CRTs with low EHT requirement. - UK Vintage Radio Repair and Restoration Discussion Forum

TNC · 25th Mar 2006, 11:09 am

Just wondering if anyone can tell me what influences the EHT requirement of a CRT of a given screen size?

I am thinking of the Emiscope TA15 (used in the HMV 1805 and others) which has a screen size of 15 inches yet only required 7kv.

Is it just a function of the deflection angle/scan power equation or is there more to it than that.

Given the scarcity of TA15s (and associated early big screen CRTs) it's an added complication when restoring sets with defunct large-screen CRTs.

Grateful for any observations.
Trevor.

Heatercathodeshort · 25th Mar 2006, 5:39 pm

Hello Trevor,
The tube you mention in your thread, the TA15 is the big brother of the smaller 10" TA10. This pair were unique as they were designed to replace a range of earlier EMI tubes. They were supplied with a 'fitting kit' that comprised a base assembly, crt heater transformer and a 'EHT BOOST' circuit that increased the EHT from 5kv to around 6.5kv.
The two tubes were rather special in that they were aluminized and would normally require around 9kv to give a bright picture. The sets that these were fitted in produced around 5-6kv of MAINS DERIVED EHT. This was about the safe and reliable [?] limit of a mains transformer derived high voltage system. The TA10/15 produced a super bright picture that would 'light the room' under normal viewing conditions and that with only around 5kv. They were just about the best EMI tubes produced and hold up well emission wise. It was not difficult for later sets fitted with RF/Flyback eht to produce around 10-12kv with ease.
Most early sets timebases were operating on the edge of technology. No Ferrite cores or high slope output valves here...! The tubes were very narrow angle [about 50 degrees] requiring a low scanning power to produce a full size picture. The maximum EHT available was as stated around 5-6kv and even if this could have been increased the timebases would not have been able to scan the tube. An increase in EHT results in a 'stiffened up' electron beam requiring more scanning power to deflect the electron stream.
It was very much a compromise cleverly engineered by the designers of the day. EMI certainly came up with a winner when they designed the TA10/15.
Their early attempt at flyback EHt was a disaster [HMV 1807] and they were one of the very last to use a mains derived system in the 1805 model range. Regards. John.

Duke_Nukem · 25th Mar 2006, 11:12 pm

Quote:

I am thinking of the Emiscope TA15 (used in the HMV 1805 and others)...

The 1805 used the smaller TA10.

I've heard mixed reports about these EMI tubes, such as their having a tendancy to suffer from seondary emission problems which ruined the picture contrast. I wonder if this was as a consequence of producing a design that certainly seems to have produced a picture brighter than anyone else acheived with such a weenie EHT.

TTFN,
Jon

Heatercathodeshort · 26th Mar 2006, 10:00 am

Hello again Jon,
The TA10 and TA15 were identical other than the screen size. They were a very special pair of tubes designed by EMI and did not suffer the horrible secondary emission problems of the 3/16 tube used in the 1807. The screens are a true white and of high brightness. Most of the surviving tubes must by now be somewhat tired and a true comparison is difficult. My 1803 certainly produced a picture that was much brighter than a good condition Mazda CRM151 operated at a higher eht.They are made from PYREX glass and therefore vacuum should still be good.
The 15" version gave a bright picture and was of very good definition and focus. Unfortunately I smashed up a 1803 15" model in the early 70's that was fitted with the TA15. It was not the original tube and had been fitted as a replacement for the 3/5 [?] it was a long time ago. One of the last old time sets I repaired for a customer was a 1806 that was the 15" version. It was installed in a huge house on Clapham Common Northside and was fitted with a band 3 converter. The owners were very odd to say the least but the picture was very good! Regards John.

25th Mar 2006, 11:09 am	#1
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	CRTs with low EHT requirement. Just wondering if anyone can tell me what influences the EHT requirement of a CRT of a given screen size? I am thinking of the Emiscope TA15 (used in the HMV 1805 and others) which has a screen size of 15 inches yet only required 7kv. Is it just a function of the deflection angle/scan power equation or is there more to it than that. Given the scarcity of TA15s (and associated early big screen CRTs) it's an added complication when restoring sets with defunct large-screen CRTs. Grateful for any observations. Trevor.

25th Mar 2006, 5:39 pm	#2
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: CRTs with low EHT requirement. Hello Trevor, The tube you mention in your thread, the TA15 is the big brother of the smaller 10" TA10. This pair were unique as they were designed to replace a range of earlier EMI tubes. They were supplied with a 'fitting kit' that comprised a base assembly, crt heater transformer and a 'EHT BOOST' circuit that increased the EHT from 5kv to around 6.5kv. The two tubes were rather special in that they were aluminized and would normally require around 9kv to give a bright picture. The sets that these were fitted in produced around 5-6kv of MAINS DERIVED EHT. This was about the safe and reliable [?] limit of a mains transformer derived high voltage system. The TA10/15 produced a super bright picture that would 'light the room' under normal viewing conditions and that with only around 5kv. They were just about the best EMI tubes produced and hold up well emission wise. It was not difficult for later sets fitted with RF/Flyback eht to produce around 10-12kv with ease. Most early sets timebases were operating on the edge of technology. No Ferrite cores or high slope output valves here...! The tubes were very narrow angle [about 50 degrees] requiring a low scanning power to produce a full size picture. The maximum EHT available was as stated around 5-6kv and even if this could have been increased the timebases would not have been able to scan the tube. An increase in EHT results in a 'stiffened up' electron beam requiring more scanning power to deflect the electron stream. It was very much a compromise cleverly engineered by the designers of the day. EMI certainly came up with a winner when they designed the TA10/15. Their early attempt at flyback EHt was a disaster [HMV 1807] and they were one of the very last to use a mains derived system in the 1805 model range. Regards. John.

26th Mar 2006, 10:00 am	#4
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: CRTs with low EHT requirement. Hello again Jon, The TA10 and TA15 were identical other than the screen size. They were a very special pair of tubes designed by EMI and did not suffer the horrible secondary emission problems of the 3/16 tube used in the 1807. The screens are a true white and of high brightness. Most of the surviving tubes must by now be somewhat tired and a true comparison is difficult. My 1803 certainly produced a picture that was much brighter than a good condition Mazda CRM151 operated at a higher eht.They are made from PYREX glass and therefore vacuum should still be good. The 15" version gave a bright picture and was of very good definition and focus. Unfortunately I smashed up a 1803 15" model in the early 70's that was fitted with the TA15. It was not the original tube and had been fitted as a replacement for the 3/5 [?] it was a long time ago. One of the last old time sets I repaired for a customer was a 1806 that was the 15" version. It was installed in a huge house on Clapham Common Northside and was fitted with a band 3 converter. The owners were very odd to say the least but the picture was very good! Regards John.