Date: Tue, 27 Mar 2012 22:20:41 -0400
From: "Gary Schafer" <garyschafer@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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|