Importance of CRT EHT voltage - UK Vintage Radio Repair and Restoration Discussion Forum

ben · 7th Mar 2016, 1:29 am

I was wondering, when studying the multiple versions of a TV chassis, how critical the voltage of the EHT supply to the anode is.

Years ago I once made a 'frankenstein' telly from two scrapped sets of the same model. I think I used a 20" chassis to drive a 24" tube ( not sure on exact sizes, it was long ago now) and the raster was basically reduced. Has anyone ever tried it the other way round ?

For example, one version of the chassis for a fairly common 4:3 28" tube has a line output transformer supplying something like 23.3 kV, while another one (used with a flatter tube) had an output of some 30kV. I think that tube was actually a bit smaller as well (25").

Presumably, if one were to use the second transformer with the first tube, the higher EHT (some 7kV higher) would result in spark gap action, or other unpleasant things. I have never tried 'overdriving' a tube in this way and am curious as to what would happen. In the old days of mains-dervied EHT I suppose there must have been the need of some range in that department to cope with fluctuations, spikes, etc.

Can't seem to find any literature on the subject of EHT voltage ranges online. Any thoughts?

Radio Wrangler · 7th Mar 2016, 2:09 am

Higher EHT means that the electrons in the beam are accelerated to a higher velocity, so you get more energy hitting the phosphor for the same cathode current. In a larger tube the cathode current has to be spread over a larger area, so the increased EHT compensates for the reduction in brightness, and allows a proven gun assembly to still be used.

Higher EHT means that the electrons are going faster through the volume in which the deflection fields are active, so the lateral acceleration produced is the same, but it has less time to act, so deflection angles are scaled down. Jigher EHT makes a smaller picture So we need more powerful deflection systems to get the same deflection angle, and much more deflection power if that higher EHT goes with a larger deflection angle and you want to avoid a longer tube.

The electrons may be faster, but as the beam current is the same, the beam experiences the same lateral force. It's the time he force acts for that makes life harder.

If EHT is badly regulated, the voltage falls on brighter pictures and this causes the size of the picture to 'breathe' in sympathy with the scene being shown.

David

Wendymott · 8th Mar 2016, 1:15 pm

I totally agree with David, however there is one more important thing to worry about... CRT Heater voltage. If you have changed the scan coils (horizontal) as part of the CRT assy, the inductance will affect the overall efficiency of the Line stage (EHT) and ultimately the heater voltage. A number of years ago (1980's) I was involved in TV manufacture, where the kit supplier (LG) used a different LOPTx from the sample, allowing the heater voltage to rise to 13.5V rms. This cost us a LOT, not just in monetary terms but our good name. When I evaluated replacement CRT's, I needed to balance the EHT, Width, Heater voltage (S) correction.

Measuring the heater voltage is very tricky, you cannot use a "True RMS" meter, as the voltage is a pulse going one way. True RMS meters need a symmetrical waveform, not necessarily a sine wave but symmetrical.
If the CRT change is just an exercise then it may not matter, but if the TV is to be used over a longer period, be aware the heater voltage, thus "Life of the CRT" may be compromised.

Nickthedentist · 8th Mar 2016, 1:53 pm

Quote:

Originally Posted by Wendymott

Measuring the heater voltage is very tricky, you cannot use a "True RMS" meter, as the voltage is a pulse going one way. True RMS meters need a symmetrical waveform, not necessarily a sine wave but symmetrical.

Couldn't the old trick of a small lamp (of approximately the same V rating as the original CRT heater) shining on a solar cell or LDR, be used to "measure" it, by comparing its brightness when the TV is driving the original and replacement tubes in turn?

Radio Wrangler · 8th Mar 2016, 3:34 pm

Proper true RMS meters can handle asymmetric waveforms correctly. The HP3400 (and onwards) family buffer the signal and use a pair of matched thermal converters to drive their meters. You're heating a resistor and measuring how warm it gets. 10Hz to 10MHz and beyond bandwidth ought to be enough.

You can do the same thing by running a light bulb of the right voltage and matching against a similar one running DC. The meters just put this in a pretty box and optimise the aaccuracy.

David

Leon Crampin · 8th Mar 2016, 4:07 pm

I've done this with CRTs and EHT rectifiers by using the light meter in an SLR camera to evaluate heater temperature. The old Pentax Spotmatic was just the ticket for this.

Leon.

Wendymott · 9th Mar 2016, 12:57 am

David...... we were poor.... and could not afford the "high end" metering stuff.....I stand corrected re the methods. I have posted before, our solution which was to drive a small lamp via an Op amp to provide high impedance input.... then an opto resistor and meter driver... Calibrated with a DC supply...... still have one if anyone wants it....

Radio Wrangler · 9th Mar 2016, 1:31 am

There have been some DIY designs for using bulbs in bolometer bridges for DC-substitution RF power meters that could be extended down in freq,,, and no sign of HP prices. There was one in radcom, I remember... not sure when.

David

McMurdo · 9th Mar 2016, 1:37 am

I've seen a few warning notices about X-rays when driving CRTs above their design EHT!

MotorBikeLes · 9th Mar 2016, 4:00 pm

When I was first learning about CTV, I was asked to look at an early Korting. It had PL509 in both scan and EHT stage. The pic was small and quite a bit of fizzing going on.
I had just acquired a new GEC EHT probe to go with my NOS GEC Selectest, which announced 31KV. Rather more than the correct 25KV. I had read all the Xray warnings, so just switched of and advised customer to either scrap it, or go to an authorised Korting service dept. (there were some back then, as they were imported locally).
Les.

7th Mar 2016, 1:29 am	#1
ben Dekatron Join Date: Aug 2006 Location: Madrid, Spain / Wirral, UK Posts: 7,498	Importance of CRT EHT voltage I was wondering, when studying the multiple versions of a TV chassis, how critical the voltage of the EHT supply to the anode is. Years ago I once made a 'frankenstein' telly from two scrapped sets of the same model. I think I used a 20" chassis to drive a 24" tube ( not sure on exact sizes, it was long ago now) and the raster was basically reduced. Has anyone ever tried it the other way round ? For example, one version of the chassis for a fairly common 4:3 28" tube has a line output transformer supplying something like 23.3 kV, while another one (used with a flatter tube) had an output of some 30kV. I think that tube was actually a bit smaller as well (25"). Presumably, if one were to use the second transformer with the first tube, the higher EHT (some 7kV higher) would result in spark gap action, or other unpleasant things. I have never tried 'overdriving' a tube in this way and am curious as to what would happen. In the old days of mains-dervied EHT I suppose there must have been the need of some range in that department to cope with fluctuations, spikes, etc. Can't seem to find any literature on the subject of EHT voltage ranges online. Any thoughts? __________________ Regards, Ben.

7th Mar 2016, 2:09 am	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: Importance of CRT EHT voltage Higher EHT means that the electrons in the beam are accelerated to a higher velocity, so you get more energy hitting the phosphor for the same cathode current. In a larger tube the cathode current has to be spread over a larger area, so the increased EHT compensates for the reduction in brightness, and allows a proven gun assembly to still be used. Higher EHT means that the electrons are going faster through the volume in which the deflection fields are active, so the lateral acceleration produced is the same, but it has less time to act, so deflection angles are scaled down. Jigher EHT makes a smaller picture So we need more powerful deflection systems to get the same deflection angle, and much more deflection power if that higher EHT goes with a larger deflection angle and you want to avoid a longer tube. The electrons may be faster, but as the beam current is the same, the beam experiences the same lateral force. It's the time he force acts for that makes life harder. If EHT is badly regulated, the voltage falls on brighter pictures and this causes the size of the picture to 'breathe' in sympathy with the scene being shown. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

8th Mar 2016, 1:15 pm	#3
Wendymott Octode Join Date: Jun 2013 Location: Huddersfield, West Yorkshire, UK. Posts: 1,795	Re: Importance of CRT EHT voltage I totally agree with David, however there is one more important thing to worry about... CRT Heater voltage. If you have changed the scan coils (horizontal) as part of the CRT assy, the inductance will affect the overall efficiency of the Line stage (EHT) and ultimately the heater voltage. A number of years ago (1980's) I was involved in TV manufacture, where the kit supplier (LG) used a different LOPTx from the sample, allowing the heater voltage to rise to 13.5V rms. This cost us a LOT, not just in monetary terms but our good name. When I evaluated replacement CRT's, I needed to balance the EHT, Width, Heater voltage (S) correction. Measuring the heater voltage is very tricky, you cannot use a "True RMS" meter, as the voltage is a pulse going one way. True RMS meters need a symmetrical waveform, not necessarily a sine wave but symmetrical. If the CRT change is just an exercise then it may not matter, but if the TV is to be used over a longer period, be aware the heater voltage, thus "Life of the CRT" may be compromised. __________________ Should get out more. Regards Wendy G8BZY Last edited by Wendymott; 8th Mar 2016 at 1:17 pm. Reason: Typo

8th Mar 2016, 3:34 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: Importance of CRT EHT voltage Proper true RMS meters can handle asymmetric waveforms correctly. The HP3400 (and onwards) family buffer the signal and use a pair of matched thermal converters to drive their meters. You're heating a resistor and measuring how warm it gets. 10Hz to 10MHz and beyond bandwidth ought to be enough. You can do the same thing by running a light bulb of the right voltage and matching against a similar one running DC. The meters just put this in a pretty box and optimise the aaccuracy. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

8th Mar 2016, 4:07 pm	#6
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: Importance of CRT EHT voltage I've done this with CRTs and EHT rectifiers by using the light meter in an SLR camera to evaluate heater temperature. The old Pentax Spotmatic was just the ticket for this. Leon.

9th Mar 2016, 12:57 am	#7
Wendymott Octode Join Date: Jun 2013 Location: Huddersfield, West Yorkshire, UK. Posts: 1,795	Re: Importance of CRT EHT voltage David...... we were poor.... and could not afford the "high end" metering stuff.....I stand corrected re the methods. I have posted before, our solution which was to drive a small lamp via an Op amp to provide high impedance input.... then an opto resistor and meter driver... Calibrated with a DC supply...... still have one if anyone wants it.... __________________ Should get out more. Regards Wendy G8BZY

9th Mar 2016, 1:31 am	#8
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: Importance of CRT EHT voltage There have been some DIY designs for using bulbs in bolometer bridges for DC-substitution RF power meters that could be extended down in freq,,, and no sign of HP prices. There was one in radcom, I remember... not sure when. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

9th Mar 2016, 1:37 am	#9
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,274	Re: Importance of CRT EHT voltage I've seen a few warning notices about X-rays when driving CRTs above their design EHT! __________________ Kevin

9th Mar 2016, 4:00 pm	#10
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Importance of CRT EHT voltage When I was first learning about CTV, I was asked to look at an early Korting. It had PL509 in both scan and EHT stage. The pic was small and quite a bit of fizzing going on. I had just acquired a new GEC EHT probe to go with my NOS GEC Selectest, which announced 31KV. Rather more than the correct 25KV. I had read all the Xray warnings, so just switched of and advised customer to either scrap it, or go to an authorised Korting service dept. (there were some back then, as they were imported locally). Les.