>R. Measures wrote:
>>>So, the operating condition definitely change during the step response test,
>>>but what's the effect on intermods? Not much. Intermod performance doesn't
>>>depend on plate voltage very much at all (for the tetrode case we thinking
>>>about here). And the grid bias changes a little bit around the AB1
>>>operating point, but that's well known to be a non critical setting.
>>
>>** It seems to me that a sag in bias potential would simply increase
>>ZSAC slightly -- which would make little difference in IMD if the ZSAC
>>was OK to start with.
>
>There's some confusion here. On the one hand, the effect of grid bias on
>ZSAC and intermodulation is not critical within quite a wide range. On
>the other hand, that statement also involves a hidden assumption that
>the grid bias remains constant wherever you set it.
>
** The grid does not and can not draw current when a competent operator
sets the grid potential just above the level where it can draw
grid-current with the max PEP drive that's available. This is the second
step in adjusting an AB1 amplifier -- the first step being: tuning out
the grid's XC with the grid roller-L. (L1 in
http://www.somis.org/D.a.05.GIF)
>If the impedance of the grid supply is very high, say 10K, then even 1mA
>of grid current will make the grid bias 10V more negative.
** This scenario is possible only when the bias is adjusted by an
operator who is not competent.
>This equates
>to a sudden decrease in gain at the onset of grid current, so the
>input/output power characteristic is no longer linear - it has a sharp
>kink, and that's bad news.
>
>When you think of the gain characteristic as a mathematical power series
>(see 'Care & Feeding', Terman etc) it is substantially linear, but it
>also contains some curvature due to higher-order terms which imply
>distortion. Gentle curvature across the whole plot implies low-order
>distortion with IMD products close to the signal. But a sharp kink in
>one place can *only* be created by quite large high-order terms, whose
>IMD appears as wideband splatter.
>
>
>(Yes, some 6146 amplifiers do have high-impedance screen and grid bias
>supplies, but there's no magic involved.
>What *is* generally involved is
>negative feedback and ALC, which Collins relied on heavily to improve
>IMD.)
>
** Indeed
>--
>73 from Ian
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