DIY: Poor people's cheap tube amplifier is also wonderful

I originally planned to make a 6SN7 two-stage tube for FD422. I checked the price online and found that it was 45 yuan per tube and it was made in China. It was almost equivalent to my pair of FD422. I had no choice but to look for other cheap tubes.

I actually found some good quality and cheap tubes. The tubes are cheaper, but the difficulty of making them is increased. It's okay, we don't have rice, but we have technology!

For voltage amplification, we use the cheapest 2J27S on the market, which costs only 2 yuan per piece; the price of the driver stage is also quite low, 2P3 only costs 4 yuan per piece; for power amplification, we use the outstanding FD-422, which costs only 25 yuan per piece; for the rectifier tube, we chose the 5Z9P180mA rectifier current, which is acceptable at 15 yuan per piece.

All these together cost only 70 bucks. Although cheap, the sound is very nice! If you don't believe me, you can try making one. I guarantee you won't regret it.

Here in the circuit diagram, the fixed gate voltage is easy to adjust. You can listen while adjusting it, and the voltage value may change.

Sound performance:

The sound is clear, transparent and spotless, especially the mid-frequency and low-frequency are not turbid at all. It is a real pleasure to pair it with the box from the forum!

The maximum residual noise is 2.5mV, and the minimum is also 2.5mV (4 ohms). You have to put your ear close to the speaker to hear a little bit of the 88BD speaker. Unfortunately, my choke coil, which was modified with a 35W transformer, can only last for less than 3H.

I dare not compare it with others, but compared with my own 6P3P, 6P3P is far behind!

This curve is a reference for selecting the working point. Impedance = (580-210) (/110-21) = 4.1

During actual debugging, it was found that the actual internal resistance of FD422 is larger than the internal resistance given in the figure. If you also want to do it, it is recommended to use a 5K output transformer.

The case was modified by myself from an old iron box, and the power transformer and output transformer were all made from unused components. Most of the resistors and capacitors were disassembled.

The iron case costs about 160 yuan each, and the output transformer of about 15W is about 300 to 400 yuan. Second-hand ones may be cheaper. The power transformer is calculated at 1 yuan per watt, which is about 200 yuan. The choke can be wound by yourself or modified with a transformer.

I can only say that this set of matching is very good and happens to be cheap, rather than deliberately pursuing cheap tubes, it is mainly matched with the lines.