- Jun 27, 2018
- Boquete, Chiriqui, Panama
Although there has been some discussion here at ASR about tube-rolling and it's usefulness or worthiness, none of those discussions focused on output tubes. The big, power-hungry tubes that feed output transformers (or not, in the case of OTL amplifiers like Atmasphere) have been mentioned, but not discussed in detail. I am aware that some amp designers and manufacturers push them to the limit for more power (which reduces life-span), some design for multiple tube type compatibility (which I assume limits the optimization for performance), and some design for excellent performance combined with lowered stress and long tube life (as with the classic McIntosh amplifiers).
It is fairly common knowledge that power transformers are a primary limiting factor, because the more powerful tubes - like the KT120 and KT150 - draw lot more filament current than an EL34 or KT88. Indeed, if I put KT150s in an KT88 amplifier that was designed with enough current capability for KT150's, will them amp become as powerful as one designed specifically for KT150s?
Some of the parameters for multiple output tube-type compatibility, including the fact that EL34, 6CA7 and KT77s are very close in specs, and seem to be very interchangeable. However, I don't know enough about the performance curves of those related tubes to understand optimization options and rules. Surely there are some parameters for optimal performance and life span that include running them conservatively.
I am awaiting the arrival of a quad of Electro-Harmonix 6CA7s next week to replace the no-name Chinese output tubes in my Yarland Class-A PP EL34 amplifier. Like the original 1960s Sylvania "fat bottle" 6CA7s (which, were designed specifically to provide a domestic American option to the British EL34), the Russian-made Electro-Harmonix 6CA7s, like the 6L6 tubes they are said to emulate, are "beam tetrodes" rather than pentodes. OTOH, most other modern 6CA7s, including JJ versions, seem to be pentodes. What's the practical difference between the two types of tubes with their different number of grids? How can a single tube type (6CA7) have a different number of grids and be either a tetrode or a pentode, depending on the brand?
Plate voltage and current ranges for EL34 KT88, KT90, KT120, KT150 are quite different. How can putting EL34s in and amplifier optimized for KT88, or KT150s in an amplifier optimized for lower-power KT88's be an intelligent option? And of course, I can imagine that putting KT88s in an amplifier optimized for KT150s would not be good - or safe!
In audio, even with vacuum tubes, most knowledgeable music lovers want flat frequency response and low noise and distortion, and many of us buy tube amplifiers for reasons other than purported "magical sound enhancement". However, guitar players are looking for other differences, and talk like this:
and...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.
Yes, tetrodes are more glassy and HiFi than pentodes. Both technologies have their advantages.
As it happens there are no true kinkless or beam tetrodes - if the number of electrodes matters. The difference between a tetrode and a pentode is the 5th electrode which in the pentode is a grid and in the tetrode are two beam forming plates. That's why some tetrodes still are referred to as pentodes. As the 4th and 5th electrodes, which are essentially performance boosting, aren't used in triode connection tetrodes and pentodes operate identically from an electrical POV.