Re: [Amps] 3.5 kV 2A REGULATED Power Supply: Schematic ?

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

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.

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 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.)


--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                           Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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