[Amps] Need for input tuning circuit

[Phil Clements]

> Consider a grounded-grid 8877 HF amplifier.
>
> I understand the need for an input tuning circuit to maintain
> linearity operating SSB, but what if the operation is CW or RTTY?
> What if the 8877 is driven directly from the transceiver's output,
> using the built-in antenna tuner for impedance matching?  Are there
> any drawbacks I'm not seeing?

Two reasons for a tuned input circuit mounted near the tube(s):

Input waveform distortion occurs as a result of grid and plate current 
loading of the input circuit on alternate half-cycles by the 
single-ended driver stage. the driving source sees a very low value of 
load impedance over a portion of the RF cycle and an extremely high 
impedance over the remaining portion of the cycle. Unless the output 
voltage regulation of the wave on the loaded part of the cycle in the 
grounded-grid stage will act to preserve a distorted waveform. A tuned 
cathode circuit with a Q of 2 or more, and resonated to the operating 
frequency will do the job of preventing this distortion.

A tuned cathode circuit provides a very short RF return path for plate 
current pulses from plate to cathode. When a tuned circuit is not 
used, the RF return path is via the outer shield of the of the coaxial 
line, through the tank circuit of the exciter or tuner, and back to 
the cathode of the linear amplifier tube via the center conductor of 
the coaxial line. This random, uncontrolled path varies with the 
length of interconnecting coaxial line, and permits the outer shield 
of the line to be "hot" compared to RF ground.

These words are not mine, they are Bill Orr's.

(((73)))
Phil, K5PC