03 March 2017

Reading Schematic

Let's have a look at the schematic



High Tension(HT) Power Supply

This is the power supply circuit. Boxed with blue box is the Main Transformer. On the left side, It was connected to the wall socket (In Malaysia 240v 50Hz) with SW-1 on/off switch and fuse. On the right side, at the top winding is 5vac which connected to rectifier filament. The second winding is 400-0-400vac, connected to the 5V4G rectifier tube which form a full wave rectification. The last winding 6.3vac connected to driver and power tube (6SJ7, 6N7, 6AS7G) filaments.

The second section which boxed with yellow is the LC filter, they are 7H 140mA choke and 40uf 450v capacitor for smoothing the voltage.The first resister after the filter is the 30K 10w bleeding resistor. It use to drain the voltage after the amplifier was switched off. The last one will be a RC filter used to filter and reduce the voltage for the driver.





Amplifier Circuit

At here, It is a long story to explain how it work. I will tell you what is the name of the components in a tube amplifier so if there a problem you can always discuss in the forum.

At the input, this is where the signal (audio) was injected to the amplifier. Follow by a 500K volume pot (variable resister). Usually after the volume pot we put a grid leak resistor around 470K ohm to 1M ohm. In this case, the volume will act as grid leak resistor. And after the grid leak resistor some circuit will have another resistor in series with the screen call grid stopper resister.

On top of 6SJ7 is the 100K plate resister. The one which connected at the 6SJ7 cathode is the 5K6 cathode bias resister. Feedback resister 270K also connected to the 6SJ7 cathode. Coupling capacitor are those with 100nf and 2uf, no voltage are show, I will make assumption from the power of the main transformer which is around 400v.

The 100uf at the 6N7 and 40uf at 6AS7G are cathode bias capacitor, no voltage are show either. The easy way is to refer to other schematic who use the same tube or just put in a 100v capacitor. The real way is to measure the cathode bias voltage and put a capacitor which is higher then the cathode bias voltage.

This circuit use two transformer, inter-stage with 10K to 80K ohm impedance and a push-pull output transformer of 3K to 8 ohm impedance. 8 ohm is not show on the circuit because it is flexible, depend of what speaker you want to use. It can be 4 ohm, 6 ohm, 8 ohm or 16 ohm.

B+1 and B+2 are where you get your power supply from the HT Power circuit. The small triangle thing below the circuit is the grounding. The amplifier circuit only show one channel, which mean you have to repeat the circuit for the other channel. Basically we don`t repeat the Power circuit unless you want to build two mono amp. The tube pins connection also not shown. So you have to refer to the tube datasheet for pins connection.



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