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Old 18th May 2019, 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 18th May 2019, 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 18th May 2019, 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 18th May 2019, 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 18th May 2019, 3:17 am   #5
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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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Old 21st May 2019, 6:07 pm   #6
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Default Re: CTV Line Output Valve Construction?

It's not just Europe that was schizophrenic over such connections: in the US they sometimes did the same.

Their 6JB6 and 6JG5 "Sweep tubes " were identical except for the 6JB6 having the beam-forming plates connected to a separate leadout and the 6JG5 having the plates tied to the cathode.

(Either way a pair of them in parallel makes a great 100-Watt-PEP HF linear amp).
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Old 21st May 2019, 7:05 pm   #7
Chris55000
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Default Re: CTV Line Output Valve Construction?

Hi!

If I remember correctly the B & O 3200/3400 both had a +32V +L line connected to the PL519 g3, and this connection as an anti–Barkhausen measure didn't occur to me simply because I've never seen it in any out of the hundreds of both b/w and colour CTVs I handled in my servicing days!

The only hybrid colour sets I did were ones where g3 of the PL509 was strapped to cathode, but I've no recollection of ever seeing BK "dangling–rope" dots or other similar artifacts on them!

Was it a manifestation of mainly new LOP valves? Virtually every PL509 I to replace had either lack of power, the anode glowing red as a result of grid emission from previous severe overheating from line–osc failure, or blown heaters from PY500 heater/cathode shorts in sets where the PL509 was at the top of the heater line!

Thanks to all those who clarified the theory for me!

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Old 21st May 2019, 7:22 pm   #8
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Default Re: CTV Line Output Valve Construction?

Were the B & O 3200/3400 sets one of those rare beasts that had a seperate EHT generator? I wonder if lack of EHT rectifier/tripler quenching on LOPT flyback conditions resulted in negative excursion shape/amplitude at the PL519 anode that were more likely to encourage Barkhausen-Kurtz parasitics? As I understand it, LOP valves starting with some versions of the PL36 had an external envelope base coating/ring that formed a lossy capture (if that's not an oxymoron!) at UHF and above, suppressing radiation from the base connections, principally the grid where high B-K amplitude was induced when the anode swung highly negative.

Does anyone know when this base coating or ring first started to be applied? Perhaps the PL36 was about concurrent with the introduction of Band III usage, when B-K parasitics might have become obtrusive.

http://www.r-type.org/exhib/aam0013.htm

http://www.r-type.org/exhib/aad0290.htm
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Old 21st May 2019, 11:09 pm   #9
Chris55000
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Default Re: CTV Line Output Valve Construction?

Hi!

Ah!

I remember some of the Japanese valves (40KD6 type) sold for the Teletons (VX1110, etc) had this ring round the inside of the glass envelope about 1–2 cm above the glass–bottom base of the valve, usually matt grey in colour, but a few of these rings were pale green in colour as well!

I had no access to internet and other data in those days, relying on what was in the OEM's manual and "Television" magazine, and it didn't twig at that time that these inside coatings were also an anti–BK measure!

As in my previous note, these various measures seemed to work very well as I never saw anything untoward that I could reasonably suspect to be B.K. artifacts on hybrid TV and CTV CRT screens in all the years I worked on them!

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Old 22nd May 2019, 3:27 am   #10
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Default Re: CTV Line Output Valve Construction?

There are several accessible articles that address the issue.

RCA Engineer 1966 April-May, p. 57ff

Technological Improvements in Horizontal-Deflection Tubes

Therein it was said: “In addition to eliminating snivets by proper load line and improved knee characteristics, snivet interference can be reduced by applying a positive DC voltage (approximately 30 volts) to the beam plate (grid-no.3) of the tube. The beam plate collects the stray secondary electrons before they return to grid-no.1 and also lowers and rounds the knee- current characteristic. Recent deflection tubes all have a separate beam-plate connection to permit the use of a positive voltage.”

The article also referred to the multi-fin plate structure, which I think is RCA’s prosaic term for the Philips/Mullard “Cavitrap” anode, as used in the EL500, etc.


RCA Review 1960 March, p.17ff

Ringing in Horizontal-Deflection and High-Voltage Television Circuits

At the end of the article, it was said: “With the elimination of transient ringing during the trace period, the problem of “snivets” is solved.”

There was no mention of positive biasing of the line output beam plates, implying that that idea had yet to enter the RCA lexicon. That I think supports the notion that Philips was first with this idea.


RCA Engineer 1958 April-May, p.35ff

Unusual Electron-Tube Effects of Concern to Circuit Designers

The last topic covered was “Snivets”, on p.39


Radio & Television News 1953 November, p.104ff

Introduction to Snivets

This covered the problem and the underlying beam tetrode characteristics that cause it.


All of the above articles are available at the usual place, namely” https://www.americanradiohistory.com/index.htm.



Cheers,
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