My SBR ONDOLINA 1926 radio: one power supply.

4V / 1A output for tube heating, 40/80/120V output for anode voltages and then -12V for negative bias. All of this voltages are not needed for the Ondolina, which needs only 120V and 4V. There is a 4.5V battery inside for negative grid voltages. But this power supply will be used for other radios, so I made it with enough possibilities. Download the schematic here , GIF 21ko file.

How it works:

The TR1 and TR2 transformers primary side are protected by F1 and F2 fuses. The NE1 indicator shows the mains presence. The fuse and neon lamp assembly are placed on a small independant PCB, for security.

The TR1 secondary voltage is rectified and smoothed by B1 and C1. IC1, with R2 and R3, regulates the output voltage, adjustable by R3. The C3 capacitor makes a small time constant when switching on, this way the output voltage slowly increases until 4V. This can avoid an electric schock to the tube heaters. C2 and C4 are additonnal smoothing caps, avoiding also IC1 going to oscillation. LD1 indicates the voltage on the PCB. All the system build with D1, T1, R5, R6, and C5 is an overvoltage protection. If the output voltage increases, the current flowing into the D1 zener (by means of R5 and the T1 gate-cathode junction) also increases. When reaching the triggering current of T1, this will become conductor, creating a short-cut on the output (crowbar system). As long as the F3 fuse will not blow, the T1 SCR will stay in conduction mode. Afer one moment, F3 will blow. C5 capacitor avoids the SCR to become conductor when switching on the system. With this values, the protection activates around 4,1V. This is depending of the components tolerances. Why this system ? I already experienced LM317 regulators wearing out, and going to shortcut between input and output. If this happens, we will have around 10V at the 4V output. The old tubes heaters will blow out... And this kind of tubes are rare, and expensive.

The 80V voltage coming from TR2 is rectified and smoothed by B2 and C7. We can find around 120V at this capacitor. R7 is used to discharge C7 when switching off. T2, R8 and C8 creates a slow increasing of the voltage when switching on. Possible to increase the time by increasing the value of C8. D6 is used to protect the components in case of accident, i.e. connecting a few outputs together. R9, D2, D3, D4, and T3 are a voltage regulator for the +80V output. R10 and R11 are a voltage divisor, to obtain a +40V output. Be careful, all the capacitors of this section must be 200V minimum. Better also in the 80 and 40V sections. If a component goes out (T3 or R11), it's better to avoid overvoltage on one of this caps... and avoid the capacitor explosion. The price difference is not huge.

The last winding of TR2 is used to create a negative voltage. A 5k potmeter can be connected to the output to adjust the voltage.

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