Prune wrote:

I read somewhere that the cheaper Svetlana 4CX800A can be used as a
substitute for the Eimac 3CX800A7. Does anyone know if the Svetlana tube
also uses beam forming? Would I notice a difference at audio frequencies?
(I'm serious)
Also, can this tube be used in class A? On Eimac's site, it says it's
designed for class B, but does that mean that it merely won't be as
efficient in class A, or it will blow up, or what?

The '3' in 3CX800 means triode, the '4' in 4CX800 means tetrode -- so
they won't be compatible.

I have no idea of the potential for high-fidelity audio of either tube,
but either one should make fine audio amplifiers otherwise.

Per basic amplifier theory no tube will be as efficient in class A --
the theoretical maximum you can get in class A with a pure sine wave
input and a plate choke or transformer is 50%, in class B this goes up
to something like 63%.

Any normal* tube can be used in any class of amplifier as long as you
don't exceed the various design limits of the tube. In designating it
for class B Eimac means that the design limits are sized for such, and
in class A operation you'll have to hold a number of them way below
maximum to keep the few (plate dissipation, mostly) in line.

Either one of these tubes should be electrically quite suitable for
class A operation -- but you have to design the circuit right, and
understand the implications of everything that you do. I don't want to
discourage you, but the nature of your question tells me that you need
to do some serious study (possibly including blowing up some lesser
tubes) before you tackle this one.

* and if you find one that can't I'll say it ain't normal!

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com