Antique Radio B-Battery Power Supply
DESCRIPTION
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.
- 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.
- One 3.3 k, 1 W resistor (see notes)
- Four 330 uF, 250 VDC electrolytic capacitors (required to eliminate zener noise, which sounds like static)
- Four 1N4748 or 1N4749 zener diodes
- One 9V battery connector / holder
- Five #8 - 1/2" brass machine screws
- Five #8 brass knurled nuts
- Five #8 brass nuts
- Five #8 brass washers
- Five ring terminals
- Suitable wire (AWG 22 or similar)
- 3 cm x 7 cm circuit board (or larger)
- Suitable case (I used a small wooden box that housed a router bit)
- M3 hex motherboard standoffs (6mm high) to mount circuit boards
- Measure the inside dimensions of the box
- Layout these dimensions on a piece of 1/8" thick of plywood or other suitable material.
- Test fit and adjust size if necessary.
- Arrange circuit boards as desired.
- Carefully mark hole positions
- Drill these out and mount standoffs.
Terminal Layout
- On top surface of box, layout five holes (evenly spaced) for the terminals.
- Using a centre punch, mark each hole.
- Drill holes with a 3/16" drill bit.
- Using the 3cm x 7cm circuit board, solder in the zener diodes, capacitors and the resistor* following the circuit diagram above.
- Solder in wires for the ground and 4 output voltage leads.
- Wire this circuit board to the ground and output voltage terminals on the nixie power supply board.
- Wire in the 9V battery.
- The power rating (Pz) of the zener diode(s), in this case 1W.
- The input voltage (Vi) (120 VDC).
- 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.
- Mount finished circuit boards (with wiring attached) on plywood mounting prepared in step 2.
- Mount component board in box with a suitable wood-screw.
- Mark ground and voltage wiring coming from the zener diode board
- Attach ring terminals to the ends of the five wires .
- Connect these ring terminals (in order of increasing voltage left-to-right) using brass machine screws and hardware.
- Double check circuit board wiring before proceeding any further.
- 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.
- 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.