The production of 6P14 small tube amplifier

With the digitization of broadcasting equipment, many electronic tubes in stock have lost their use, especially small and medium power electronic tubes, which are still in large quantities. It would be a pity to discard these electronic tubes, so why not use them to make a low-power electronic tube monitoring amplifier. The electronic tube amplifier introduced below is made of the most common electronic tubes. Its circuit diagram is shown in the figure: The front stage uses 6N2 to connect to the SRPP circuit, that is, "current adjustment push-pull circuit" also known as "single-ended parallel push-pull circuit:" This circuit has high input impedance, low output impedance, wide frequency band, low distortion, and excellent overall performance. The rear stage 6P14 is connected to a standard pentode Class A single tube output. The stability and efficiency are very high. And it is a Class A single tube output, with perfect sound quality.

The tube amplifier circuit is simple, does not require a printed circuit board, and uses "scaffolding" type welding.

The chassis is made of aluminum alloy square tubes with a cross section of 10×4.5 cm as the base of the whole machine, and holes and grooves are drilled and grooved on it. After spray painting, the tube holder and power transformer are installed. The output transformer is installed inside the base. The front and back panels of the chassis are made of floor blocks. The appearance of this machine is shown in the figure (omitted).

During the specific production process, we must pay attention to:

First: The layout of components should be optimized with the shortest routing, minimum interference, and strict symmetry of the two channels.

Second: The power transformer should be as far away from the pre-amplifier as possible, and the installation angle should be perpendicular to the output transformer to reduce electromagnetic induction and further reduce noise.

Third: The ground wire must be strictly grounded. It is best to gather the ground wires at all levels at the negative end of the power filter capacitor and then connect them to the casing.

In addition, if the output transformer only has 4Ω output terminals, in order to adapt to 8Ω speakers, we can use the following method to modify it: disassemble the EI core, note the gap between E and I without damaging the insulation paper, carefully disassemble the secondary, note the number of turns, and use the same diameter enameled wire to wind the square root of 2 multiplied by the number of turns of the original secondary coil to get 8Ω output. The winding direction must be consistent. Reinstall the transformer. Dip in varnish and let it air dry.

The capacitor Ca in the figure is a tantalum capacitor; Rc can be modified with a soldering iron core. Except for Rc, all resistors are 1W metal film resistors.

This circuit uses AC power supply instead of DC regulated power supply. Although DC regulated power supply has lower noise, the filament of the tube will soon turn white and corrode, which will shorten the life of the tube. AC power supply does not have this problem. If you try to ground both ends of the filament of each tube separately and select the end with low noise to connect firmly, it is a completely acceptable method.

The output power of this amplifier is about 4.2W. Although the power is small, it is a pure Class A tube amplifier. According to some information, its undistorted power is about 9 times that of a transistor amplifier. The same is true for actual listening. I use a two-way closed speaker with a self-made "Silver Flute" 5-inch carbon fiber woven basin woofer and a 1-inch soft dome tweeter. When listening to large percussion instruments, it is full of dynamics, and the treble is also very transparent and clear. Use the "audio test CD" to test the machine. The subjective feeling is that the frequency response is basically flat between 65Hz and 16.4KHz. The noise of this machine is also very low. The noise can only be vaguely heard when the ear is 20 cm away from the tweeter.

Note: 6N2, 6P14 filament pins are both 4 and 5