[AMRadio] Smoothing Choke & RX filter audio resonance


Mark Foltarz Foltarz at rocketmail.com
Thu May 4 14:06:07 EDT 2006


Jim,

   You may have just provided me an answer to an issue I have with a BC779 !

 de KA4JVY
 Mark


--- Jim candela <jcandela at prodigy.net> wrote:

> 
> Rick,
> 
>    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:
> 
> http://pages.prodigy.net/jcandela/CE20AQRO/PS.JPG
> 
> Regards,
> Jim
> JKO
> 
> -----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
> 
> 
> Jim,
>     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.
> 73,
> Rick/K5IZ
> 
> 
> 
> Jim candela wrote:
> 
> >Rick,
> >
> >   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
> >years...
> >
> >
> >
> 
> 
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