Pre war CRT photo - UK Vintage Radio Repair and Restoration Discussion Forum

ppppenguin · 8th Dec 2006, 5:54 pm

This photo was taken in 1986 at Display Electronics in Uxbridge during the rebuild of an EMI 6/6 CRT. The man is Charlie Bradley, a highly skilled glassblower, who worked at EMI before the war. The gun assembly has been removed from the 6/6 and is sitting separately between the 2 CRTs. The 6/6 has been rough pumped and temporarily sealed because it was going to be some time before the gun was rebuilt. It looks like Charlie had done the graded seal, a tricky bit of glassblowing, to interface between the Pyrex of the 6/6 and the soft glass of the modern pinch that would be fitted. The modern CRT is, I think, an A63-11X 25" colour showmask tube.

ppppenguin · 8th Dec 2006, 6:30 pm

This photo was used in my article in September 1987 Television. This article describes the re-gunning of the 6/6 used in the HMV900 in the Thorn-EMI Central Research Labs museum.

The CRT wasn't re-screened. The old phosphor was in pretty good condition and Display Electronics had little experience of re-screening. There was no significant ion burn. The tube worked very well indeed, probably the brightest, sharpest 6/6 I have ever seen. Though one of your sets must be comparable.

There were a few heart stopping moments. One of the getters fell off when it was fired. Luckily it landed somewhere harmless. There was a nasty high voltage sound when we put the tube back in the set. This turned out to be a breakdown between the EHT and 4V windings on the mains transformer. A separate 4V supply sorted that. When we got the set running the picture was dim and soft. A lot of people had done a lot of work to get it that far and this was a big disappointment. An hour's running gave the cathode the extra activation that it needed and the reward was a superb picture.

ppppenguin · 8th Dec 2006, 8:03 pm

AFAIK Charlie Bradley is no longer with us and Display Electronics no longer exists. At present there is nobody in the UK or elsewhere in the world who can rebuild these early Pyrex CRTs. One of the big problems is the Pyrex. This needs specialist glassblowing skills which are not found at CRT rebuilders who haven't had to touch it for over 50 years. The glassblowers who do know about Pyrex and graded seals can probably only be found in university physics departments, assuming they still employ high grade technicians. Possibly also in organisations like CERN and JET where I'm sure they have to do a lot of work with vacuum in glass.

There is certainly one company in the USA which will, at a price, rebuild early CRTs but they won't touch Pyrex.

I hope that even the dud tubes will be kept. One day somebody may be able to rebuild them.

Steve_McVoy · 8th Dec 2006, 8:16 pm

In the 1980s a U.S. company successfully rebuilt a 9AP4 using the graded glass approach.

A company in the U.S. recently spent a great deal of effort trying to rebuild a 12AP4 (Pyrex) tube. They found glass for the tubing, but gave up when they couldn't find a stem that had characteristics close enough to the Pyrex. A custom stem would cost several thousand dollars in tooling cost, and a large minimum order.

For now, the prospects of rebuilding this type of tube is bleak. We are still exploring other options, though.

FERNSEH · 8th Dec 2006, 10:20 pm

Quote:

Originally Posted by Panrock

Wouldn't it be more practicable to have a new tube hand-blown from scratch ?

I've thought about that as well.

In the early 1990s Display Electronics repaired two Emiscope 3/2 CRTs for me. The 3/2 is the 7" CRT which is fitted into the Marconi 707 and the HMV 905. One tube was a total success but the other is prone to internal flashover.
For the repair to the gun assembly the graded glass technique was employed. The gun mount is soft glass and then three types of graded glass tube are used to match the remainder of the CRT neck which is Pyrex hard glass.

I know that the graded glass job was not an easy task even for experts.

There is a company in the United States called Hawkeye who undertake certain old type CRT rebuilds. I'm not sure if they do hard glass tubes.

Of special interest is a replacement for the RCA 12AP4 CRT using a Brimar radar tube. For all intents and purposes the Brimar tube has similar dimensions to the 12AP4. Visit: http://earlytelevision.org/12ap4_substitute.html

The 12AP4 is employed the RCA TRK12 and TRK120 TV sets.

DFWB.

8th Dec 2006, 5:54 pm	#1
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Pre war CRT photo This photo was taken in 1986 at Display Electronics in Uxbridge during the rebuild of an EMI 6/6 CRT. The man is Charlie Bradley, a highly skilled glassblower, who worked at EMI before the war. The gun assembly has been removed from the 6/6 and is sitting separately between the 2 CRTs. The 6/6 has been rough pumped and temporarily sealed because it was going to be some time before the gun was rebuilt. It looks like Charlie had done the graded seal, a tricky bit of glassblowing, to interface between the Pyrex of the 6/6 and the soft glass of the modern pinch that would be fitted. The modern CRT is, I think, an A63-11X 25" colour showmask tube. Attached Thumbnails Last edited by Mike Phelan; 4th Jun 2007 at 4:23 pm. Reason: Merged attachment with text

8th Dec 2006, 6:30 pm	#2
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Pre war CRT photo This photo was used in my article in September 1987 Television. This article describes the re-gunning of the 6/6 used in the HMV900 in the Thorn-EMI Central Research Labs museum. The CRT wasn't re-screened. The old phosphor was in pretty good condition and Display Electronics had little experience of re-screening. There was no significant ion burn. The tube worked very well indeed, probably the brightest, sharpest 6/6 I have ever seen. Though one of your sets must be comparable. There were a few heart stopping moments. One of the getters fell off when it was fired. Luckily it landed somewhere harmless. There was a nasty high voltage sound when we put the tube back in the set. This turned out to be a breakdown between the EHT and 4V windings on the mains transformer. A separate 4V supply sorted that. When we got the set running the picture was dim and soft. A lot of people had done a lot of work to get it that far and this was a big disappointment. An hour's running gave the cathode the extra activation that it needed and the reward was a superb picture.

8th Dec 2006, 8:03 pm	#3
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Pre war CRT photo AFAIK Charlie Bradley is no longer with us and Display Electronics no longer exists. At present there is nobody in the UK or elsewhere in the world who can rebuild these early Pyrex CRTs. One of the big problems is the Pyrex. This needs specialist glassblowing skills which are not found at CRT rebuilders who haven't had to touch it for over 50 years. The glassblowers who do know about Pyrex and graded seals can probably only be found in university physics departments, assuming they still employ high grade technicians. Possibly also in organisations like CERN and JET where I'm sure they have to do a lot of work with vacuum in glass. There is certainly one company in the USA which will, at a price, rebuild early CRTs but they won't touch Pyrex. I hope that even the dud tubes will be kept. One day somebody may be able to rebuild them.

8th Dec 2006, 8:16 pm	#4
Steve_McVoy Pentode Join Date: Feb 2004 Location: Columbus, Ohio, USA Posts: 150	Re: Pre war CRT photo In the 1980s a U.S. company successfully rebuilt a 9AP4 using the graded glass approach. A company in the U.S. recently spent a great deal of effort trying to rebuild a 12AP4 (Pyrex) tube. They found glass for the tubing, but gave up when they couldn't find a stem that had characteristics close enough to the Pyrex. A custom stem would cost several thousand dollars in tooling cost, and a large minimum order. For now, the prospects of rebuilding this type of tube is bleak. We are still exploring other options, though.