[R-390] Current direction

[2002tii]

Jay wrote:

>IIRC the supressor grid was to attract electrons that actually 
>bounced off the plate and then impacted again causeing noise.This 
>grid would draw off those errant electrons and thus improving the 
>noise figure of the tube.

The suppressor grid is at a very negative potential with respect to 
both the screen grid and the plate (generally cathode 
potential).  Thus, it repels the electrons that are re-emitted from 
the plate ("secondary emission"), sending them back to the plate 
instead of letting them continue to the screen grid.  This is not 
done to reduce noise, but (1) to suppress the negative-resistance 
regime of a tetrode, which causes the undesirable "tetrode kink" in 
the transfer characteristic, and (2) to reduce the screen grid 
dissipation (mainly in power pentodes).

Best regards,

Don