> Roy,
> You got it! Most of the old sweep tube amps used a form of
> grid leak bias.
> Best,
> Will
> *********** REPLY SEPARATOR ***********
> On 7/22/06 at 10:50 AM Roy Koeppe wrote:
>>"...If it's open, the most it might do would be to attract
>>a few
>>electrons being negative like was mentioned...(etc.)"
>>In the old days this was called 'contact bias,' and was
>>the principle
>>incorporated intentionally in a grid-leak biased stage.
>73, Roy K6XK
Actually both of you are mixing applications and very
different systems. Contact bias or space charge biasing is
NOT the same as grid leak biasing.
Grid leak biasing is dependent on grid current to establish
bias. A common application is oscillators where we might
want zero bias at start and automatically adjusted bias that
increases with oscillator output. This type of bias works by
grid current from a positive going grid with respect to
cathode placing the grid/cathode path into conduction!
Space charge bias, which is more commonly called contact
potential bias, depends on the control grid having no
leakage to the anode (no gas and low anode voltage) and no
grid emission (cold grid) to allow a very small amount of
charge buildup from the space charge around the cathode to
develop self-bias. From "Electronic Communication" page
13.15
" This type of bias is useful with high-u tube operated with
very small input-signal levels only. The diagram resembles
grid-leak biasing used in oscillator and RF amplifiers, but
the theory of operation is not the same." It goes on to
warn inexperienced designers NOT to use this type of bias
unless they are experienced since there are few applications
where it is useful. It is useful with small tubes in very
low level stages with low bias requirements, primarily where
AC from a heater might modulate the cathode.
The stuff Will remembers from cheap ratty poorly designed CB
amplifiers is NOT contact potential bias, but rather bias
developed when a grid is driven positive with respect to the
cathode during part of an RF cycle in an amount that exceeds
any negative bias. Don't hang your hats on that as something
justify gross misstatements that the grid in a large power
grid tube will go negative when floated! I especially
wouldn't point to grid current biased sweep tube amplifiers
as panicles of good communications system engineering.
73 Tom
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