KT’ stands for in the KT77 valve. Well, these valves, like many power valves are basically TRIODES. That means they have three main electrodes. The cathode, the anode (plate) and the grid. So the ‘T’ stands for ‘Triode’. The ‘K’ stands for ‘kinkless’. And no, that’s nothing to do with the sexual preferences of this valve…
Many valves exhibit a ‘kink’ in the sine wave and this manifests itself as slight distortion (“Oh no… distortion in a a guitar amplifier…. Whatever will we do???”) The KT77 is designed to have no kink in the output sine way. If that’s confusing they are actually pentodes with 5 electrodes! Ah well. Too much info.
Ok, so what does that mean? All other things being equal it will give you a cleaner sound with more clean headroom. If you play any kind of distorted sound (metal etc) then you don’t need KT77’s in your amp. Use EL34. If you’re into Hi Fi or want a very clean guitar sound you should go for KT77.
GEC optimized the KT77 valve for use in audio application. This makes KT77 the ultimate ‘EL34’ for audiophile.
Here’s what JJ say about their KT77 Valve
“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.”
Here is what one user says: “Tonewise they’re somewhere between an EL34 and a 6L6…
bigger bottom end, nice detailed highs… more headroom too.”
You Can buy a matched pair of KT77 here.
You Can buy a matched quadof KT77 here.