Jim,
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 ,
Rick/K5IZ
Jim candela 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
