claudepaz: Système 'Vidéo-balance' (breveté)

ID: 379056
This article refers to the model: Selectro-phone Stéréo 6 (Claude Paz & Visseaux (CPV), Paz & Silva; Paris, Boulogne)

? claudepaz: Système 'Vidéo-balance' (breveté) 
18.May.15 22:59
34
Dirk Bladt (B)
Articles: 31
Count of Thanks: 8

Hello fellow members,

In the above model, a prospect states: Système "Vidéo-balance" (breveté). See the complete text in the public notes with this model.

I can't find any reference to it on the internet. Does anyone know what this "Vidéo-balance" means and how it works?

Thanks for your attention,

Dirk

 

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 2
Vidéo-balance 
19.May.15 13:49
34 from 2423
Mark Hippenstiel (D)
Articles: 1250
Count of Thanks: 8
Mark Hippenstiel

From Toute La Radio, No. 233, Feb. 1959:

[...] Deux autres touches, encliquetées entre elles mais séparées des trois premières, constituen une commande de « Relief » creusant plus ou moins la réponse dans le médium. Leur action est completée par un potentionmètre de relèvement progressif des graves et un de relèvement ou d'atténuation des aiguës (voir courbes).

Les deux touches dont le rôle n'a pas encore été précisé front partie d'un dispositif de contrôle visuel de l'équilibrage (« Vidéo-Balance ») des deux branches amplificatrices. Ce dispositif, dont nous ne pouvons encore fournir le schéma car sa protection par un brevet est en cours, comporte deux ampoules de cadran, logées sour la grille quadrillée à la partie supérieure du couvercle de matière moulée : en même tems que ces ampoules se trouvent raccordées à la sortie des amplificateurs, les H.P. sont coupés et un signal composite, comportant une tension à 100 Hz es des harmoniques, est applicqué aux entrées des préamplificateurs. La commande d'équilibrage est alors ajustée, et nous anvons pu nous assurer que le précision de réglage obtenue était tout à fait remarquable de même, d'ailleurs, que la réponse aux signaux rectangulaires de diverses fréquences.

 

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 3
What is 'Vidéo-balance' ?  
19.May.15 16:34
53 from 2423
Dirk Bladt (B)
Articles: 31
Count of Thanks: 6
Mark, thanks for putting me on the right track. I have found the pages from Toute La Radio No 233 page 62 and 63 where the concerning amplifier is described in somewhat enigmatic terms because the writer wouldn't reveal too much detail in view of the pending patent. Those pages are uploaded to the schematic section of this model. 
 
The title or name of this amplifier "Le stéreo-push à vidéo-Balance" is of course a fantasy name. 
 
The power amplifier is a Push-Pull type with extra taps on the primary coil of the output transformer for the screen grids of the output tubes;  A design called "Ultra-Linear".
 
Description of the environment and its problem where "vidéo-balance" provides the solution:
In Push-Pull amplifiers the output tube at the top generates the positive going half of the sinewave, the other one at the bottom does the negative going half and the two halves are summed to feed the loudspeaker. The problem with such Push-Pull amplifiers in general is that the output tubes must have the same characteristics in order to avoid distortion of the amplified signal. They must be "matched". If not, the amplitude of one half of the amplified sinewave will not be equal to the other half and the resulting output will contain distortion. 
That matching between tubes is only temporary at best due to aging of the devices which they undergo at different rates. In order to get a renewed match after some time, it is necessary to adjust the balance of the amplifier. Those "error compensation" parts are left out of the schematic in Toute La Radio to protect the pending patent.
 
What is "Vidéo-balance"?
It is a balancing method where the resulting amplifier output has to be monitored while fine-tuning the amplifier. While most users won't have an oscilloscope to do the monitoring, the patent-keeper invented a method with a light bulb and a built-in 100 Hz square wave generator to make the adjustment. Hence adjusting the amplifier balance by means of a visible distortion signal.
 
How does it work ?
(This explanation is derived from the today's method we use for balancing a Push-Pull amplifier. Ref: Elektuur, Moderne High-End buizenversterkers, Ir. Menno van der Veen)
 
One amplifier at a time. 
 
When the button of the "Vidéo-balance" is pressed, a square wave generator is connected at the input and the loudspeaker is disconnected and replaced by a lightbulb. 
Then the dedicated balancing potentiometer must be turned in either direction to make the light bulb go dark. This process has to be repeated at succesive higher output levels until the light bulb can't be darkened while turning the balancing control. This would be just past the maximum power level of the amplifier.
 
Each of the 2 amplifiers has its own "Vidéo-balance" button and light bulb at the output.
 
There are of course some other issues involved here but they fall beyond the scope of this article.
 
 

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