Date: Wed, 19 Jun 2013 18:36:23 -0400
From: "Carl" <km1h@jeremy.mv.com>
To: "Jim W7RY" <w7ry@centurytel.net>, <amps@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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|