[AMPS] Re: [super cathode]

Sat, 17 May 1997 14:19:08 +0000

Hi Rich,

I refuse to get into personal arguments, but I have no problem with 
technical disagreements.

> >Rich, I can fax you a copy of the article. 

> Thanks, but no thanks, Tom.  The SEMI or SUPER semantics thing is a 
> non-issue.

You asked for and I offered the data.

>The issue is why did Collins connect a 220pF cap. from each 
> grid to ground. 

The article covers that, as did private conversations with 
the system designers.

> When and if a 30L-1 owner takes a dipmeter and compares 
> the grid resonance with a directly-grounded 811A grid versus grounding 
> through the factory stock 220pF capacitor, the truth will out. 

A GDO is not the correct instrument for telling what really goes on 
in the system. The best way to measure the grid's effect is to 
measure the transmission loss from the anode to the filament. 
Where ever the grid "looks like" a high impedance, the transmission 
loss will be minimal. 

The article by Orr in the previous month contains curves of grid 
resonance. I have no idea how he measured them, but his results are 
almost the same as the measurements I posted and viewed on my 
network analyzer. There is only one sharp null moved into the VHF 
range by the cap, and everything else is made worse. 811's and 572's 
like to oscillate near ten meters in many PA's (the FL-2100 has this 
problem, as well as the Dentrons) when non-neutralized. The correct 
cap is in the thousands of pF, but it makes lower frequencies worse.

> >Do you think non-linear RF feedback, providing maximum gain 
> >compression when peak levels are reached, is a good idea?

> No, but this is non-issue because the amount of negative feedback is 
> miniscule.  I believe that the Collins engineers added the C for raising 
> the grid resonance and increasing VHF stability.  

Feedback that varies due to a non-linear voltage divider between a 
few percent and  50% voltage (-6dB) feedback is NOT miniscule in a PA 
capable of over 15 or 20 dB gain (at low power levels).

Even -20 dB feedback could and probably will destabilize the PA.

73, Tom W8JI 

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