Encyclopedia of radio electronics and electrical engineering / Civil radio communications
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Changing (due to mains voltage fluctuations) the filament voltage of expensive ceramic-metal lamps used in power amplifiers can significantly reduce their service life. For this reason, either means of monitoring and manually adjusting the filament voltage, or stabilizers of this voltage, are introduced into high-quality amplifiers.
The figure shows a circuit diagram of such a stabilizer developed by PAOLMD (Technical Topics. - Radio Communication, 1990, October, p. 29). It is intended for use with a 4CX250 lamp, which consumes a current of about 6 A at a voltage of 6,3 V. 1 B). This is necessary to ensure a margin for adjusting the filament voltage due to a drop in part of the mains voltage on the regulating element included in the primary circuit of the transformer.
The control element is made on the transistor VT1, included in the diagonal of the diode bridge VD1 - VD4. The bridge ensures that the voltage polarity on the regulating transistor remains unchanged during both half-cycles of the mains voltage. This transistor operates in a linear mode, so noticeable power can be dissipated on it. The VD5 zener diode for a voltage of 100 V protects the control transistor from breakdown in case of possible voltage surges.
Transformer T2 provides an alternating voltage of about 8 V on the secondary winding. This voltage is rectified by the VD6 diode and is used to power the control unit. Part of this voltage is supplied to the base of the transistor VT2 through an adjustable divider (R1R2). The emitter voltage of this transistor is fixed at about 0,7 V (forward voltage drop across the VD7 diode). When the mains voltage fluctuates, the voltage at the base of the transistor VT2 changes and an error voltage occurs, which is amplified by the transistors VT2-VT4 and fed to the base of the regulating transistor VT1. The required value of the filament voltage is set by the tuning resistor R2.
The adjustment unit can be performed on the KT848A transistor and KD202K diodes (see, for example, V. Bannikov. "Protection of electric lighting devices" in Radio, 1990, No. 12, p. 53). Transistors VT2 and VT4 are from the KT3102 series, and VT3 - KT3107. Diodes VD6 and VD7 can be replaced with KD522A, aVD5 - with D817G.
It must be remembered that the control unit is galvanically connected to the network, therefore, when manufacturing it, placing it in the amplifier housing and setting it up, it is necessary to take appropriate precautions.
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