Este amplificador fue utilizado en la década de 50 en muchos tocadiscos portátiles como los de Zenith, siendo acoplados a un pequeño tocadiscos con cápsula de cristal. Actualmente, podemos usarlo en proyectos experimentales de intercomunicadores, toca cintas, tocadiscos y otras aplicaciones.

El circuito utiliza tres válvulas con los filamentos conectados en serie y la alta tensión se obtiene directamente a partir de la red de energía, sin la necesidad de utilizar transformador.

El único componente crítico, además de las válvulas, es el transformador de salida, que puede obtenerse de aparatos de chatarra. Se trata de un pequeño transformador de 2 a 10k ohms de impedancia. En la figura 1 tenemos el circuito completo del amplificador.

 

Pequeño amplificador valvulado
Pequeño amplificador valvulado

 

 

La potencia de este amplificador es pequeña, del orden de 2 W, pero la calidad del sonido es muy buena. El circuito todavía prevé la conexión de un motor de tocadiscos que puede ser sustituido por un resistor de 120 ohms x 10 W.

El capacitor electrolítico es de alta tensión y cómo podemos ver, el circuito es indicado para la red de 110-120 V. La impedancia de entrada es muy alta lo que posibilita su uso con fuentes de señales de baja intensidad.

Observe sin embargo que el circuito no está aislado de la red de energía, ya que no utiliza transformador de alimentación. Es una desventaja, pues exige que el aparato quede encerrado en caja a prueba de contactos, pero al mismo tiempo, elimina la necesidad de transformador de fuerza.

 

 

Pequeño amplificador valvulado
Pequeño amplificador valvulado

 

 

Lista de materiales:

Válvulas

35W4

12AV6

50C5

 

Resistores:

3,9 M ohms x 1, 8 W

560k ohms x 1/8 W

150 ohms x ½ W

10 k ohms x ½ W

180 ohms x 7 W

2,2 M ohms x 1/8 W

100 k ohms x 1/8 W

 

Capacitores:

60 nF (47 nF) - poliéster (2)

0,1 MFD (100 nF) - poliéster

100 pf (100 mmd) - cerámico

0,1 mfd (10 nF) - poliéster

20 + 20 mfd (20 + 20 uF o 22 + 22 uF) - 150 V

 

Potenciómetros:

2 M - log

50 k - lin

 

Transformador de salida

Altavoces de 15 a 20 cm x 4 ó 8 ohms

Cable de fuerza, cables, chasis, soldadura, etc.

 

 

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