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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." |
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