Date: Fri, 15 Dec 2017 10:12:23 -0800
From: "Jim Thomson" <jim.thom@telus.net>
## The ripple current ratings on these modern day electrolytics is
a quantum improvement from years gone by. Per the various tech notes from
hammond, sprague, CDE, united chemi-con, etc, the consensus is the CCS
DC load current should be no more than the... CCS ripple current rating
divided
by 2.56
Agreed.
## On paper, a string of lytics with a 10A CCS ripple current rating should
be good for
a max of 3.90 A CCS plate current. These days, you can get ripple current
ratings a lot
higher than 10A, in a high C, 450 or 500 vdc lytic. You can also parallel a
2nd or 3rd string
for more CCS ripple current. For something like a FM broadcast TX, running
24 hrs, 365 days
per year, you would want it ultra conservative. The tech notes depict
formulae for determining
how many years to expect from modern day electrolytics. CCS current drawn,
ambient air temp
surrounding the circumference of the lytics, loaded B+..as a percentage of the
max 450 vdc rating,
EQ resistor value, internal leakage current, cap value in UF, etc, etc, is
about all you require to come up with a good
estimation of electrolytic life. If you make sure the ambient temp inside the
HV cabinet is low, like
the same as the ambient room temp, and also run the lytics at only 70-75 % of
their max stamped V rating,
and dont max out the ripple current, they will last a long time, like > 15
years.
## I remember you mentioned that the resonant choke would not filter out the
harmonics of the rectification
freq...and that additional LC sections were required. It was to do with
the..... AM noise spec on a FM broadcast TX.
The 240-360-480- 600 etc, etc, had to be reduced to an acceptable min level.
Yes, AM noise on analog FM signal will be evident when there is weak
noisy reception, when the receivers of the day were not fully limiting.
It would sound like a faint audio note, in the background. Lots of
harmonics in it, like a Hammond organ.
## A simple high C filter will kill the fundamental..and also all the
harmonics. The last B+ supply I had used 4
CDE oil caps in series parallel. Each was stamped 47.6 uf at 4.5 kvdc..and
each weighed 30 lbs,so 120 lbs for the 4 x oil caps.
So ended up with 47 uf at 9 kv. The 4.8 kv sec of the 10 kva hypersil pole pig
and FWB, netted +6700 vdc no load.
We paralleled an additional 100 uf via a string of 20 x 2000 uf lytics. Its
now being used on a hb 80-10m YC-156.
The biggest capacitor we have (at work) is 230 uF, running at 23 kV. It
used to be 30 kV but I eliminated a series tube modulator and now the
tetrodes run continuous DC on the plate, and use grid pulsing. They are
linear grounded grid amplifiers. In the main operation for the particle
factory, the capacitor banks are in a steel room, and we have ~50 each
at 4.6 uF oil filled GE caps, for each transmitter. In the test set we
run-in new tubes and test coaxial components like loads and combiners,
There we have 18 each of 12 uF made by Maxwell. Right now we are testing
a new solid state crowbar system there, as don't like depending on
mercury ignitron tubes to short out the stored energy during an
overcurrent fault. We can drop a 31 AWG magnet wire about 2 feet long,
across the output terminals of the capacitor bank, and it will survive.
This is whats required with 61 kJ of stored energy, to prevent blowing
out tubes if there is a burp inside. There are a few ohms of
non-inductive series R between the tubes and the capacitors too. That
resistor is very large and air-cooled as the peak plate current is as
high as 250 Amps during the pulses.
No chokes are used in the pulsed high power amplifier filters.
Resonances between chokes and capacitors would cause mayhem when the
pulse repetition rate was varied over 20-120 pulses per second. This is
similar to the bounce that is evident on old L-C filters when used with
varying load modulation like SSB.
## Some outfit in Halifax, in eastern Canada makes a 53 kw FM TX, consisting
of 55 x 1 kw SS modules and a
myriad of combiners. They can be configured from anything from 1 kw to 11 kw in
a bay, and multiple bays for
a high powered unit. One RF module craps out, the remaining RF decks take up
the slack, and po divides equally.
RF modules and mating switchers can also be pulled hot.
Tubes are history for FM broadcast, except for legacy equipment. AM broadcast
is barely alive in Canada, they
all moved to the FM band years ago. We only have 1 AM station left here on
vancouver island...well established
talk radio format.
Its Nautel. They make some very decent FM and AM rigs, all solid state.
They are practically the leader worldwide now, although Gates Air and
others are still quite competitive.
73
John
K5PRO
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