Amplification factor

[catstrat]

Hello everyone. I had a question asked of me over the weekend by my wife , of all people, about the amplification factor of different tubes in the 12a_7 family that i didn't know how to answer. How do they design a tube to have a higher factor of amplification over another? For example, a 12ax7 has an amplification factor of 100 while a 12au7 has a lower level of amplification factor. We all know about the external componants that have effect of this. Plate resistors, grid stoppers and cathode circuits. But what internally in the tube makes one weaker than the next? Is it the coating used on the cathodes? Is it the way the control grids,plates are constructed? Or do they put the tube in a machine of some kind and run it until it just wears down? Looking forward to finding out. Thanks everyone.

[loudthud]

Grids are usually constructed as a spiral wire wound in a circle or elipse supported by two or more vertical rods. The pitch or spacing between the wires and how close the structure is to the cathode are the most significant details that govern the amplification factor. Other factors are the amount of electrons liberated by the cathode, including chemistry and the amount of heat from the filament, and the size, proximity and voltage on the plate.

Look at the data sheets for the 12AT7 and the 12AZ7. The specs look identical except the 12AZ7 needs more filament current but it can also stand more voltage between heater and cathode.

[eschertron]

But what internally in the tube makes one weaker than the next?

Don't think of a 12AU7 as a 'weaker' version of a 12AX7 just for the lower amplification factor. It is constructed differently for different functions. A 12AU7 can (trans)conduct enough current to destroy a 12AX7 without breaking a sweat. That's the design.

OTOH, there does seem to be a school of thought with power tubes that out-of-tolerance or weaker tubes can be graded and sold as having 'special qualities'. That's not what is going on with preamp tubes, though.

[Tom Phillips]

The attached shows a nice illustration of the design features explained by loudthud.

Triode Types.pdf