Using an old tube radio to make a 6N1+6P14 tube amplifier

I bought a Chunfeng SC-1 6-light dual-band tube radio from a scrap collection station, and I was overjoyed. I had always wanted to have a tube amplifier to listen to the radio in my bedroom, and with this radio, I didn't have to spend too much effort, just repair and modify it.

The machine has been stored for decades and looks very worn out. First, wipe off the dust, pull out the movement, and clean it with alcohol. The circuits and components are not too old. Then check the components, circuits, and solder joints, and no problems are found. This is a single-ended Class A small tube amplifier, using 1/2 of 6N1 for voltage amplification and the other 1/2 for driving. The power amplifier tube is 6P14, with an output power of 5.4w, and high and low tone control, which is very suitable for room audio.

After the machine was powered on for testing, the brightness of the filaments of each tube was normal, there was no sparking phenomenon, the temperature of the power transformer was normal, and there was no sound. In order to facilitate the inspection, all the circuits in the front part were disconnected and removed from the detector. The circuit diagram was drawn according to the actual object (as shown in the figure). The R and c numbers in the figure were compiled by the author, and the parameters of R and c were originally marked.

Because there is no sound, the first step is to check the speakers. The battery test has a gurgling sound, indicating that the two speakers are good. The second step is to measure the voltage at various locations. The voltage of each filament is normal, the three DC high-voltage outputs are normal, the two screen voltages of 6N1 are normal, and the screen grid and grid negative voltage of 6P14 are normal. Only the screen high voltage of 6P14 is abnormal, which must be the output transformer coil is broken. Remove the output transformer and measure the primary coil. Sure enough, it is not connected. What should I do?

In order to test the power amplifier, I found a small transformer with a primary of 220V and a secondary of 6v to replace the output transformer. I connected the coil and used a screwdriver to hit the input terminal CD. There was a clang sound in the speaker, indicating that it has amplification capability.

It is inevitable to make an output transformer. The original output transformer is relatively small, with a cross-sectional area of ​​only 14mm×18mm. A bad inter-line transformer used in broadcasting, with a tongue width of 38mm, a fold of 200mm, and a thickness of 20mm, is used to make the output transformer.

In the production of tube amplifiers, you have to work hard on the output transformer to get a low enough bass component. The output transformer is required to have a large enough inductance, and small enough distributed capacitance and leakage inductance to achieve a good playback effect.

First, make the skeleton of the output transformer coil and divide it into five sections. The primary and secondary coils form a 3+2 structure. The primary coil occupies three slots, and φ0.14mm enameled wire is used to randomly wind 1000 turns. The three coils are connected in series head to tail for a total of 3000 turns. The secondary coil occupies two slots, and φ0.68mm enameled wire is used to randomly wind 60 turns. The two coils are connected in series head to tail for a total of 120 turns. The secondary impedance is 8Ω. After the insulating paint is dipped and dried, the wires are connected and tested. Use SONYSRF-H35 FM stereo radio as the sound source, tune to 89.8MHz Baoding FM station, listen to the radio and songs, and the sound is very strong. Then adjust the bass control, the sound is soft, sweet, clear and pleasant. The only drawback is that there is a little AC noise and rustling noise when the bass is increased.