I’d like to use a dc to dc step up converter to provide 90 VDC B+ to power a zenith transoceanic tube radio. I’ve tried a number of different converter boards but switching noise can clearly be heard when the radio is turned on. Does Digikey have a filter board that can be connected to the output of the converter that will eliminate the switching noise?
Welcome to the forum.
The old tube radio will only have capacitors suitable for DC power that was generated from low frequency AC. So you may just need to add some high frequency suitable capacitors to the power rail.
I’d try a combination of a 100pF (0.1nF), 0.01uF (10nF), 0.1uF (100nF), and 1uF all rated for high frequency use.
I have not worked with a tube radio in nearly 50 years so maybe somebody who has, like @APDahlen, can provide other suggestions.
1 Like
Sorry mcurtis848,
This is not an easy task, at least not if we consider safety.
The 90 VDC battery was once common for portable radios. I’ve seen many battery eliminators that allow the radio to be powered from the 120 VAC line. Some 90 VDC “solutions” are accidents waiting to happen with nothing more than a diode and a dropping resistor. Others are more reasonable as they include an isolation transformer to provide at least some measure of safety. The hazard comes from the 120 VAC line power and the high likelihood that the radio chassis will be at 120 VAC. This is one of those fork-in-the-toaster situations.
Your AC to DC step up converter is an interesting solution. However, I’m not sure the electrical noise can be eliminated. True, as @PaulHutch suggests, capacitors may be able to clean up the output. However, I wonder if some of the noise is being radiated from the power supply itself. Stated another way, filter capacitors may take care of the noise for the 90 VDC source but they will not eliminate the noise is being radiated by the power supply and picked up by the antenna.
Options:
-
It may help to shield the power supply in a metal enclosure. This may be complicated as we would need to shield the AC input power along with DC output power.
-
Perhaps a better solution is to construct a period correct battery box. I did find one provider that offers a good looking relatively simple solution. They also sell a few of the necessary connectors:
Please let us know if you would like to further explore the shielding solution. On the other hand, kindly post a picture of your radio if you used the period correct battery.
Best Wishes,
APDahlen
Public domain image: Edwin and Esther Armstrong listening to their portable radio at the beach in 1923. Ref: Edwin Armstrong wife and portable superhet radio - Edwin Howard Armstrong - Wikipedia
1 Like
Good morning,
Thanks so much for your reply. I did construct two period correct battery boxes. Both with six D cells for the filament voltage and one with ten 9 volt batteries and the other with sixty AA batteries for the 90 V B+. What I’m attempting to do is build a third box using six D cells for the filament voltage and a DC to DC step up converter for the B+. The DC to DC converter would be powered by fewer batteries than used in a period correct battery box and therefore more convenient and less costly to power the radio. I’ve tried putting a 10uf ceramic capacitor across the output rails of the DC to DC converter then a 300uh toroid inductor on the positive rail and then another 10uf capacitor across the output rails of the converter. I then enclosed everything in a plastic project box covered with several layers of copper foil. This works pretty good but not perfect. Still some high frequency noise on certain frequencies of the AM band of the radio. If I put the wave magnet antenna of the radio right next the copper coated project box, the noise increases. Any thoughts? Thanks so much for any advice you can provide. Have an excellent day!
Regards,
Mark Curtis
Hello Mark,
Now I see! Rather than uses the 9V or the 60 AA batteries, you are using large low voltage battery with a DC-to-DC converter to step up the voltage.
Your comment about “Still some high frequency noise on certain frequencies” tends to support the radiation theory. These are likely harmonics of the DC converter’s switching regulator. Then again, your converter may already be hermetically sealed.
Perhaps using a metal box will help:
I would recommend feedthrough capacitor, but these can be very expensive. They may not be necessary as your capacitors and inductor serve the same purpose. My only advice is to ensure that each wire entering and leaving the device have a solid bond to the box.
Sincerely,
Aaron
P.S. PCB material is an alternative to the box. It’s relatively easy to cut. The sides ends can then be soldered together to make a single electrically isolated enclosure. The only problem is that heat dissipation will not be as good as all metal box.
1 Like
Hi Aaron,
You’ve been very gracious sharing your knowledge. I’ll work on this and let you know how I make out. Have an excellent day! Enjoy the eclipse!
Regards,
Mark
1 Like
Hi Aaron,
What do you mean by wires in and out of the box should have a solid bond to the box? Thanks again for all of your help. Have an excellent day!
Regards,
Mark
Sorry for not being clear.
The grounds should have a solid bond to the box. The positive input and output wires should include a bypass capacitor with one end of the capacitor to the wire and the other with a solid electrical bond to the metal box.
/r
APD
Hi Aaron,
I get it. Thanks again. Enjoy the day!
Regards,
Mark
1 Like