Big screen 405 line TV set.

[Geordie McBoyne]

Let's fnd out what a 405 line picture looks like on this 24" 405 line TV set?
Took delivery today of this unique 24 " CRT console TV set. The set is a Kolster-Brandes PV100 in which the chassis has been installed into a specially made console cabinet. The seller told me that the set was purchased in 1958 by an American Gentleman who lived in Kent.
Before any attempt is made to power up the set the line output transformer will be warmed up to drive out any moisture. The method used to do this has been discussed in vintage TV forums. A current of 20 to 30 mA will be passed through the EHT overwind and primary winding.
The chassis of the PV100 is nothing special being simply a modified version of the popular OV30 model. The 90 degree magnetic focus CRT is a Brimar C24KM which was imported from the Continent. It has a pentode gun so it's likely it was made by Philips or Telefunken.
The line output valve is the Brimar 50CD6G, the valve with twin heater-cathode assemblies. The common PL36 can be used as a replacement as long as an 83 ohm resistor is inserted in the heater chain. PL36 has 25 volt heater.
The cabinet is solid and beautifully made only let down by the awful pegboard back.

Geordie McBoyne.

[Refugee]

The tube looks like it is just held in place by being pushed through the front panel and the neck. Is that right?
It does look like there is a front plate glass shield on the front.
The 50CD6 does not just have two cathodes. If it is anything like the 6 volt 6CD6 it will be two pentodes in parallel with a shared anode.

[Geordie McBoyne]

Hi Refugee,
              There is a metal band around the periphery of the CRT faceplate to secure it to the chassis.
Evidently the 6.3 volt version the 6CD6G was redesigned and re-designated as 6CD6GA.
https://www.radiomuseum.org/tubes/tube_6cd6ga.html
As a precaution to the line output transformer failing when the mains full supply voltage is supplied to the set the windings are presently being warmed up by passing a small current between the R19 top cap connector and the anode connection of the 50CD6G. Winding resistance is about 220 ohms. The power supplier is set to 20 volts,  this will give just under 2watts, sufficient to drive out any moisture.
The attachment shows the pegboard back.  That awful thing will be replaced with something better.

[Refugee]

That peg board does not look original. It looks more like the tube neck guard has been salvaged from the old back and added to the peg board.
It was speak of the devil when you put this post up on the same day that I was using the signal generator that actually has the 6CD6G in it. It was glowing away nicely deep inside the cabinet while I got on with my circuit.

[Crackle]

With radios I find them fascinating and whilst restoring one and listening to it for the first time after dozens of years of silence, I imagine all the important issues and worldwide news that they used to deliver to us in real time. They were the first means for instant communication to the population in real time. Making it possible for the whole world to be told of a happening at the same time.
But for some reasons, mainly because of their size and I suppose that despite having an Aurora I just don't find TV's fascinating, I don't get involved much with TV restoration, but I am really pleased to see this one being done by someone who is such an expert.
Well done David.
Mike

[Jac Janssen]

Hi David,

Yes, it would be interesting to see if a 405 line picture still is acceptable on such a large CRT.
On my 9" and 12" sets I think it is fine, at the right viewing distance that is. This size would probably be good for a large room.

Jac

[ppppenguin]

Jac, David, it's really a question of viewing distance. Too close and you see the lines (and pixels for a flat panel), to far and you don't see all the quality in the picture. For 625 the recommended viewing distance is 6 or 7x picture height. For 1080 HD it's 3 or 4x. So for 405 the optimal vewing distance should be 9x or 10x PH. About 11 feet away for your 24" set. Whether that's practical in the average living room is another question.

On occasion I've been in cinemas, sitting a bit too close, to the point where the pixel pattern has been visible on some scenes. I reckon that was 2K projection, in other words no more than domestic 1920x1080 HD.

At the other extreme I don't see the point in cramming more and more pixels on to a smartphone screen. You're going to be very close indeed to make use of 2K+ resolution.

[Jac Janssen]

That's right Jeffrey.

RCA, but also EMI and Philips, have done extensive studies to determine the required number of lines for a given screen diameter and viewing distance, which adds to the same as viewing distance as a function of the screen diameter at a given number of lines.
The higher anode voltage of larger tubes gives a sharper picture, helping in a better overall picture, but averse to the invisibility of the lines.

Jac

[Geordie McBoyne]

After studying the characteristics of the 567 line system proposed by Philips I find this was an excellent compromise between transmission bandwidth, horizontal definition and visible line structure. Line frequency 14,175cps.

The warm up of the line output transformer continues, drivng out any moisture in the windings.

The origins of that Brimar C24KM 24" are most likely to be Philips. A tube type MW61-80 was available to Continental manufactures between 1956 to 1959. From Franks Electron Tube Data Sheets:
https://frank.pocnet.net/sheets/030/m/MW61-80.pdf

Geordie McBoyne.

[ppppenguin]

Having just looked at the parameters of the 567 line system it's clear that the vision BW would be too narrow. The sound carrier is only 4.5MHz from the vision carrier. This would be like a 625 line system with 5MHz sound carrier spacing. Since 5.5MHz (Systems B/G) spacing is only just about adequate this really wouldn't be enough. Not as bad as Belgian 819 which was ludicrously short of vision bandwidth.