[Heathkit] HX-11 Question
sbjohnston at aol.com
sbjohnston at aol.com
Wed Feb 3 09:26:32 EST 2010
I've seen several supplies with a cap across the filter choke (for
example my old WRL Galaxy 300) , and I assumed it was to resonate the
filter choke on some useful frequency, perhaps the ripple frequency of
the DC from the rectifier - 60 or 120 Hz.
Let's see if I can find any references online...
In an old posting on the AMRADIO mailing list, Geoff/W5OMR
quoted from the book "SSB Circuits and Systems" (1964):
"The shunt capacitor value is selected to tune the choke at the ripple
freq
and with the load consisting of the highest value of Reff which will be
encountered. Filter choke inductance normally decreases to some extent
as
the direct current through it increases. The effective L at low load
currents may be twice as much as the full-load rating. This is partially
due to the inductance increase caused by the large AC voltage across it.
When the load is increased and Reff is reduced, the choke may change
inductance enough to detune the circuit, but the critical inductance
needed
is then lower, so regulation does not suffer. Proper tuning can permit a
several-times reduction in filter choke size...
Tuning the input choke will reduce the fundamental ripple component on
the order of 10 dB. The atten. of harmonics of the ripple freq. is less
with a tuned input choke however. The reduction is limited by the
voltage
division of the tuning capacitance and the filter capacitance. With 0.1
uf
and 8 uf, this amounts to 38 dB. THe peak voltage of the second
harmonic of
the ripple frequency is down 17.5 dB from the dc voltage. The ripple
second
harmonic component, therefore will be down approx. 55 dB."
Webpage http://www.webace.com.au/~electron/tubes/power.html states
"The value of this capacitor should be fine-tuned to create a tuned
shunt filter. The values of the choke and capacitor will determine the
tuned frequency, ideally being the fundamental or first or second
harmonic of the rectified mains frequency. This simple design
modification will also improve the power factor of the first choke
stage, thereby delivering more usable power to the output tubes. The
optimum size of capacitor should restore the power factor to 1."
Steve WD8DAS
sbjohnston at aol.com
http://www.wd8das.net/
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-----Original Message-----
From: Bill Stewart <cwopr at embarqmail.com>
To: heathkit <heathkit at mailman.qth.net>
Sent: Wed, Feb 3, 2010 7:23 am
Subject: [Heathkit] HX-11 Question
The HX-11 has a .1uf/1200vdc polarized capacitor (C26) across the
filter choke.
What is its purpose and any problem if taken out? This capacitor in
mine has the
ends puffing out some and may be alright. However, I would like to
replace it
(or remove if ok). I did replace the 'lytics. Anybody got a good one
laying
around?
Tnx for any info..Bill K4JYS
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