Battery-powered radios used during the 1920s required up to 5 DC voltages to operate:
+4V |
heating of tube filaments |
-12V |
grid bias (variable from 0 to -12V) |
+40V |
power supply for screen grids of tetrodes and power supply for plates of low-voltage tubes (A441N) |
+80V |
HT plate power supply |
+120V |
loudspeaker excitation |
To power a 7-tube radio with a margin of available power, the following currents must be supplied:
| 4V | -12 to 0V | 40V | 80V | 120V |
| 1A | 0 | 5 mA | 50 mA | 20 mA |
Solutions
4V : This is the critical voltage. Exceeding it risks burning out these rare and expensive tubes, so regulation via an LM317T will be chosen. A filtering capacitor of 4700µF will give 2V of residual voltage, so the peak rectified voltage must be at least 7V. The secondary of the step-down transformer must be at least 7/√2 or 6Veff minimum.
-12V : This source does not supply current. A half-wave rectification followed by regulation by a zener diode will be sufficient. The zener will consume ≈ 2 mA.
40V : This source will be derived from the 80V via a zener diode (~ 2 mA).
120V : This source requires neither regulation
nor significant filtering. It will power the 80V regulation. The RMS voltage
on the secondary of the step-down transformer should be around 85V.
80V : This 'High Voltage' must be filtered and
stabilized. A ballast circuit based on a power transistor (or MOSFET)
will therefore do the trick.
A 25VA transformer must therefore supply 85V/0.1A — 8V/1.5A —
12V/5mA. Small independent transformers can also be used.
HT Sources
4V Source
Negative Bias Source
Practical Implementation
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For the record, I used my spare parts as follows:
- The transformer is a recovery from a Jouef transformer. The iron core section corresponded to a 40VA transformer and its no-load output voltage was 18V. It could supply 2A. I then unwound the secondary winding, counting the turns to determine the number of turns/volt. Here 7 Turns/volt. I then rewound 3 secondaries to obtain 85V (595 turns of 0.2mm diameter wire), 12V (84 turns of 0.2 mm wire) and 8V (56 turns of 0.8 mm wire).
- In the photograph above, the NPN transistor and the LM317T are visible, both in TO220 packages mounted on a heatsink.
- The 12V zener dissipates less than 25mW.
- The 40V and 80V zeners each dissipate less than 1/4W. They can be composed of zeners in series.
