Wanted: education RFC thoughts

[Murray Leshner]

Hello:

My thoughts:

22 uH is about 250 ohms reactance at 1.8 MHz and 4k at 30 MHz. I'll address
GG amps since I don't have much experience with the other configurations. I
have seen GG amps with grids DC grounded (as a choke would be), and RF
grounded (capacitors only).

The inpedance at the cathode of a GG amp is relatively low, say anywhere
from 30-300 ohms, but probably several K off the plate before the tank
circuit. After the pi or pi-L network it's 50 ohms.

I think the purpose of the two inductor examples you give holds the hidden
answer(s). Whatever choice the designer made in grounding the grid, whether
directly, thru capacitors or thru chokes plays a role. There might be
metering thru the cathode which wouldn't work with one grounding
arrangement.

A relatively small reactance probably plays a role in the impedance at that
point.

At the output of the output tank circuit the relatively large choke is
supposed to look like a high impedance to RF - it's purpose there is easier
to identify - it's a DC safety feature. If the blocking capacitor (coupling)
off the plate were to short, a pi or pi-L network would pass anode voltage
to the feedline, a real safety hazard. The output choke goes to ground here
to be a short circuit to DC, blowing the fuse or breaker. The wire should be
heavy enough to withstand this long enough to trip the protection device. A
low inpedance here would dissipate too much RF current and fail during
normal operation.

Murray

It may also fail in some cases if the fuse or breaker

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