[Amps] Grounded Screen 4CX1500B

[Ian White G/GM3SEK]

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