[AMRadio] Smoothing Choke

Jim candela jcandela at prodigy.net
Thu May 4 11:34:55 EDT 2006


   I understand your feel for the nostalgia, and passion for pursuing it.
All of us more or less on this group have some common ground here.

I'd like to relate a story however to make a point. I have an RCA AR-88
receiver. This 'heavy' boat anchor has multiple chokes, and oil capacitors
in the 250 volt power supply. I don't recall the values, or whether the
first choke was swinging or not, but I do recall the problem. I started a
project to beef up the audio from the receiver. This included extending the
LF response to 50 HZ, and multiple negative feedback paths around the audio
output tube (6K6?). What happened really surprised me. The power available
below 300 hz steadily dropped off, but much steeper than I anticipated. It
turned out that I had over 50 volts P-P ripple on the B+ whenever I cranked
the audio up when listening to AM broadcast 'male' voices. I looked at the
B+ on a scope, and I'll never forget what I saw. The waveform was going up,
and down with the audio, and sometimes exploding in amplitude when a certain
audio pitch was being sent to my loudspeaker. It was kind of like a
automobile driving over speed bumps, and not having any shock absorbers!

The problem was filter resonance excited by the audio rate draw by the audio
output tube. In the end, a single 100 uf 450V capacitor across the B+ cured
the problem, and I had nice clean bass down to 50 hz as desired.

As far as resonating a smoothing choke, this is not new stuff, so you might
keep that option on the table. At the following link I have an example of
where I did this with good success. The supply was already choke input, but
the critical inductance was not met, so the regulation was poor, and the
ripple was higher than I wanted. By simply resonating the choke, both
problems were solved. Here is the link:



-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Rick Brashear
Sent: Thursday, May 04, 2006 9:58 AM
To: Discussion of AM Radio
Subject: Re: [AMRadio] Smoothing Choke

    I appreciate your take on the power supply and agree for the most
part.  Even though this particular ART-13 will see little if any CW
operation I would like the supply to be as stable and clean as
reasonably possible.  You are absolutely correct about the difference of
availability of capacitors and chokes in today's world versus the days
of W.W.II.  I considered using more modern components and at one time
even thought about an all solid state power supply.  It would be much
easier and cheaper to construct and less expensive to operate with less
heat.  However, the lure of the olden days kept calling me back to
glowing tubes and swinging chokes.  So, even though I agree that with
modern technology applied I could have a cleaner, lighter and less
expensive supply, I will likely stay with the old school thinking on
this one.  That is IF I can get the ripple to a low enough level so as
not to cause problems on the transmitted signal.
    Thanks again for your advice and explanation, you make a very valid
point and one which would serve my project well.

Jim candela wrote:

>   Sorry but I am going to throw a wrench into this discussion. Those
>swinging choke power supplies, although useful, have been out of favor for

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