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Monde cruel

Monde cruel

par Le Grincheux  


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Il n'y a pas à dire, le monde du tube électronique est plein de bricoleurs. Certains, sans aucun complexe, se permettent même de vous apprendre votre métier alors qu'ils devraient vraiment faire attention à l'endroit où ils mettent les pattes.

Sur un forum, je suis tombé sur un individu me prenant de haut parce que j'avais osé dire qu'une E80F n'était pas une EF86 mais une EF80 à tolérance spéciale. Je n'avais pas ouvert mes docs, j'avais dit cela de mémoire. Mais j'ai une excellente mémoire et, une chose est sûre, dans un poste Grundig des années 1950, j'ai remplacé des E80F qui étaient câblées comme des EF80 (en particulier le branchement du filament) par, justement, des EF80. Une autre chose est sûre, l'E80F standard de la grande époque est brochée comme une EF86.

J'ai donc battu ma coulpe, rien n'y a fait. Ce ne serait pourtant par la première fois que je tombe sur des tubes avec un pinout sortant des sentiers battus. En effet, j'ai une boîte de NE407 fabriquées pour Radio Canada qui ne sont pas des 407 classiques, deux broches sont inversées. Dans le monde moderne des transistors, cela arrive aussi de temps en temps. Rien à fait, mon interlocuteur me soutient mordicus qu'une E80F remplace directement et en mieux une EF86.

Sauf que ce n'est pas vrai. Si la transconductance est la même entre les deux tubes, le point de polarisation n'est pas du tout le même (le courant au repos de l'une est 50% supérieur à celui de l'autre). Cela déplace un tout petit peu le point de fonctionnement, un tout petit peu, et on peut obtenir assez rapidement des choses bizarres. J'ai donc sorti les datasheets des deux tubes, exposé en quoi les deux tubes étaient différents, rien à faire. J'ai sorti les réglages de mon lampemètre pour les deux tubes, même réaction. Il faut dire que la personne en question essayait de vendre des E80F et qu'il était très intéressant de dire que c'était des tubes parfaitement identiques aux EF86 mais en mieux. Forcément, ce que je pouvais dire risquant de mettre la puce à l'oreille du chaland, je dérangeais.

Pire, en remplaçant une EF86 par une E80F, on peut avoir un meilleur résultat. Mais par hasard, du fait du changement du point de fonctionnement du tube. Là, on entre dans des grands débats idiophiles permettant de savoir si tel tube est meilleur que tel autre, si les NOS sont meilleurs que la fabrication courante et j'en passe. Il est juste impossible de comparer une EF86 et une E80F sans refaire la polarisation du circuit.

Chose amusante, le même individu a demandé comment tester un tube de sa collection sur son lampemètre Métrix. Je vous le donne en mille, le tube était un thyratron, ce qui prouve le sérieux de cette personne et ses connaissances approfondies des tubes électroniques. Ce qui rend a posteriori très savoureux le fait de se faire traiter de buse par cette personne et sa garde rapprochée.

Catégories: L'idiophile en folie
 08.08.17 10:24,

Aucun commentaire pour le moment


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Formulaire en cours de chargement...


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