[Amps] Question about 4CX800 amplifiers

[Larry Benko]

Morel,

I would not call a series LC circuit a "rejection" in this case. 20pF in 
series with 150nH has a resonant frequency of 91.9MHz.  In this 
application I am nearly certain they are for impedance matching.

20pF in series with 150nH acts like:
20pF at low frequencies
20.5pF @ 14MHz
22.1pF @ 28MHz
30.5pF @ 54MHz

The wiring of the input circuit, the capacitance of the grid to ground, 
the 50 ohm 100W resistor etc. probably did not match well enough with 
just fixed capacitors.  It is a very common impedance matching technique 
to use LC circuits above or below the resonant frequency to act as 
frequency variable capacitors or inductors.

73,
Larry, W0QE

On 5/17/2015 4:01 AM, Morel Grunberg wrote:
> Hello guys,
>
> I wonder if someone can explain the presence of two LC rejections (20pF
> series with 150nH) in the input board of ACOM amplifiers with 4CX800 /
> GU-74B tubes (ACOM 1000, 2000).
>
> Thanks in advance,
>
> 73
>
> Morel 4X1AD
>
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