< As a retired broadcast engineer, if it arcs, isolating the choke is
classified
as <NOT a safe practice, the safe procedure is to move the choke to the
negative output <of the rectifier, between the two capacitors if it is a π
network. ("Pi" if the <server and or your E-mail client doesn't support
symbols)
<--Ron, W4BIN
## Zero issues floating any Choke from the chassis.
Just treat it like a string of lytics or anything else with HV on it.
On that 10 kw 6M amp, it uses 2 x 60uf @ 4 kv oil caps in series. When
putting oil
caps in series, you have to float them from each other, AND the chassis. Back in
the 90's, I built a 6800 vdc supply using a 10 kva pole pig (4800 vac
secondary).
It used 4 x CDE, 48 uf @ 4.5 kv rated oil caps in series parallel.
Again, the 4 x caps
were isolated from each other...and chassis.
## On my latest HV project for myself, I use a C-L-C filter setup, with the
127 lb Dahl 4H choke (3 amps CCS) in the B+. The choke uses the same pair of 10
kva hypersil cores as the Dahl 10 kva plate xfmr's. It's rated for (I think)
10+ kv. It draws no leakage current, with my hi-pot tester maxed out to 15 kv.
## Any C-L-C is NOT a choke input. It's a simple C filter, followed by a LC
section.Sometimes called a PI filter. In software, it's modeled as a C filter
...followed by a LC section.
## If you wanted to install a resonant choke in the B-, the single oil
cap is wired
between the B+ and the outboard end of the choke.
## On my C-L-C setup, the 127 lb choke sits on a pair of 17" long, 4" wide
inverted channel aluminum pieces, butted up tight. At the far ends of each
channel is the 2" diam x 2.5" tall 'glastic' insulators, 4 in total. They are
just red HV micarta, with 3/8" steel embedded threads on each end.
Bottom end of
each glastic is bolted to the stacked 3/4" plywood. (1.5" total
thickness to take
the weight. Choke sits at the bottom of the new 24" x 24" rack....
with a mess of
series 10,000 uf lytics above the choke...and wired with caps on both sides of
the choke. Floating the choke above chassis relieves the stress on the choke.
Borrowed a 30 kv hi-pot tester, and it still reads 0 ua. I'll see if I can
borrow the 100 kv hi-pot tester from the electrical dept, where I used
to work for
the telco.
## installed a HB adjustable spark gap across the choke terminals. Just a pair
of 1/4-20 solid brass machine bolts, with solid brass lockdown nuts..to set
the gap. Solid brass acorn nuts, with their domed heads face each
other, where the
gap resides. If domed acorn nuts get trashed, easily replaced.
Putting a spark gap
across a choke is an old Harris trick. Under certain conditions, the peak V is
through the roof, like with any B+ fault. I can see why chokes can
bite the dust.
I modeled all of this in software before building it, and also verified the
software, once built. The spark gap is cheap insurance.
## Ok, here's the real problem with any choke, resonant or not... in
series with
the B-. When current drawn through the choke, you get a V drop
across the choke.
ANY V drop in the B- is bad news. It just adds extra, 'yo-yoing' bias to the
existing bias, in any GG triode amp. Typ B- setup in any GG amp is.... B-
wired to the negative terminal of the plate current meter. Positive terminal of
plate current meter wired to positive terminal of grid current meter. Neg of
grid meter wired to chassis.
## At the junction point of the 2 x meters...it's then wired up to the
bias cutoff
resistor (relay contacts across it)..then through either a zener bias
diode, or a
series string of diodes..then to the CT of the fil xfmr.
## My 4H dahl has 9.56 ohms across the choke terminals....when choke
is cold,and
rises to 11 ohms when hot. With 3 amps CCS flowing, the choke is at room
ambient temp. It gets pretty warm..and hot where the gap is, with 6 amps CCS.
With normal 1.5 to 2.5 amps flowing, the V drop across the choke is
15 V to 25V.
And 30 vdc with 3 amps flowing.
## That V drop just reduces the B+ by 15-30 vdc..if wired in series
with the B+.
IF the choke is instead wired in series with the B-, all hell breaks
loose, with an extra, fluctuating 15-30 vdc of bias, on
additional 15-30 vdc of extra, unwanted, yo-yoing bias. That just
screwed up the
load line on the high mu triode.
## don't believe me, try it urself.
Jim VE7RF
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