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Resistor voltage ratings
Resistor voltage ratings
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Old 6th March 2018, 11:24 PM   #1
Marksd is offline Marksd  United States
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Default Resistor voltage ratings

I have a beginner resistor voltage rating question. I have a Dynaco Pas 3 and ST 70. I rebuilt the ST70 with a Dynakit board and parts and it works great. I bought replacement boards for the PAS 3. While resistor shopping for the PAS 3 I noticed the working voltage of the resistors I ordered are rated for 350 volts. I bought all the original correct wattage values in metal film resistors, but are at the 350 volt rating. I checked carbon resistors and they too are rated at 350 volts. How can these possibly work in an the PAS 3 where B+ is 405 volts or so and how did these work in the ST70? And I need to add that I soldered the resistors into the boards already before I noticed this 350 volt working voltage rating. Thanks to this community for any help with this.
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Old 6th March 2018, 11:44 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by Marksd View Post
I bought all the original correct wattage values in metal film resistors, but are at the 350 volt rating.
How can these possibly work in an the PAS 3 where B+ is 405 volts and in the ST70?
That's a typical voltage rating for most such resistors. In actual operation, the voltage across them
is much less than the power supply voltage. The resistor with the most voltage across it in these units
is the ST70's 330k, which has one end at 305VDC, and the other end at 1VDC, but this is still within its rating.

Last edited by rayma; 6th March 2018 at 11:52 PM.
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Old 7th March 2018, 03:02 AM   #3
BillWojo is offline BillWojo  United States
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Interesting, so it's not the voltage it "see's" but the voltage it drops that determines the voltage rating if I read that correctly.

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Old 7th March 2018, 07:03 AM   #4
Alllensoncanon is offline Alllensoncanon  United States
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Resistor voltage ratings
The voltage it see is the voltage it drop. It take two ends (points) to define a voltage potential.
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Old 7th March 2018, 10:42 AM   #5
DF96 is offline DF96  England
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You may need to watch out for any resistors which have more voltage across them when the valves are cold. Exceeding the voltage rating does not necessarily make the resistor fail straight away, but it may gradually become noisy and intermittent before failing a few months later. Using a higher power rating may get you a higher voltage rating too.

Like everything, a resistor cannot see a voltage - it just sees a voltage difference between two points. (Yes, I do mean "everything": there is no such thing as 'a voltage').
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Old 7th March 2018, 11:38 AM   #6
Marksd is offline Marksd  United States
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All thanks for all your responses and input. I have checked all the resistors on the working PAS 3 voltage drop and they are all below the working voltage and wattage rating. I had a hard time understanding this as I was reading articles and notes from Vishay, ohmite and others.
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Old 7th March 2018, 01:40 PM   #7
Lingwendil is offline Lingwendil  United States
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Resistor voltage ratings
Good rule of thumb for most resistors, capacitors, etc used in tube electronics is to rate them for at least the peak voltage that the transformer can supply (remember, peac AC can be much higher than the DC voltage before smoothing, choke input, etc) except in certain circumstances where you are really sure that it will ever only see low voltages (cathode bypass components on gain stages, for example) and even then a good bit of headroom is recommended to account for variations in line voltage or parts tolerance.
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Old 7th March 2018, 02:00 PM   #8
kodabmx is offline kodabmx  Canada
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I find if you buy cheap parts you need to double rate them, and if you buy quality parts you have some room built into the design.

2W metal film resistors by Stackpole are the size of 1W generic Chinese resistors from eBay, but they will actually handle 2W, where as the 1W resistor will discolour from overheating at 1W (but still hold the correct value). Also a 350V resistor won't arc over at 351V, there is a sizeable safety margin built in.
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Old 7th March 2018, 02:01 PM   #9
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by DF96 View Post
Like everything, a resistor cannot see a voltage - it just sees a voltage difference between two points. (Yes, I do mean "everything": there is no such thing as 'a voltage').
Ohhh really?? You never had a DMM like this one, then?
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Last edited by jan.didden; 7th March 2018 at 02:05 PM.
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Old 7th March 2018, 02:35 PM   #10
DF96 is offline DF96  England
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No, never had one of those. It seems that some people here are proud owners of one, though.

Interesting that you see the inverse issue on RF/antenna websites: people believe that you can input a current with just one connection.
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