On 7/22/06 at 4:25 PM Tom W8JI wrote:
>> 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.
No, this actually forms the basis for grid leak biasing where the grid collects
electrons. In a grid leak circuit, those electrons are fed to a capacitor to
ground which charges up with those same electrons during the positive half
cycle. Generally, a resistor is in parallel with the capacitor making the grid
leak circuit (grid leak capacitor and grid leak resistor). One can also add
some negative voltage from a power source giving it a minimum amount of bias.
The grid still bleeds off into the capacitor when doing this. Most of the old
amps in mention and other equipment used this method unless all grids were tied
to ground like a few did.
>
>Grid leak biasing is dependent on grid current to establish
>bias.
Not necessarily. A small negative voltage can be fed to the circuit giving it a
minimum amount the bias can go to. One can do it either way, but adding the
minimum bias voltage is done a lot in some of the older amps and other
equipment I'm speaking of.
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!
Correct, but see above.
>
>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
Correct, but if using a grid leak circuit, the grid can still obtain some
electrons from the space charge around the cathode. You just said the grid can
charge, so the grid leak capacitor can be charged to a small value by it also.
>
>" 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.
Yes I mentioned them for others who may not know of this method. However, don't
degrade this method by calling it "cheap" or "poorly designed" as it was used
in numerous pieces of military and commercial tube equipment. The control grid
in a power tube can be charged negative the same as any other tube. Just
because your wrong in some areas, makes no need to try and belittle others by
your tone you use to try to prove a point that's partially incorrect.
>
>73 Tom
>
>
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Will
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