Rich Measures wrote:
>>
> 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.
OK, that makes sense, but until now I don't recall you mentioning the
need for a range of input transformer ratios to bring everything into
roughly the right ball-park.
>>>>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.
Ah...
>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?
>
I do not interpret that as a recommendation to deliberately choose a
poorly regulated grid supply. IMO it's merely a recommendation not to
allow the grid to "float" and charge up electrostatically. All it means
is that you cannot use a high-value grid resistor for "contact-
potential" biasing, as you can with small receiving tubes.
>>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. .
>
Yes, but there's another way to achieve that, namely to adjust the
drive, either manually or by ALC.
>>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.
Nor could I.
> 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?
As the Scots say, "I hear you" - which means that I appreciate what
you're saying, but don't agree that it's the only "right" way to do it.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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