[Amps] SB200 Parasitic Chokes

[Jim Thomson]

Date: Wed, 19 Jun 2013 18:36:23 -0400
From: "Carl" <km1h at jeremy.mv.com>
To: "Jim W7RY" <w7ry at centurytel.net>, <amps at contesting.com>
Subject: Re: [Amps] SB200 Parasitic Chokes


Of course it requires less drive for the same output. Read up on negative 
feedback and notice that without it the stage gain is higher which may lead 
to instability.

You can add some non inductive R in series with the drive at the filament to 
restore the NFB and the original drive versus output. You also made a big 
circle for a net gain of zero.

Carl
KM1H

## IF the grids are bonded to chassis and gain increases...so what ?  If it then becomes
unstable, the re-do the suppressor design. 

##  heath et all is telling us the NFB comes from the V divider action.   The grid caps are one
half of the divider, and the stray C from grid to cathode is the other half of the V divider. 

## KM1H tells us that the NFB is actually coming from the 25 ohms of DC resistance of the grid 
chokes on a SB-220.   Which is it ?? 

##  the grid chokes in all 4 of my drake L4B amps measure .91 ohm of dc resistance.  Just under ONE ohm..each. 
Bond the grids in any L4B...and toss the grid caps + chokes... and drive requirements drop by 20-25 watts. Any NFB
in the drake amp did not come from the  grid chokes !  

##  with the xcvr  now operating at 20-25 watts LESS  PO, the xcvrs  IMD is way down.   Total system imd  from the
kw amp is also way down because the xcvr is a lot cleaner. 

##  Plenty of both   SB-200  and  SB-220 amps on 6m.....and all stable as a rock.....and these are the ones  with grids
bonded to chassis.   The slight increase in gain  resulting from bonding grids to chassis doesn’t mean the sky is gonna fall in
any time soon.   If the amp in question becomes unstable,  fix it.  Re - design the suppressor if needed, no big issue. 

##  For folks who just have to have NFB in a GG amp.... the correct method is to insert some non inductive resistance between
the OUTPUT of the tuned input..and the cathode.   3-15 ohms will usually suffice.  Beware, the input Z to the cathode will  INCREASE
if you do this...and by an amount equal to the value of cathode resistor used.   Which will of course means the PI net tuned input will have to be 
tweaked, or re-designed.   On any multiband amp.... that becomes a pita. 

##  IMO, GG tube amps  dont  require NFB,  what is needed is a xcvr with cleaner IMD to begin with.   

later........ Jim   VE7RF