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Choix des régulations

Choix des régulations

par Le Grincheux  


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Lors d'un article précédent, j'ai présenté un circuit de régulation performant pour les différents filaments de chauffage. Chacune de ces cartes est capable de fournir 6,3V et 1,5A. Le refroidissement des composants de ces cartes, principalement des transistors de puissance se fait par convection naturelle.

Les doubles triodes peuvent être chauffées en 6,3V 300mA ou en 12,6V 150mA. Pour des raisons de symétrie et parce que rien n'assure que les filaments soient rigoureusement identiques, ils seront alimentés en 6,3V 300mA parfaitement régulés. Cette régulation permet d'assurer une durée de vie des tubes correcte et indépendante de la tension du secteur ainsi que d'éviter une ronflette à 50Hz.

En revenant au schéma de principe de l'amplificateur, un œil averti constatera d'une part que les deux canaux se partagent les tube V4 et V9, et d'autre part que les tensions de cathode de tous les tubes sont faibles à l'exception des tubes V9, V3B et V7B. Les tubes V3 et V7 sont disponibles chez JJ sous la référence ECC832. Mais la tension de cathode de leurs deux triodes est très différente (plus de 100V d'écart). Le schéma est donc remanié de la façon suivante :

  • V1, V2, V5 et V6 ne changent pas. Ces quatre tubes nécessitent 1,2A pour leurs filaments de chauffage. Les tensions de cathode de tous ces tubes étant inférieures à 2V, le point froid de la régulation des filaments sera fixé à 30V. Une carte de régulation est suffisante ;
  • V11, V12, V13 et V14 ne changent pas non plus. Ces quatre pentodes de puissante requièrent chacune 1,5A sous 6,3V. Les cathodes étant à une tension inférieure à 1V, il faut utiliser quatre cartes de régulation dont les points froids sont fixés à 30V ;
  • les parties gauches de V3 (V3A) et V7 (V7A) sont des demi-ECC83 qui ont intérêt à être couplées avec V4A et V4B. Leurs tensions de cathode sont identiques. De la même manière, les tensions de cathode des pentodes V8 et V10 sont du même ordre de grandeur.. Le chauffage de l'ensemble de ces tube demande ainsi 1A et une carte de régulation dont le point froid est fixé à 30V ;
  • V3B et V7B sont deux demi-ECC82 qui peuvent être couplées respectivement avec les inverseurs de phase V9A et V9B. Les tensions de cathode de ces deux tubes sont respectivement 115V et 72V. Une carte de régulation avec un point froid aux alentours de 130V suffit aux 600mA requis par les filaments.

Cette modification de la répartition des tubes permet d'une part d'assurer une polarisation positive à tous les filaments de chauffage ainsi qu'une totale isolation entre les deux canaux. Le prix à payer est la présence de sept cartes de régulation juste pour l'alimentation des filaments.

Dernière constatation : le push-pull de sortie est monté en mode pentode et non en configuration ultra-linéaire. Dans un tel cas, lorsque le signal demande à l'alimentation haute tension un fort courant comme lors d'un coup de grosse caisse, l'alimentation va bien entendu le lui donner. Or cette tension, prélevée après un simple filtrage en pi derrière la haute tension, va s'écrouler durant un bref laps de temps dépendant de la constante de temps de l'alimentation.

Comme les caractéristiques d'une pentode dépendent de la tension écran, la restitution sera moyenne. Il faut noter qu'il s'agit là de l'un des facteurs qui est responsable du son rond des tubes, souvent décrié par les défenseurs des transistors. À savoir : graves mal tenus et ampleur artificielle du son.

Un bon amplificateur à tubes n'est pourtant rien de tout cela. D'une part, les triodes (comme les 300B) n'ont pas ce problème, d'autre part, il est possible de réguler la tension de l'écran de manière indépendante de la haute tension d'anode des pentodes de pusisance. Sur une pentode, un écran régulé signifie une stabilité des caractéristiques du tube, ce qui se traduit par des attaques plus franches, une dynamique accrue et des graves beaucoup mieux tenus.

Ainsi, après avoir étudié une régulation pour les filaments de chauffage, il convient d'étudier des régulations pour la haute tension des push-pull d'EL34, une seconde régulation pour les tensions de grille écran de ces pentodes de puissance et une troisième régulation pour tous les autres tubes du montage. Naturellement, il faut ces trois régulations pour chacun des deux canaux.

Catégories: Bonnes manières électroniques
 07.10.13 12:41,

Aucun commentaire pour le moment


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Formulaire en cours de chargement...


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