[AMPS] re:4-250A

George T. Daughters gdaught6@leland.Stanford.EDU
Mon, 21 Jul 1997 13:47:03 +0008


re the ongoing debate,

> >Whenever grid current 
> >flows in a grid driven amplifier, linearity is compromised.  
> 
> Rich, you keep saying that in such sweeping, all-inclusive terms. 
> You may believe it most sincerely, but that still doesn't make it true.

my unsolicited $0.02 worth is... there are tubes where even a teensy 
bit of grid current will result in compromised linearity.  there are 
tubes where a fair amount of grid current can be drawn, with no 
compromise.  it ain't grid current that's the defining 
characteristic!  it's the linearity!  the "transfer characteristic" 
should be a straight line.  if it bends, then it ain't linear.  

with many modern tetrodes, even those with handles, there is a serious 
bend that occurs just about at 0 volts on the grid.  the control 
grids are really good at intercepting electrons on their way from the 
cathode to the plate.  if the grid starts grabbing a significant 
amount, then linearity goes to pot.  what is "significant"? it 
depends on how much "to pot" you're willing to accept.  if the 3d and 
higher order products are legal, then maybe it's not so bad.  

but it depends on the tube!

ok, maybe that was only a penny's worth.

73,
George T. Daughters, K6GT

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