Resistor Replacement in Series With HV Lead

[Polaraligned]

The existing one is a carbon comp. It went down in value 50k, which is an uncommon direction I assume was due to the HV.

I have either a carbon film or a metal oxide in stock. Would one of these work? I only ask because of the high voltage and I know that resistors have voltage ratings. I have no idea how much current the anode draws so I am not able to calculate what the voltage drop may be on this resistor. I am sure the current is very low.

[Yamamaya42]

Originally Posted by Polaraligned

The existing one is a carbon comp. It went down in value 50k, which is an uncommon direction I assume was due to the HV.

I have either a carbon film or a metal oxide in stock. Would one of these work? I only ask because of the high voltage and I know that resistors have voltage ratings. I have no idea how much current the anode draws so I am not able to calculate what the voltage drop may be on this resistor. I am sure the current is very low.

any one will work, as long as they are of the same power rating.

From what I understand, they are not there to give any kind of voltage drop, but put in there to act as a safety device if the anode were to ever to happen to get shorted to ground somehow, to protect the FBT and the rectifier.

[Retrovert]

Originally Posted by Yamamaya42

any one will work, as long as they are of the same power rating.

That is not always true and caution is suggested when using ordinary resistors in high-voltage circuits.

Resistor types have specific voltage limits for particular versions. Film and the spiral-cut metal film typically are not rated for much above 500 VDC, lest they arc over. Tube amplifiers often use the wrong kind and don't blow up, but this is not a ringing endorsement for exceeding the specifications by a factor of 100. Not sayin', just sayin'.

Only specialized resistors are suitable — safe and reliable — for operating in the tens of kiloVolt range. So either a resistor specifically rated for the desired voltage must be selected or it must be constructed using a string of smaller resistors since, given n resistors, the voltage across each resistor is 1/n of the total.

To demonstrate the issue, I dug out some of Vishay's offerings. Any resistor manufacturer would, of course, be suitable.

This page lists all of the Vishary carbon-film resistors. Pick any of the ordinary, i.e. not high-voltage, flavors and look at the PDF specifications. The voltage rating show the resistor is typically rated to about 500 VDC to 600 VDC, sometimes into the low kiloVolts for pulse operation:

www.vishay.com/resistors-fixed/carbon-film/

And here is an example of the high-voltage equivalents good for many tens of kiloVolts:

www.vishay.com/resistors-fixed/high-voltage

So best to order the appropriate resistor and know that it won't blow up. As an aside, many regulatory authorities like UL or CE, won't certify equipment using strings of resistors because the potential for arc-over can be high without proper layout.

[Electronic M]

Retrovert. I don't think you properly grasp how the circuit or ratings work.
The voltage rating of a resistor only matters for the voltage drop across it, and the voltage between the leads and anything else in close proximity to the case.

The voltage drop across the resistor will normally be well below 500V. A typical color CRT firing 3 guns into a shadow mask that blocks %80 of emissions from reaching the phosphor (thus requiring that much more current for a given brightness) draws around 1mA of current. This 12LP4 Black and White CRT in the original posters set has 1/3 the number of guns and no shadow mask so HV current is likely under 100uA....But let's say he wants to REALLY crank the brightness and draw 500uA HV current to give a worst case normal opperation scenario...500uA through a 680K resistor yields (by ohm's law) a 340V drop across that resistor...That's well below the 500V max rating you quoted for average carbon film resistors.

The case is never kept anywhere near ground (only insulators and other conductors at the same 11KV voltage it's leads are connected to.

Normal carbon film resistors are well within their voltage specs in this application in normal opperation.
The only way their specs would be exceeded would be if HV were shorted to ground (a fault condition). The original resistors had a similar rating to the new carbon film parts, and we're meant to burn up and or arc if HV shorts the same way the modern ones will if you short it out.

The way I see it the new parts will be equivalent for the application.

[Yamamaya42]

Originally Posted by Electronic M

Retrovert. I don't think you properly grasp how the circuit or ratings work.
The voltage rating of a resistor only matters for the voltage drop across it, and the voltage between the leads and anything else in close proximity to the case.

The voltage drop across the resistor will normally be well below 500V. A typical color CRT firing 3 guns into a shadow mask that blocks %80 of emissions from reaching the phosphor (thus requiring that much more current for a given brightness) draws around 1mA of current. This 12LP4 Black and White CRT in the original posters set has 1/3 the number of guns and no shadow mask so HV current is likely under 100uA....But let's say he wants to REALLY crank the brightness and draw 500uA HV current to give a worst case normal opperation scenario...500uA through a 680K resistor yields (by ohm's law) a 340V drop across that resistor...That's well below the 500V max rating you quoted for average carbon film resistors.

The case is never kept anywhere near ground (only insulators and other conductors at the same 11KV voltage it's leads are connected to.

Normal carbon film resistors are well within their voltage specs in this application in normal opperation.
The only way their specs would be exceeded would be if HV were shorted to ground (a fault condition). The original resistors had a similar rating to the new carbon film parts, and we're meant to burn up and or arc if HV shorts the same way the modern ones will if you short it out.

The way I see it the new parts will be equivalent for the application.

exactly!

thus the reason I said it was in there as a safety device to protect against shorting the anode to ground, to do as Retrovert said an use a higher rated volt resister would defeat that feature.

think of it a fusistor!

[Polaraligned]

Thanks Tom. As I stated I had no idea how much current the anode would carry therefore was not able to calculate the voltage drop across it. Google was not help trying to find out a typical anode current. It looks like an ordinary Allen Bradley 1/2 watt resistor and the part list doesn't say it is anything special.

[Notimetolooz]

Some resistor manufacturers make lines of resistors now that are smaller than the usual size of resistors of a particular wattage. They are made of materials that can withstand higher temperatures. These unusually small resistors may not withstand the usual voltage for that wattage. Make sure the replacement is similar sized.

[Phil]

Given the choices mentioned in the original post I would use the oxide resistor. Metal film will serve no useful function as it will simply arc over if the 2nd anode lead is grounded. The oxide probably will also but it should be more likely to burn into like the carbon resistor would do. If it were mine I would spend the 25 cents for a carbon resistor to replace it.

[Penthode]

Why change it? You will not notice any performance change. It is only bad if it goes up in value.

[old_tv_nut]

I reread the first post. You say it "went down" 50k. How do you know it went down? Did you measure it as 680k before? I'd guess it's a 10% tolerance 680k resistor that is within tolerance, nothing wrong.

[zeno]

I would use an old school carbon. Its the only thing I remember seeing
in HV & often focus circuits that can run upwards of 7KV.
Note also C93, R120 & the CRT dag make a Pi network.
I wouldnt worry about fire etc. It cant supply the current. Thats why it wont kill
you but lower voltages can.

73 Zeno
LFOD !

[Polaraligned]

I have never measured a 70 year old carbon comp and had it low. They always are at least a bit over value. I assume that the high voltage had something to do with it going down in value, and if so, it will only get worse. I can't think of a good reason not to replace it considering the whole HV cage and rectifier assembly were already apart.

Anyway, I have a raster on this set now. Someone put a 10 watt resistor in series with the factory 6K resistor across the damper tube. The added resistor was open and I can't read the value on it as the print is gone. Any clue why someone would want to increase that value across the damper? Changing this resistor value would probably effect the linearity. It is R118 on the schematic. I just eliminated it and it now matches the schematic. Just scratching my head as to why....I guess I will find out if there are linearity problems someone was trying to fix when I get to that point.