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Old Yesterday, 12:05 am   #1
Chris55000
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Default CTV Line Output Valve Construction?

Hi!

Can anyone suggest, from a theory/design point of view, why colour TV line output valves nearly always had separately pinned supressor (g3) pins/beam plates rather than being internally connected to cathode?

Also, what was the reason, again from a design point of view, for the +32V g3 bias used on the PL519 110 CTV line o/p pentode found in continental TV sets?

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Old Yesterday, 12:31 am   #2
Argus25
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Default Re: CTV Line Output Valve Construction?

Quote:
Originally Posted by Chris55000 View Post
Hi!

Can anyone suggest, from a theory/design point of view, why colour TV line output valves nearly always had separately pinned supressor (g3) pins/beam plates rather than being internally connected to cathode?
That is a very interesting question, but it might just be a European thing.

Looking at RCA's designs (who were the world leaders in both early monochrome and color receiver design, ignoring issues of PAL & NTSC of course) their primary H output valve starting immediately post war was the 6BG6 and in their first color TV, the CT-100 model from 1954, was the 6CD6, both of which have their suppressor grids internally connected to the cathode and both these valves created specifically for horizontal output use.

Also, RCA did most of the pioneering work on energy recovery line output stages, most was from their Deflection Engineer Otto Schade who elaborated the maths & physics (giving credit to Blumlein for the idea of the energy recovery diode). I have all of those early RCA research papers. There is no mention about an independent suppressor connection that I have seen, I'll have another look through them.

So it would be interesting to know what, if any, advantage could be attained with a separate suppressor connection in the H output valve of a CTV.
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Old Yesterday, 3:02 am   #3
Maarten
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Default Re: CTV Line Output Valve Construction?

European line output tubes are usually beam tetrodes even when the datasheet calls it pentodes. This may not be relevant as I suspect USA ones are beam tetrodes as well.

I can possibly imagine that any voltage applied to the beam plates will affect the shape of the beam, maybe some efficiency gain can be had from applying such voltage?
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Old Yesterday, 3:10 am   #4
Argus25
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Default Re: CTV Line Output Valve Construction?

Maybe the answer is that immediately after flyback the horizontal output valve's plate can go negative momentarily. Electrons get attracted from the cathode by the screen grid are repelled from the plate so they oscillate around the screen grid causing RF radiation and Barkhausen oscillations sometimes evidenced as dark lines on the left side of the picture on VHF channels. Maybe by keeping the suppressor grid more positive than the cathode these are inhibited to some degree.

The common trick to eliminate them though was to put a magnet on the valve's exterior and rotate it until they disappeared.
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Old Yesterday, 3:17 am   #5
Synchrodyne
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Default Re: CTV Line Output Valve Construction?

As I understand it, the separate suppressor grid/beam plate connection in line output valves was done to allow the application of a positive bias (typically around 30 volts) that in turn helped with the suppression of Barkhausen oscillations, the effects of which on the picture were called snivets in American practice. I am not sure who was first with this idea - it might have been Philips. RCA mentioned it in later publications.


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