Rich Measures wrote in response to Tom Rauch:
>>The important point I was making is the grids belong grounded in a
>>grounded grid PA. Any time any grid is RF coupled directly or
>>through a divider to the input or output circuit, stability is decreased.
>
>Apparently, Collins didn't think so.
Well, put it this way... any time you apply feedback to an amplifier, it
partially connects the output to the input, making each port more
sensitive to the termination at the other. Feedback that is negative in
the operating frequency range will almost certainly turn positive in
some different frequency range.
>I have never heard of an
>instability problem with the Collins 30S-1 amplifier, and the grid is RF
>coupled directly to the cathode.
Ahem: not in the circuit I'm looking at. The control grid is connected
to a capacitive divider between anode and ground, but not connected to
the cathode by any external components. Drive is applied between cathode
and ground through the normal switched pi-networks.
Here's another one to chew on. The 30S-1 has a parallel R/L network
connected to the anode, but it does NOT go to the tank circuit! The tank
circuit connects directly to the anode and the R/L goes to the top of
the capacitive feedback divider. I wonder what happens without the R/L?
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
Still struggling to understand this topic...
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