Editing Control grid (section)

== Grid variations ==
[[File:GRIDS_TM11_662_FIG_39.jpg|thumb|Illustration representing various control grid configurations]]
A variation of the control grid is to produce the helix with a variable pitch.  This gives the resultant valve a distinct non-linear characteristic.<ref>[http://www.tpub.com/content/neets/14178/css/14178_74.htm Variable mu valves] {{webarchive|url=https://web.archive.org/web/20070310142129/http://www.tpub.com/content/neets/14178/css/14178_74.htm |date=2007-03-10 }}</ref>  This is often exploited in R.F. amplifiers where an alteration of the [[grid bias]] changes the mutual conductance and hence the gain of the device.  This variation usually appears in the [[pentode]] form of the valve, where it is then called a variable-mu pentode or remote-cutoff pentode.

One of the principal limitations of the triode valve is that there is considerable capacitance between the grid and the anode (C<sub>ag</sub>). A phenomenon known as the [[Miller Effect]] causes the input capacitance of an amplifier to be the product of C<sub>ag</sub> and amplification factor of the valve.  This, and the instability of an amplifier with tuned input and output when C<sub>ag</sub> is large can severely limit the upper operating frequency.  These effects can be overcome by the addition of a [[screen grid]], however in the later years of the tube era, constructional techniques were developed that rendered this 'parasitic capacitance' so low that triodes operating in the upper [[very high frequency]] (VHF) bands became possible.  The [[Mullard]] [[EC91]] operated at up to 250 MHz. The anode-grid capacitance of the EC91 is quoted in manufacturer's literature as 2.5 pF, which is higher than many other triodes of the era, while many triodes of the 1920s had figures which are strictly comparable, so there was no advance in this area.  However, early screen-grid tetrodes of the 1920s, have C<sub>ag</sub> of only 1 or 2 fF, around a thousand times less. 'Modern' pentodes have comparable values of C<sub>ag</sub>. Triodes were used in VHF amplifiers in 'grounded-grid' configuration, a circuit arrangement which prevents Miller feedback.

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