Re: Stereo Amplifier
Yes, I think Langford-Smith is talking about the same thing as me - non-linear small grid current in the negative bias region where most people expect to see no grid current at all. See also Ch 2, sec 2.2(iii) - especially fig 2.9 on page 19 (curve D).
When you mention grid current to most people they think of Class C or AB2, where tens of mA of grid current can be quite normal. They know that a low impedance drive is needed. The region I am talking about is not so well known, despite being in Langford-Smith. I suspect, but have no evidence to back this up, that this effect is responsible for some of the subtle differences between various good amplifiers, and some of the changes which are heard when valves are swapped ("tube rolling"). The size and position of the grid current curve varies from valve to valve, and changes with age. It is affected by residual gas, contact potential (depends on exact metals used), cathode temperature. It is quite common to bias a small signal valve right in the middle of this region, especially high mu ones like ECC83. You need to be below about -1.5V before the probem can be ignored, or have a low impedance source.
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