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.
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.
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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').
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').
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.
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.
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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I started the same thread almost 10 years ago.
Importance of resistor working voltage spec?
2 things I learned from that thread: a lot of DIY builders ignore resistor voltage rating, and the DIY designs where you nothing lower than 1W rating on resistors are because the builder/designer does not ignore the voltage rating.
Importance of resistor working voltage spec?
2 things I learned from that thread: a lot of DIY builders ignore resistor voltage rating, and the DIY designs where you nothing lower than 1W rating on resistors are because the builder/designer does not ignore the voltage rating.
if you work whole life with µC and semiconductors, you will never cross the 300-500V max rating
but with tubes.. yikes
df96...One thing I did with my tube equipment was add a thermistor in the AC line to keep the inrush voltage lower at cold turn on. Where I live my AC line voltage caries from 114.5 VAC to 122 VAC. I think most of this spread is due to A/C loading and other such demands in the summer and furnaces in the winter. I also set my bias voltages so that my ST70 amps steady state at about 40MA per tube instead of 50MA. This way I know whether the AC line is high or low I will always be under the 50MA plate current.
Let's lash-together an iPod, your Dyna, and a speaker. Oh, and an isolated battery inverter supply. Get a well-insulated stick (Do Not Try This At Home!!) and hook it onto the 13,000V wire at the end of your street. The one the birds sit on. Everything is fine.
Inside the Dyna, most resistors have one foot on a 250V line and the other foot on the head of a 12AX7 dropping 150v. So it only "feels" 100V.
Violating the 350V rating is usually not quickly fatal. 1,000V on a 1/8W part, yes, a little surge may start an arc and burn it up. 500V on a 1/2W part sits quietly but drifts in value over the years. Truly high-voltage guitar preamps often use 2W-rated parts for plate resistors for the better voltage drift, even when the happy-amp dissipation is under 1/4W.
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