[Amps] Grounded Screen 4CX1500B

[Shawn Tayler]

Wow Ian,

I had totally missed that.  The input circuits were right next to the grid
circuit and I had simply assumed.  Well how about that.  Hmmm, very
interesting.  Kinda burst the bubble for me.  I must be blind and deaf.....


n Mon, 7 Nov 2005 14:43:00 +0000 Ian White G/GM3SEK
<gm3sek at ifwtech.co.uk> exclaimed:

> Shawn Tayler wrote:
> >
> >Hello Rich,
> >
> >Interesting that you say this.  I was under that same impression from
> >conversations through the years.  I was happy to find that only the
> >screen is grounded, for stability.  The 30S-1 is a grid driven tetrode
> >amp otherwise.
> >
> The 30S-1 is cathode driven (I'm looking at the schematic now). G1 is 
> mostly grounded to RF by a 220pF capacitor, but RF negative feedback is 
> also applied at this point by a 5pF cap from the anode.
> 
> > I could never see how a 4CX1000A could function well in a
> >grounded grid mode.
> 
> G1 has normal negative bias, the same as it would have in grounded 
> cathode,  so the tube operates in normal class AB1. The first traces of 
> grid current will operate a transformer-coupled ALC circuit - but grid 
> bias supply has quite a high impedance, so if the ALC output has not 
> been connected back to the exciter, any grid current will cause a large 
> shift in operating point with resultant IMD.
> 
> These features make the 30S-1 very different from the "triode-connected 
> grounded grid" operation of 4-250/400/1000 jamjars with G1 and G2 both 
> connected to chassis ground.
> 
> 
> -- 
> 73 from Ian G/GM3SEK
> http://www.ifwtech.co.uk/g3sek
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