Vintage tube substitution in Baird T18 - UK Vintage Radio Repair and Restoration Discussion Forum

beery · 10th Jul 2006, 12:36 pm

Hi All,
over the weekend I tried fitting about four different tubes in a Baird T18 pre-war TV.
In the end I fitted a Philips MW31-74, which unlike the Mullard version is not aluminised.
I was able to find A1 sources of 150, 200 and 300 volts. I tried different settings to find the optimum compromise between focus and brightness.
The original tube was a triode (Cathodevisor 12MW2) and required a stronger focus field.
The owner of the set is going to make a new wooden tube support (a bit like the arrangement in the Bush TUG24) and after that is done I shall wire the tube up properly.
Then I can try and lower the current through the focus coils, probably by shunting them.

See details of this exercise at:-
http://freespace.virgin.net/andy.valve/

I have found this experiment very useful as at shows the benefits of fitting a Tetrode tube as well as the focussing problems encounted with a Tetrode.

Cheers
Andy

Panrock · 10th Jul 2006, 12:56 pm

Good luck with this Andy.

A working Baird T18 (with original tube) was the very last image broadcast at the final closedown of the 405-line service from Crystal Palace.

The set in question had passed through my hands and then had been very thoroughly restored by Canadian BBC engineer Terry Harvey. I seem to remember at some social event connected with the closedown he bumped into the original designer of the Baird's circuit, and was able to discuss the finer points of the peculiar field oscillator circuit, etc...

I remember this set gave a good picture but was rather sensitive to variations in mains supply voltage. I wonder where it is now ?

Steve

Heatercathodeshort · 11th Jul 2006, 8:16 am

Quote:

Originally Posted by beery

In the end I fitted a Philips MW31-74, which unlike the Mullard version is not aluminised.Andy

The Mullard MW31-74 is not an aluminized tube. As far as I can remember Mullard were very late to use this technique. I think the MW36-20 was one of the first together with the MW53-20 around 1955/6 Regards John.

beery · 11th Jul 2006, 12:13 pm

Hi all,
John is right about the MW31-74, not sure where I got that idea from.
I only know of two surviving T18 sets. One is the console belonging to the ETF and the other is this table set. This set is the same one in Andy Emmerson's 'Old Television' book.
Neither of the two sets has any evidence of any protection glass being fitted. How did they get away with it??

The frame timebase is the same as all the pre-war Baird sets. I have attached the circuit from the T5 for the interest of others. The frame transformer (on the right-hand side of the circuit with octal connector) is also the oscillating transformer and the scan coil. The laminations of this transformer have arms that sit either side of the tube neck (shown in attatched photo of T18).

Interesting to hear about the T18 on the last day of 405. If you watch the film of TV at Radio Olympia in 1936 you can see a Baird T5 being used in the control room at AP.

Cheers
Andy

Duke_Nukem · 11th Jul 2006, 1:06 pm

Quote:

Neither of the two sets has any evidence of any protection glass being fitted. How did they get away with it??

There was no rule to say that they had to. The RMA (IIRC) came out with some guidelines stating that sets ought to be fitted with safety glass and quoted increasingly thick glass for larger tubes. I think this was in mid '39.

TTFN,
Jon

Panrock · 11th Jul 2006, 1:27 pm

Didn't this set have a so-called 'flame lamp' - a pygmy lamp wired in series with the mains eht transformer primary ? It was supposed to illuminate when there was an overload or short on the eht, and shine red light through the clear wall of the tube onto the back of the screen.

It could have been the source of this story, quoted from page 210 of the book 'Here's Looking at You' by Bruce Norman...

Cyril Fletcher: After a programme people would ring up. There were so few of them that you would get to know them. The phones were in the corridor outside the studios and on one occasion this old girl rang up and said: 'Oh, it's marvellous tonight, Mr Fletcher, I have enjoyed it. It's all in colour.' And so I said: 'What sort of colour?' She said: 'It's all in marvellous red shades.' And I said quickly: 'Switch off. Your set's on fire!' As, indeed, it was.

Steve

10th Jul 2006, 12:36 pm	#1
beery Heptode Join Date: Jun 2006 Location: Ware, Hertfordshire, UK. Posts: 988	Vintage tube substitution in Baird T18 Hi All, over the weekend I tried fitting about four different tubes in a Baird T18 pre-war TV. In the end I fitted a Philips MW31-74, which unlike the Mullard version is not aluminised. I was able to find A1 sources of 150, 200 and 300 volts. I tried different settings to find the optimum compromise between focus and brightness. The original tube was a triode (Cathodevisor 12MW2) and required a stronger focus field. The owner of the set is going to make a new wooden tube support (a bit like the arrangement in the Bush TUG24) and after that is done I shall wire the tube up properly. Then I can try and lower the current through the focus coils, probably by shunting them. See details of this exercise at:- http://freespace.virgin.net/andy.valve/ I have found this experiment very useful as at shows the benefits of fitting a Tetrode tube as well as the focussing problems encounted with a Tetrode. Cheers Andy

10th Jul 2006, 12:56 pm	#2
Panrock Nonode Join Date: Feb 2005 Location: Worcestershire, UK. Posts: 2,533	Re: Vintage tube substitution in Baird T18 Good luck with this Andy. A working Baird T18 (with original tube) was the very last image broadcast at the final closedown of the 405-line service from Crystal Palace. The set in question had passed through my hands and then had been very thoroughly restored by Canadian BBC engineer Terry Harvey. I seem to remember at some social event connected with the closedown he bumped into the original designer of the Baird's circuit, and was able to discuss the finer points of the peculiar field oscillator circuit, etc... I remember this set gave a good picture but was rather sensitive to variations in mains supply voltage. I wonder where it is now ? Steve

11th Jul 2006, 12:13 pm	#4
beery Heptode Join Date: Jun 2006 Location: Ware, Hertfordshire, UK. Posts: 988	Re: Vintage tube substitution in Baird T18 Hi all, John is right about the MW31-74, not sure where I got that idea from. I only know of two surviving T18 sets. One is the console belonging to the ETF and the other is this table set. This set is the same one in Andy Emmerson's 'Old Television' book. Neither of the two sets has any evidence of any protection glass being fitted. How did they get away with it?? The frame timebase is the same as all the pre-war Baird sets. I have attached the circuit from the T5 for the interest of others. The frame transformer (on the right-hand side of the circuit with octal connector) is also the oscillating transformer and the scan coil. The laminations of this transformer have arms that sit either side of the tube neck (shown in attatched photo of T18). Interesting to hear about the T18 on the last day of 405. If you watch the film of TV at Radio Olympia in 1936 you can see a Baird T5 being used in the control room at AP. Cheers Andy Attached Thumbnails

11th Jul 2006, 1:27 pm	#6
Panrock Nonode Join Date: Feb 2005 Location: Worcestershire, UK. Posts: 2,533	Re: Vintage tube substitution in Baird T18 Didn't this set have a so-called 'flame lamp' - a pygmy lamp wired in series with the mains eht transformer primary ? It was supposed to illuminate when there was an overload or short on the eht, and shine red light through the clear wall of the tube onto the back of the screen. It could have been the source of this story, quoted from page 210 of the book 'Here's Looking at You' by Bruce Norman... Cyril Fletcher: After a programme people would ring up. There were so few of them that you would get to know them. The phones were in the corridor outside the studios and on one occasion this old girl rang up and said: 'Oh, it's marvellous tonight, Mr Fletcher, I have enjoyed it. It's all in colour.' And so I said: 'What sort of colour?' She said: 'It's all in marvellous red shades.' And I said quickly: 'Switch off. Your set's on fire!' As, indeed, it was. Steve