[AMRadio] Smoothing Choke

Rick Brashear rickbras at airmail.net
Thu May 4 12:03:08 EDT 2006

    That is very interesting and I may very well have to incorporate 
such a scheme to achieve my needs.  Needless to say, I want the cleanest 
and most stable source I can get.  These old gals certainly weren't made 
with fidelity in mind and that is one aspect on which I'd like to 
improve, so it may be that I'll need to employ your tactics.
    I appreciate your comments, link and time.  Hopefully, using a 
little of the new in with the old will satisfy my desire to keep it true 
to the period and have a good sounding signal to boot.

73 ,

Jim candela wrote:

>   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:

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