Mike Knudsen wrote:

Can someone explain how a tube can show good cathode emission, but not be up to
snuff in transconductance? A tube's performance depends on teh geometry and
structure of its grids and plate and their spacing. If hte tube hasn't been
physically knocked around, the geometry hasn't changed. So if the cathode is
putting out the electroncs, how can the tube not be up to snuff?

Let's say you have a beam power tube that was used on an amplifier with
a screen grid supply problem, and the screen grid drew current and burned
completely away. There's nothing left any more.

That tube will test just fine on an emission tester. The cathode is putting
out plenty of electrons, but the tube Gm and mu are way off because the
geometry itself has been radically altered. It tests fine, but it will
not even come close to working in a circuit.

Okay, it's an extreme example, but plenty of things go wrong with tubes
other than the cathode getting stripped.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."