Antique Radio B-Battery Power Supply




Introduction: Antique Radio B-Battery Power Supply

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Early vacuum tube (valve) radios ran off of batteries instead of house current. These antique radios required two different batteries for operation, an "A" and a "B" battery. Some models even required a third "C" battery supply.

The "A" battery provided power to the tube filaments (typically 1.5 to 6 volts) and the "B" battery provided the plate voltage to the tubes. Unfortunately, vacuum tubes (with the exception of space charge tubes) require high plate voltages on the order of 90 V or higher to work. Back in the day, B-batteries were common place and relatively cheap. However, these large voltage sizes are no longer readily available, so an alternative is needed.

Typically, radio enthusiasts power their radios using a special power supply that runs on house current. Though they work well, they are expensive to buy (or build) and they don't allow for portability. Because of this, I decided to construct a similar power supply that runs off of a single 9V battery. Not only is it cheaper to build, it is cheap to operate and it allows me to listen to my radios where I like. Of course, I could have just wired up ten 9 V batteries in series, but that would not be so much fun.

Step 1: Gather Materials

  1. A suitable nixie tube power supply (These are relatively cheap and can be bought online. I used a Lumos PS 180) or you can build your own DC boost converter.
  2. One 3.3 k, 1 W resistor (see notes)
  3. Four 330 uF, 250 VDC electrolytic capacitors (required to eliminate zener noise, which sounds like static)
  4. Four 1N4748 or 1N4749 zener diodes
  5. One 9V battery connector / holder
  6. Five #8 - 1/2" brass machine screws
  7. Five #8 brass knurled nuts
  8. Five #8 brass nuts
  9. Five #8 brass washers
  10. Five ring terminals
  11. Suitable wire (AWG 22 or similar)
  12. 3 cm x 7 cm circuit board (or larger)
  13. Suitable case (I used a small wooden box that housed a router bit)
  14. M3 hex motherboard standoffs (6mm high) to mount circuit boards

Step 2: Prepare Box for Mounting Circuit Boards

Circuit Board Layout
  1. Measure the inside dimensions of the box
  2. Layout these dimensions on a piece of 1/8" thick of plywood or other suitable material.
  3. Test fit and adjust size if necessary.
  4. Arrange circuit boards as desired.
  5. Carefully mark hole positions
  6. Drill these out and mount standoffs.

Terminal Layout

  1. On top surface of box, layout five holes (evenly spaced) for the terminals.
  2. Using a centre punch, mark each hole.
  3. Drill holes with a 3/16" drill bit.

Step 3: Solder Components

  1. Using the 3cm x 7cm circuit board, solder in the zener diodes, capacitors and the resistor* following the circuit diagram above.
  2. Solder in wires for the ground and 4 output voltage leads.
  3. Wire this circuit board to the ground and output voltage terminals on the nixie power supply board.
  4. Wire in the 9V battery.
*Note the minimum size of the current limiting resistor (Rs) depends on the following:

  1. The power rating (Pz) of the zener diode(s), in this case 1W.
  2. The input voltage (Vi) (120 VDC).
  3. The total voltage drop (Vz) across all four diodes (for 1N4728 it's 22 V each, for 1N4729 it's 24 V each)

Rs = (Vz/Pz)*(Vi-Vz) = 88(120-88) = 2.8 k.

I used a 3.3 k resistor for a bit of margin.

    Step 4: Mount Circuit Boards in Suitable Case

    1. Mount finished circuit boards (with wiring attached) on plywood mounting prepared in step 2.
    2. Mount component board in box with a suitable wood-screw.
    3. Mark ground and voltage wiring coming from the zener diode board
    4. Attach ring terminals to the ends of the five wires .
    5. Connect these ring terminals (in order of increasing voltage left-to-right) using brass machine screws and hardware.

      Step 5: Test and Enjoy

      1. Double check circuit board wiring before proceeding any further.
      2. Insert 9V battery into power supplyMeasure DC voltage from ground to each of the four terminals. It should read approximately 22, 44, 66 and 88 VDC.
      3. Assuming everything checks out well (and there is no smells of frying components), hook up the power supply to your radio.

      Note this supply can supply multiple voltages at the same time to the radio (such as 45V and 90V as is typical with many 1920's TRF radios).

      Fortunately the plate circuits for most tube radios draw very little current, so you should have no trouble powering radios with up to 5 tubes.

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        11 Discussions


        Question 2 years ago on Step 1

        I need to just have a 90v B supply output. Can I delete three of the capacitors and 3 or 4 of the diodes from the circuit?


        Answer 9 months ago

        You can, but you will probably need one of the capacitors across the output to filter the noise. You can experiment with it.


        1 year ago on Step 5

        By changing two resistors I have been able to change the output range to 75-120 volts which means I can run my TransOceanic directly by using a hefty 9 volt supply (6 D cells?) to power the both the Lumos board and the filament. I changed R4 to 430k and R3 to 4.3k. Waiting on the rest of my parts. I'll keep you posted.


        Reply 9 months ago



        9 months ago on Step 3

        The 3.3 k ohm current limiting resistor is too much resistance. It needs to be 820 to 1000 ohms, rated at least 2 watts, with at least 3/4 inch lead length and mounted to allow free air flow around it
        It would be better to put a 1N4749A zener between the 22 and 45 V taps, with the other 3 zeners 1N4748A.
        The current going through the zener string with no load on the output taps is 30 V divided by the current limiting resistance. The maximum current the zeners will handle is 45 mA, but the Lumos supply will only provide 41 mA at 120 V, so 41 mA is the maximum permissible current through the zeners.
        The minimum current through the zener string with full load on the taps should be 11 mA, to permit good regulation.
        The maximum total current drawn from all four output taps added together needs to allow the 11 mA through the zeners.
        So for example with 820 ohms for the current limit resistor and taking the nominal voltage across the zener string as 90 V, the zener current at no load is 36 mA and the maximum total current drawn from all four output taps added together should be not more than 25 mA.
        The zeners need to have about 3/4 inch lead length on each end, and be mounted to allow free air flow around them.
        The enclosure needs allow free air flow through it.
        RF filtering and shielding will probably need to be applied to the 120 V switching supply that is used in the project.


        Reply 9 months ago

        Thanks for the advice. I threw this together as an experiment to see if it would work, but haven't fine tuned it. I will revisit it and try out your suggestions.


        3 years ago

        Shouldn't the capacitors be between output and ground to filter properly ?

        Yes, plates need only very small current, but filaments need power and I doubt a single battery could supply 5 tubes.


        Reply 3 years ago

        You are correct. I had inadvertently uploaded the wrong schematic. It has been corrected.

        This supply is not for the filaments, it is for the plate voltage only. The filaments can be powered by ordinary batteries (1.5V or 6V).


        3 years ago

        Thanks for sharing this! I have a 1920's radio that I would like to try and restore. I already know that a major problem with old radios is that the capacitors need to be replaced. That said, my radio needs A and B power and I probably would not have attempted to get it working if I had to design my own power supplies.

        DIY Hacks and How Tos

        Nice power supply. Also I really like the look of that housing. You should write up an Instructable on how you made that too.


        Reply 3 years ago

        Ididnt make this particular box (other than replacing the window with some black makrolon. It came with a router bit I bought some years ago. However, I did build similar box jointed boxes for some other projects. I will try and post one of those.