Loftin-White amplifier. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers
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The amplifier is made on lamps, 6g2, which serves as a voltage amplifier (driver) for the lamp of the final stage - 2A3 with a direct connection of the 2A3 grid to the 6G2 anode circuit. The non-working legs of the 6G2 lamp are connected to the body. To comply with the operating mode of the output lamp 2A3, the power supply of the 6G2 lamp is made symmetrical with respect to the cathode of the output lamp with a "virtual ground", the resistor R7 serves to adjust the quiescent current of the output stage 60mA, which is determined by a voltage drop of 0,06V across the resistor R6.
The preliminary setting is done as follows: The grid of the output lamp is disconnected from the anode of the driver and closes to a 1 ohm cathode resistor. At the anode of the driver lamp, a voltage of -50-55V is set relative to point B (the midpoint of the anode winding of the power transformer). Then the grid of the output lamp is connected to the anode of the driver and the final adjustment is made according to the current of the anode of the output lamp, which should be about 60mA with a voltage drop of 0,06V across the 6 Ohm resistor R1. Diodes D311 in the filament-cathode circuit of a 2A3 lamp can be replaced with 10 ohm resistors. Electrolytic capacitors 200uF are designed for an operating voltage of 400V, C2 for an operating voltage of 10V.
(click to enlarge)
The power supply circuit is made on a 5Ts3S or 5Ts4S kenotron, which serves to rectify the voltage in positive direction, and a 6X2P double diode, which rectifies the negative voltage to power the 6G2 driver lamp. Turning on the anode power with a toggle switch in the kenotron filament circuit after a five-minute warm-up of the lamps.
The amplifier is easily tuned when using serviceable lamps and parts. Moreover, it is highly desirable to use parts of the highest quality. What details are applied by me, I do not write consciously, giving the possibility of self-selection of components according to sound. Despite the apparent simplicity of the circuit, the amplifier has a good sound. Amplifier sensitivity - 0,7V, output power - 2,8 W with THD up to 1,5-2%.
The power transformer is made on Sh30x50 iron. The output transformers are made on ShLM 32x50 iron, the anode winding has 2502 turns in three sections of 834, wire 0,25, the secondary winding for a load of 4 ohms consists of 4 sections of 100 turns of wire 0,71, the first section of the secondary winding is divided in half for the possibility load connection 8 ohm. When connecting all four sections in parallel-series by two, it is possible to connect a load of 16 Ohm. Non-magnetic gasket in the gap of the output transformers-0,12mM.
The amplifier is designed to be built by amateurs with some experience in building and tuning tube designs. good luck!
Authors: Anatoly Manakov, Evgeny Razumov; Publication: cxem.net
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