Reconstruction of the tube pre-stage by using low voltage difference power supply

Reconstruction of the tube pre-stage by using low voltage difference power supply

A tube preamplifier purchased by a friend two years ago has a lot of AC noise recently. Please help me take a look. The amplifier circuit of this machine is composed of 6N11SRPP+6N8P cathode output; the high voltage power supply is 220V full-wave rectified and filtered by RC to get 260V high voltage: the filament voltage is AC 6.3V and supplied through a balance potentiometer.

After careful inspection, no obvious damage was found. Three electron tubes were replaced. The fault still exists. Based on experience, it is judged that the fault should be caused by the power supply.

Use a multimeter to check the high voltage capacitor. There is a slight leakage phenomenon. The high voltage and large capacity electrolytic capacitors on the market are not only expensive, but also of poor quality.

In order to completely eliminate this fault, a low voltage dropout power supply was made, as shown in the attached figure.

The high voltage 220V is rectified by RL206 full wave, filtered by RC, and then output by active filtering. Adjust the resistance value of R to make the output voltage 260V. Do not use a high power filter tube, because the Hfe of high voltage and high power tubes is very small. In order to ensure that there is a voltage drop of about 8V on the filter tube (the filtering effect is best at this time). Use a medium power color TV tube 2SC237l. Hfe=160, bias resistor 100kΩ. At this time, the high voltage delay is about 1 minute. Use a small power tube KSP44 (500V/300mA/625mW) with a small tube voltage drop, Hfe is also 160, and test the machine after a long time of power on.

It proves that it is also very safe to use a low-power tube. The relationship between the tube voltage drop of the filter tube and the bias resistor is U=0.6+R×load current/Hfe. The tube voltage drop cannot be too large, otherwise the power consumption will increase. It cannot be less than 2V, so as not to make the tube enter a saturated state and lose the filtering effect. This circuit uses active filtering. Therefore, the capacity of the high-voltage capacitor does not need to be too large, and the quality requirements of the capacitor are not strict.

Considering safety and noise, the filament voltage was also changed to a voltage regulator. The original filament voltage was measured with a load and it was AC 6.4V. After rectification, this voltage is only about 8V. How can it be stabilized to 6.3V? The ordinary three-terminal regulator does not meet the requirements in terms of voltage difference or current. After testing, the low voltage difference voltage regulator circuit composed of VMOS tube 40N03 and TIA31 was finally adopted. The minimum voltage difference of this circuit can reach 0.2V after testing. In order to reduce voltage loss, three Schottky half-bridge S20C40s are used to form a bridge stack. The voltage drop is less than 0.5V. In addition, there is a set of voltage doubler rectifier circuits that output 14V to provide bias for TL33l and 40N03 to ensure that TLA31 has a cathode current greater than 1mA and 40N03 has a Vgs voltage greater than 3V. The output voltage of this circuit is 2.5x (1/4.7/3.1) = 6.3V. The upper voltage divider resistor of TLA31 is connected in parallel with a 220μF capacitor. This capacitor and the lower resistor form a 5-second filament delay circuit. It can be said to kill two birds with one stone. Since the filament has a suspended high voltage to the ground, this filament circuit is strictly prohibited from being grounded.

The circuits are all soldered on a 4cmx6cm board. S20C40 and 40N03 share a small heat sink. The circuit board is fixed in the empty space of the chassis and connected to the main circuit board and transformer one by one. After everything is normal, the power amplifier is connected, and the AC sound disappears, even the original background noise no longer exists. The transformation is very successful. This power supply can be applied to all AC-powered tube fronts with just a slight change.