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Old 17th Sep 2018, 8:40 am   #1
Heatercathodeshort
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Default The Emiscope 3/6A 15" CRT 1946

I thought you might like to see in detail the EMI 'EMISCOPE 15" CRT type 3/6A from 1946.

The 3/6A is a very early aluminised CRT. The tube is a very narrow angle with a triode gun. It has the pre war side wire base and stepped diameter neck, exclusive to EMI. It has an overall length of 26.5" It appears to be constructed with PYREX glass, the front face plate ressembling a 15" pie dish welded to the cone. It has a 'lip' as can be seen in one of the pictures.
The data is interesting requiring just 4kv on the final anode but despite this the 15" tube is capable of producing very bright pictures. The heater is the pre war standard 4v at 1.3A. How EMI developed this is remarkable considering an aluminised CRT requires at least 7kv for the beam to penetrate the micro aluminium layer that backs the internal surface of the cone together with the luminescent screen.

It was employed in the immediate post war HMV receivers released in early 1946, the 1803 console and the 1901 tele/radio/gram.

This was basically the pre war chassis the difference being the replacement of the KTZ41 B7 based valves with the post war octal based Z66. These valves were very variable and gave all sorts of odd effects measuring ok on the valve tester. Together with the B36 double triode so infamous in the HMV 1807, MOV certainly had some quality control problems to solve.

The tube became obsolete in 1948 replaced by the TA15 together with a kit that included a new shaped mask to cater for the bulb shape as opposed to the cone of the 3/6A. Also included was a B7B socket suitable for the TA15 and all the odd bolts, screws etc.

There was also a modification to increase the EHT from 4kv to around 7kv by adding the voltage available from the anode of the line output valve [KT44] via an additional rectifier and capacitor to the 4kv mains derived system but the few I encountered when these receivers were at the end of their life, had never had this modification carried out. I think the TA15 gave a good picture at 4kv similar to the 3/6A.

Another odd factor is the stepped neck. It was usual to do this the other way round i.e. reduced at the point were the scanning coils met the cone. Being closer to the beam the scan coils would require less power to scan the tube.

TV development including of course the important CRT made incredible leaps during the decade 1946-56.

I will attempt to test this 3/6A and report results.
John.
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Old 17th Sep 2018, 9:04 am   #2
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Default Re: The Emiscope 3/6A 15" CRT 1946

That's a marvellous (and slightly terrifying....) piece of glassware there- hope it has some go left in it. Seems curious that the setmakers didn't plump for flyback-derived EHT earlier on in designs if there was the awareness that the existing EHT could be bumped up substantially like this- after all, a mains EHT transformer must have been an appreciable chunk of the overall cost. Even if the flyback pulse had to be doubled, the cost of extra rectifier diodes and HV capacitors would surely be offset,

Colin
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Old 17th Sep 2018, 10:03 am   #3
Peter.N.
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Default Re: The Emiscope 3/6A 15" CRT 1946

Was that the tube used in the Cossor with the radio in the top? It looks similar to what I remember. The set also had a huge neck protector on the back.

Peter
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Old 17th Sep 2018, 11:10 am   #4
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Default Re: The Emiscope 3/6A 15" CRT 1946

That is fascinating! I've never seen one before. Do you have a set that uses that tube?

Peter
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Old 17th Sep 2018, 12:36 pm   #5
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Default Re: The Emiscope 3/6A 15" CRT 1946

Quote:
Originally Posted by Peter.N. View Post
Was that the tube used in the Cossor with the radio in the top? It looks similar to what I remember. The set also had a huge neck protector on the back.
Nope, the Cossor one is far more bulbous, scarier and is even longer (IIRC, when I did pluck up the courage to go near the beast, it measured nearer 30 inches).

TTFN,
Jon
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Old 17th Sep 2018, 1:08 pm   #6
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Default Re: The Emiscope 3/6A 15" CRT 1946

Quote:
Originally Posted by peter_scott View Post
That is fascinating! I've never seen one before. Do you have a set that uses that tube?

Peter
Hi Peter,
maybe John is starting on this set...

https://www.vintage-radio.net/forum/...d.php?t=117152

Cheers
Andy
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Old 17th Sep 2018, 2:35 pm   #7
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Default Re: The Emiscope 3/6A 15" CRT 1946

Thanks Andy, that would be good.

Peter
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Old 17th Sep 2018, 4:56 pm   #8
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Default Re: The Emiscope 3/6A 15" CRT 1946

Scary things these early 15" CRTs. Perhaps the most scary of all was the Baird-Cathodevisor 15MW1 that was fitted in the Baird T5 of 1936.
Link to my Baird T23 which has a Cossor 65K 15" CRT.
https://www.vintage-radio.net/forum/...d.php?t=132064
The Emiscope 3/6A soon became obsolete and was was replaced by type TA15, this tube has a slightly wider deflection angle which means certain modifications are required to the timebase circuits to increase scanning power.

The Dumont company in America made an all glass 20" diameter CRT.

DFWB.
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Old 17th Sep 2018, 5:05 pm   #9
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Default Re: The Emiscope 3/6A 15" CRT 1946

Very interesting John.

When I was restoring my Marconi 702, I did not want to risk the CRT, and made a small unit with a modern 5" CRT in it. It is the 14SX3Y4, and is also aluminised.

I tried various EHT voltages, to see from which point the CRT was usable for my experiments. For this tube it turned out that from 3.5 kV upwards a reasonable picture could be obtained.
So 4 kV indeed might be the minimum practical EHT with an aluminised screen.

It would be interesting to see at which EHT the 3/6A would fall off.

Jac
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Old 17th Sep 2018, 7:54 pm   #10
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Default Re: The Emiscope 3/6A 15" CRT 1946

Quote:
Originally Posted by FERNSEH View Post
Scary things these early 15" CRTs. DFWB.
'Scary..scary David? I thought the lads from the North East were a tough lot. I thought it was only us Southerners that were 'Wendy Woo's'

Flyback EHT came in a big way late 1948 by way of the Pye B18T. A very rare receiver, the KB CV40 of 1947 actually arrived earlier but it was a rarely seen and somewhat quirky model.

I have both the HMV 1901 and 1803 models. The 1803 has it's original tube and produced a bright folded up mess of a raster when tried on first acquisition many years ago. It is very clean and originated from a World famous surgeon that carried out his skills at the Middlesex hospital. It was presented to him upon his retirement in 1946.

The 1901 has a very long story only suitable for another thread, given time that is.

I Will do an article on the 1803 when time allows. Once I start on it I'm sure it will be fun.

I have checked the 3/6A. It has a dead heater/cathode short [not common, usually an intermittent leak]easily overcome with an isolating transformer but it does have a fair bit of emission, more than enough I hope for presentable picture. I will stuff a bit of EHT on it's anode and see how bright a 'blob' we can obtain.

Regards, John.
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Old 17th Sep 2018, 7:55 pm   #11
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Default Re: The Emiscope 3/6A 15" CRT 1946

Quote:
Originally Posted by FERNSEH View Post
Scary things these early 15" CRTs. Perhaps the most scary of all was the Baird-Cathodevisor 15MW1 that was fitted in the Baird T5 of 1936.
I think I've told the story before of how the tube in the BBC Wood Norton T5 seemed to be jammed in its neck collar. I had to get it out so grabbed under the bowl and h-e-a-v-e-d as directly upwards as I could. Fortunately it shifted!


Steve
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Old 17th Sep 2018, 10:18 pm   #12
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Default Re: The Emiscope 3/6A 15" CRT 1946

Quote:
Originally Posted by Heatercathodeshort View Post
Quote:
Originally Posted by FERNSEH View Post
Scary things these early 15" CRTs. DFWB.
'Scary..scary David? I thought the lads from the North East were a tough lot. I thought it was only us Southerners that were 'Wendy Woo's'

Regards, John.
Ha ha us Cornish-men like to think of Northerners as just being more dispensable after all there are just loads of em

I hate to think of the number of tubes I chucked down mine shafts by the necks in the early 70's around the Liskeard area it was in the hundreds and I was still at school.

Cheers

Mike T
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Old 17th Sep 2018, 11:51 pm   #13
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Default Re: The Emiscope 3/6A 15" CRT 1946

That's fascinating, John:- thanks for posting about it. Your experiences from 'back in the day' are always interesting to read and give an extra dimension to the sets we are restoring.
Please keep them coming!!
Cheers
Nick
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Old 18th Sep 2018, 5:38 am   #14
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Default Re: The Emiscope 3/6A 15" CRT 1946

Wow !! That CRT is a beautiful combination of science and art. Such a creation will never be repeated. What a total gem.
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Old 18th Sep 2018, 5:50 am   #15
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Default Re: The Emiscope 3/6A 15" CRT 1946

I gave a couple of similar tubes to BVWTVM in 1992. Both were in wooden crates with the
tube supported in a elasticated webbing arrangement.
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Old 18th Sep 2018, 8:04 am   #16
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Default Re: The Emiscope 3/6A 15" CRT 1946

I would have very much like to have seen those. We need a TA15 for an HMV 1851. I have tried a Mazda CRM151 but the EHT is just too low and the mechanical fixing is very different. I thought I had struck lucky with a 3/32 the AC/DC version but it was utterly and completely flat! Oh well, some day my tube will come..

I didn't get involved with the museum on a regular basis until around 2002 and didn't see those tubes around. Pity. Regards, John.

PS I will have a play with the 3/6A at the weekend if my curiosity can last that long.
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Old 18th Sep 2018, 9:52 am   #17
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Default Re: The Emiscope 3/6A 15" CRT 1946

The first time I saw the Cossor tube it terrified me, I felt that if I went anywhere near it the neck would break off. Mind you I did repair one or two of them so I must have overcome my fear.

Peter
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Old 22nd Sep 2018, 5:15 pm   #18
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Default Re: The Emiscope 3/6A 15" CRT 1946

With 4V on the heater and 4kv applied to the anode, this quite bright blob appeared. This CRT would certainly give a viewable picture. John.
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Old 22nd Sep 2018, 9:13 pm   #19
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Default Re: The Emiscope 3/6A 15" CRT 1946

Does this mean we can all look forward to you restoring another old television ?

Robin
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Old 23rd Sep 2018, 3:56 pm   #20
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Default Re: The Emiscope 3/6A 15" CRT 1946

That's great.

I hope to see what it will be going in.

Cheers
Andy
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