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Loubardes

Cambriolage

par Le Grincheux  


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Rien à dire d'autre. Je viens de me faire voler une station à souder Weller et un multimètre de table au milieu de broutilles. Je parie que le ou les voleurs ne sauront même pas s'en servir. Si j'avais su, je n'aurais pas investi dans un oscilloscope quatre voies et un générateur de signaux à adjoindre à mes deux analyseurs de spectre puisqu'il me manque maintenant du matériel de base.

Lampemètre

par Le Grincheux  


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Se dire qu'on va utiliser des tubes électroniques sans avoir un minimum de matériel pour les tester est une gageure. J'ai à cet effet trouvé à un prix défiant toute concurrence un lampemètre B&K 607 dans un état acceptable.

Ce lampemètre fabriqué aux USA au milieu des années 1970 est un testeur transistorisé. Ses différents culots permettent de tester efficacement l'immense majorité des tubes tant américains qu'européens.

La ligne de boutons notée « lockout » permet d'associer un connecteur d'un culot aux différentes onctions du tube à tester. Le connecteur d'anode flottant permet de tester les tubes à anode au sommet du verre. Attention donc, danger…

Il n'y a que deux petits problèmes. Le premier est purement mécanique et va trouver une solution promptement. En effet, le sélecteur permettant de tester les courts-circuits tourne dans le vide et il va me falloir tailler un détrompeur pour qu'il puisse entraîner correctement l'axe. La réparation de fortune faite par l'ancien propriétaire était pour le moins légère et s'arrêtait à un coin de papier que l'on voit encore sur la photographie.

Le second point est assez ennuyeux. Cet appareil fonctionne en 115V/60Hz. Les spécifications techniques indiquent qu'il ne faut pas le faire fonctionner longtemps en 50Hz. J'ai testé en faisant attention et, en effet, le transformateur sature à 50Hz et chauffe anormalement. Il me faudra donc concevoir une alimentation 115V/60Hz, ce qui passera par un onduleur 60Hz. À l'heure où j'écris ces lignes, le schéma de cette alimentation est jeté sur le papier. J'espère ne pas perdre trop de temps sur ce détail. Pour que ce circuit puisse être réutilisable, j'envisage un onduleur 400VA avec une entrée 230V ou 12V continu et une sortie en 115V ou 230V et en 50Hz ou 60Hz.

Dernier point, cet appareil comportait encore ses deux fascicules. Le premier est un mode d'emploi et de maintenance. Le second, en revanche, comporte les réglages de l'appareil pour tous les tubes courants.

Du X-1000 au X-1000-B

par Le Grincheux  


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Reprenons donc.

Les alimentations sont entièrement à revoir pour d'une part augmenter la durée de vie des composants et d'autre part pour limiter tout bruit inutile. Chaque filament aura droit à son propre circuit de stabilisation, de limitation de courant et de limitation des appels de courant lors de l'allumage. Tous les condensateurs de fortes valeurs seront doublés par des condensateurs de valeurs plus faibles pour encaisser plus facilement les hautes fréquences. Le potentiel le plus bas de chaque filament sera d'une trentaine de volts supérieur au potentiel de la cathode (sauf pour V3 et V7 où ce sera impossible). Une marge de sécurité importante sera prise sur tous les composants critiques.

Le circuit de polarisation négative sera entièrement revu puisqu'il fait partie de la sécurité de l'ensemble. Un défaut de ce circuit doit aboutir au blocage des tubes de sortie et non à leur destruction.

Le transformateur d'entrée sera protégé par une diode transil en parallèle de ce dernier permettant de faire sauter le fusible en cas de surtension.

Deux références de tensions indépendantes seront utilisées, une par canal. Ces deux références seront toutefois connectées par une résistance pour éviter qu'elles ne flottent l'une par rapport à l'autre. Une temporisation retardera l'application de la haute tension sur les tubes. Pour éviter les boucles de masse, les entrées seront toutes dans la mesure du possible protégées par un transformateur de ligne de bande passante suffisante.

Le système de haut-parleur central ne sera pas repris. À sa place, un système de commutation entre deux systèmes de hauts-parleurs sera étudié.

Les entrées existantes seront gardées :

  • microphone ;
  • tête magnétique 9,5 cm/s ;
  • tête magnétique 19 cm/s ;
  • phono magnétique 1 correction Columbia ;
  • phono magnétique 1 correction RIAA ;
  • phono magnétique 2 correction RIAA ;
  • tuner ;
  • aux1 renommé en cd ;
  • aux2 renommé en aux1.

À ces neuf positions vont se rajouter aux2, aux3 et aux4, chaque entrée auxiliaire ayant un double réglage de niveau sur la face arrière de l'amplificateur. Aucune décision n'est encore prise sur la position des boutons « low level control » qui pourraient finir sur la face arrière.

Le X-1000 possède une entrée moniteur. Possédant plusieurs systèmes d'enregistrement dont un ReVoX A77, le X-1000-B aura deux moniteurs avec la possibilité de cooupler ces deux systèmes.

Le réglage des impédances de sortie des systèmes de hauts-parleurs se fera avec un sélecteur.

L'implantation des composants doit être impérativement revue pour éviter tout bruit parasite. En particulier, tout sera fait pour isoler magnétiquement le transformateur d'alimentation, les transformateurs de sortie et la partie amplificateur proprement dite. Comme tout sera fait pour approcher le plus possible les étages de préamplification des entrées de l'appareil.

Schéma des entrailles du X1000

par Le Grincheux  


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Voici le schéma complet des entrailles d'un X1000 originel. Plusieurs points de ce schéma sont aujourd'hui contestables. Il y a même quelques erreurs manifestes, sans doute pour tromper l'ennemi. J'y reviendrai plus loin.

Fig. 1 : schéma complet (en haute résolution)

Dans un premier temps, les filaments des tubes de préamplification sont sous-alimentés puisque la tension de chauffage n'est que de 10,5V contre 25,2V nominaux. Par ailleurs, le concepteur a fait l'hypothèse que les résistances des filaments sont exactement les mêmes. Je vais donc commencer par alimenter ces filaments avec une tension constante et régulée de 12,6V. Même chose pour les filaments simples qui seront alimentés en 6,3V stabilisés.

D'autre part, la tension d'alimentation des tubes n'est pas assez positive par rapport à leurs tensions de cathode. Dans la mesure du possible, je vais donc garder une différence de potentiel entre 20 et 30V entre le potentiel de la cathode et le potentiel le plus bas de l'alimentation du filament. Les seuls filaments posant problème sont ceux de V3 et V7, les deux triodes étant polarisées différemment et n'accepteront certainement pas une tension de l'ordre de 150V entre leur cathode et le filament correspondant. De plus, ces deux doubles triodes n'ayant pas d'écran interne, il m'est impossible d'utiliser deux demi-ECC par canal comme par exemple les V4 et V9.

Un dernier point me chagrine, la tension au repos de 300V relevée aux bornes de C5 (qui est d'ailleurs infirmée par la liste des composants qui indique un condensateur de 250V). Sans doute une coquille.

Quant aux étages d'entrées, ils rapportent des masses sur toutes les entrées non utilisées. C'est un peu violent et je risque de charger les entrées non utilisées par des résistances de 47k. Ils risquent aussi de provoquer des boucles de masse et j'hésite encore pour les supprimer entre utiliser des transformateurs d'isolation sur chaque ligne ou isoler chaque circuit de masse active. En tout état de cause, si le châssis sera bien à la terre, le 0V des alimentations sera connecté à la terre au travers d'une résistance de forte valeur.

La polarisation négative des grilles de l'étage de puissance est mauvaise car elle n'utilise qu'une seule diode. Elle sera remplacée par une alimentation régulée et le système de réglage de la tension des grilles (bias adjust et balance) sera revu pour rapporter une tension la plus négative possible en cas de dysfonctionnement de cette alimentation à seule fin de ne pas détériorer l'étage de puissance.

Dernier point, l'alimentation haute tension est appliqué sur les tubes froids. Un système de relais temporisé sera donc ajouté sur cette alimentation pour retarder l'application de la haute tension sur les tubes.

Caractéristiques

par Le Grincheux  


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Depuis l'annonce de l'ouverture de ce blog, vous avez été nombreux à me poser des questions. La première d'entre elle est sans nul doute pourquoi un amplificateur à tubes. Je ne fais pas partie de la secte des idiophiles capables d'acheter au prix de l'or un préamplificateur ou un amplificateur mal bricolé en Chine sous prétexte qu'il se trouve une double triode bien visible sur le châssis et quelques transformateurs qui rayonnent joyeusement pour faire joli. Le tube a ceci de supérieur au transistor : son impédance de sortie est plus forte, ce qui fait qu'il sature moins vite et de façon beaucoup plus agréable pour l'oreille.

La seconde est indéniablement pourquoi avoir choisi un Fisher et précisément un X-1000 ? La réponse à cette question est assez simple. C'est purement subjectif et motivé par un certain nombre d'articles sur ces amplificateurs. J'aurais très bien pu jeter mon dévolu sur un ReVoX, mais étant déjà l'heureux propriétaire d'un A77 sauvé des eaux avec amplificateur intégré, il me fallait passer à autre chose.

Quant aux autres questions, elles tournaient autour de la disponibilité des composants. Rassurez-vous, on fabrique à ce jour bien plus de tubes électroniques qu'il y a ne serait-ce que dix ans. Même des tubes comme les EM84 sont à nouveau fabriqués pour en faire des vu-mètres et, hier, en furetant sur des sites de fabricants, j'ai même trouvé des tubes nixies. Tout n'est pas perdu et l'histoire est un éternel recommencement.

Pour clore ce billet, voici ce que publiait à propos du X-1000 le magazine 'High Fidelity Magazine' en novembre 1961.

AT A GLANCE: The Fisher X-1000 is the highest-powered and most elaborate in a new line of Fisher integrated stereo control amplifiers. It provides a multitude of operating features and controls (for stereo and mono) as well as two power output channels of 55 watts each, with an optional "centre channel" output without the need for an additional power amplifier. The unit is very well built and, despite its many facilities and high power, is relatively compact and handsomely designed. In tests conducted at United States Testing Company, Inc., it met its specifications in all important respects and, in fact, proved to be an outstanding performer in its class. Dimensions are 16 15/16 in. wide by 5 13/16 in. high by 13¾ in. deep (exclusive of knobs). Weight is 44 pounds. Price: $329.50. Cabinet extra.

IN DETAIL: The X-1000 has 8 pairs of inputs for microphone, tape head (with equalization for either 3¾ ips or 7½ ips tape speed), magnetic phono (two inputs, one of which is equalized for RIAA and the other for either RIAA or Columbia), tuner, tape monitor, and two separate auxiliary inputs, one pair of which has individual channel level controls located on the rear panel.

After passing through the input selector, low level input signals are fed to a twin triode (type ECC83/12AX7) for initial preamplification and equalization. These signals then are fed, as are high level input signals, to the function selector. Provisions are made for normal stereo, reverse stereo, left input to left amplifier alone, right input to right amplifier alone, both inputs to both amplifiers monophonically, or either input to both amplifiers.

A second ECC83/12AX7 is used for the next stage, which furnishes the tape monitor inputs, recorder outputs and inputs, and outputs for the Fisher "Spacexpander" reverberation unit. Also associated with this stage are a centre channel (monophonic) recorder output, low frequency cut-off filter, and phase reversal switch. Bass, treble, volume, balance, and variable channel separation controls, as well as high frequency cut-off filters come next, with each amplifier using a 7247 twin triode, half an ECC83/12AX7, an EF86/6267 pentode, half of an ECC82/12AU7, and two EL34/ 6CA7 pentodes in push-pull outputs. Output impedances of each channel are either 4, 8, or 16 ohms, and provision for connecting a "centre channel" speaker directly to the amplifier is provided, as well as a centre channel high impedance output jack and a low impedance stereo earphone jack.

Tests indicate that the Fisher X-1000 would make an excellent choice for an all-in-one stereophonic amplifier and preamplifier. Each channel will deliver approximately 55 watts of clean signal at 1,000 cps with a total harmonic distortion of only 0.5%. At half power, or 3 db down from maximum power output, the total harmonic distortion is less than 1% from 50 cps to well above 20 kc.

At full rated power output (55 watts per channel), the frequency response of the amplifier was measured as flat within 2 db from 24 cps to 20 kc. Response at half-power level was essentially the same.

All frequency response measurements were made with the tone controls in the mechanically flat position, as indicated by the dot or arrow on the control knob. However, manufacturers rate their amplifiers and preamplifiers with the tone controls in the electrically flat condition, but since most owners of high-fidelity equipment do not have the facilities, to check accurately the frequency response of their amplifiers, USTC operates the amplifiers as the consumer would. Slight variations between mechanically centered and electrically flat are normal in most amplifiers, and many users eventually find the exact control setting that suits their listening needs. In any case, the high frequency response of the X-1000 could be greatly improved by setting the treble control at "1 o'clock," and a slight improvement also could be made in the low frequency response with a small amount of bass boost.

The bass control provides a maximum of 14.4 db of bass boost and cut at 50 cps, and the treble control provides up to 15 db of treble boost and cut at 10 kc. The low frequency rumble filter operates at a slope of 10 db/octave below 80 cps and the scratch filter operates at the rate of 16 db/octave above 5 kc. These are desirable filter characteristics since they permit the filter to suppress most of the noise with virtually no loss of musical quality.

The equalization provided for RIAA recordings and NAB tapes (7½ ips) is very good, being quite close to professional standards for those media. Similarly, the equalization for 3¾ ips tape resembles the EIA standard for that speed.

The intermodulation distortion was extremely low, less than 0.2% up to 10 watts, less than 0.5% up to 40 watts, and 2.9% at full power.

The channel separation of the X-1000 was better than 56 db at 1 kc, and was down to 38 db at 10 kc, which is more than adequate for all stereo program sources. Channel balance, once adjusted, remained excellent at all volume levels, with a measured variation in balance from full power to minimum power of only 0.6 db.

The amplifier's signal-to-noise ratio, measured at maximum gain, was 78 db on the high level inputs, 52 db on the RIAA phono input, and 48 db on the tape head inputs. The amplifier sensitivity (for 55 watts output at 1 kc) was measured at 170 millivolts at the high level inputs, 2.2 millivolts at the RIAA inputs, 0.85 millivolts for microphone, and 1.25 and 1.4 millivolts for tape head inputs, 3¾ and 7½ ips respectively. A speaker-damping factor of 10 was measured at both the 8 and 16-ohm speaker taps. The amplifier, incidentally, appears to be quite stable and should encounter no difficulties in driving electrostatic speakers. With its clean sound, operating versatility, and fine workmanship and appearance, the Fisher X-1000, in sum, would make a very suitable foundation block for those people who are just getting into stereo or high fidelity, as well as for many others who are considering up-dating their system.


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