>Rich Measures wrote:
...
>>Why not adjust the grid bias V so that the grid can not draw current with
>>max. drive applied?
>>
>Not all tetrodes require grid bias voltages that conveniently match up
>to the available drive power and the grid swamping resistance. Adjusting
>the grid bias may work for Rich's amplifiers because they use big
>tetrodes that require more than 100V of grid bias. Since the peak RF
>voltage from the nominal 100W exciter is already pretty close to the
>manufacturer's recommended bias voltage, a relatively small adjustement
>to the bias voltage may make sense. But that strategy obviously can't
>work for tetrodes that only need 35-50V of bias.
>
My strategy, Ian, is to choose a value of grid termination resistance
that roughly affords the needed peak RF drive V to the grid with exciters
that develop 100v-p (100w rms) to 141v-p (200w rms) across 50 ohms.
-- For tubes that require 50v to 70v of grid bias, like the 4CX800A, a
voltage-halving bifilar stepdown transformer driven, 12.5 ohm grid
termination is used.
-- For tubes that require 100v to 140v of grid bias, a directly-driven
50 ohm grid termination would be used.
-- For tubes that require 200v to 280v of grid bias, like the 5cx1500a
and 8169, a voltage-doubling bifilar stepup-transformer driven, 200 ohm
grid termination is used.
-- For tubes that require greater than 300v grid bias, like the 8171, a
trifilar stepup-transformer driven, 450 ohm grid termination is used.
>>>I also like Ian's suggestion of building the grid supply
>>>like the PA is for AB2 (regulated with a stiff shunt regulator), even
>>>though you plan on running it on AB1.
>>>
>>Eimac says you don't need a regulated bias supply..
>
>Rich, please can you quote exactly what Eimac says on this topic, and
>where.
Eimac does not state such in so many words. The max. allowable R of the
supply is found in the technical specifications for each tube. For
example, on page 4, column 1. of the specs. for the 4CX15,000A, the
maximum R is stated to be 100k ohms. In your opinion, Ian, is a
regulated supply, or an unregulated supply, most likely to have an output
R of 100k?
>Similarly, please can you quote what and where Eimac says that the
>correct method to configure a class AB1 amplifier is to adjust the grid
>bias for zero grid current,
You've got to be putting me on, Ian. AB1 means zero grid current. .
>and the adjust the screen current for the
>recommended ZSAC.
I thought I read it in *Care and Feeding....*. After spending about 15
minutes therein, I couldn't find it. However, the practice makes sense
to me. To explain: There are two means of setting ZSAC: by adjusting
grid V, and by adjusting screen V. Grid V adjustment is constrained by
the requirement that zero grid current be maintained---which leaves
choosing to adjust the screen V. Does this make sense to you?
Rich---
R. L. Measures, 805-386-3734, AG6K
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