[Amps] question for you experts...

[Jim Thomson]

Date: Tue, 27 Mar 2012 22:20:41 -0400
From: "Gary Schafer" <garyschafer at comcast.net>
Subject: Re: [Amps] question for you experts...

Well Carl, which is it? You seem to be waffling both ways.

The only "safety benefit" that the choke offers is it keeps the DC voltage
at zero on the output terminal of the amplifier so you won't get zapped if
you should touch the antenna connector or an antenna connected that does not
have DC continuity in normal operation.

73
Gary K4FMX

##  I have seen and heard those so called mickey mouse 1- 2.5 mh chokes explode,
in cases where a plate block cap actually did short out.   I even measured the dc resistance
of some of them..and quite a few were as high as 40 ohms.     Then I measured the rf chokes
I pulled from my drake L4B amps, use   from grid pins to chassis.    Those drake pi wound chokes are
only .916 ohm..go figure.    And I have  pulled a total of 8 chokes...from 4 x drake amps now. 
(all 3 x grid pins bonded to chassis with cu strap etc).

##  On my hb amps,  I use a hb safety choke, wound with 18-22 ga magnet wire, on a 1” solid teflon
rod, with the base  drilled and tapped  for a .25-20   or  .25-28  SS bolt.  Typ value  for the safety choke
is 45 uh.   You DON’T  require 2.5 MH nonsense.   That’s just more arrl fubar.   Some bozo plucked the 1st
thing he had in his junk box... and everybody copied it. 

## the 2nd benefit is the 45 uh safety choke will DC grnd  BOTH tune + load caps   + the tank coil (and the
L coil if a PI-L is used)...  + the bandswitch itself. 

##  OK, here’s the kicker.   The safety choke  will NOT  grnd the hot side of the coax on RX..... ONLY  on TX. 
We had a case at the VE7ZZZ  MM contest site years ago, on 160M.. the nosie level would slowly build up on RX
to something just wicked.  Hit the PTT  just once..and RX noise dropped like a rock.  Then  it would start building
up..yet again.   Hit the ptt, and poof, no more nosie... the cycle would just  repeat.    On TX, the safety choke is  no dc
grnding the hot side of the coax. 

##  OK, here’s the real fix.    Install a 2nd 45 uh safety choke..and wire it between the hot side of EITHER  coax connector
and chassis.  Then the 2nd choke is ALWAYS in the circuit on RX..and also TX.     Meanwhile  safety choke #1’s  real job is
to short out the B+..in the event a plate block cap fails shorted.   u still  need BOTH chokes to do the job right.   The 2nd choke
will not help you on RX..... if the plate block cap fails. 

##  another superb  idea for a safety choke is a torroid, aprx 45-70 uh..wound with 12-16 ga magnet wire.    ICE  uses a 35 uh
torroid, wound with 12ga magnet wire inside their  lightening arrestors.   That puts the center conductor of the ant coax at dc grnd.
That ICE choke is also directly in parallel with the safety choke across the load cap..... but only on TX.   

##  so u can either add the 2nd safety choke internal to the amp, between hot side of either coax connector ( they are in parallel
on RX anyway...IE: bypass mode)   or  add it external.    or do both and have a total of 3 x chokes.  The last thing you want is
2.5Mh oversized choke..with a 20-40 dc ohm winding..rated for 100ma.    If a plate block cap ever let go, the fubar safety choke will
literally explode.   been there, done that. 

##  If u look at any commercial 5-20 kw HF amp.. you will see they all use a 45 uh safety choke, wound solenoid style,  and typ
18-22 ga.   Some will use a 40-50 uh torroid..and  12-16 ga magnet wire.   And all of this is across the load cap.     You still 
need the 2nd choke to get rid of rx noise.    you can also use a 3 meg resistor instead.. for a dc bleed off. 

Later........ Jim   VE7RF