For valve circuits I don't trust the voltage handling capacity of resistors from the likes of Jaycar

Is there anybody in Oz these days selling resistors for use in high voltage circuits?

I have warned people about that. the lower wattage resistors are the lower voltage ones. Jaycar did respond to my question re that at one stage.

I will not use below 1 watt on HT. You can get away with lower watt ones on grids & AGC in a pinch, but know the volts.

I did ask EVATCO ... Thus far zero response. You can always ask possible supplier & vote with your feet if the answer fails to appear.

I did see one UK company selling 800V types.

Marc

I have just purchased a bunch of them of various wattages from Just Radios in the USA but it would so much better and faster to be able to get them onshore. With their range of caps, etc, Evatco would be the logical place to go, so I'm surprised they didn't respond. I'll email them and see if I can get a response.

http://www.justradios.com/resistors.html.

Is it an issue of buying 5% tolerance compared with 20% tolerance ones? A resistor carrying 800 volts across it, where it is rated at 1/2 a watt maximum should have a minimum resistamce of 1.28Mohms. The current then would be a maximum of 0.66mA. Given that currents are higher than that in some (most) circuits, then the wattage rating of the resistor dpends what voltage drop you are dealing with, doesn't it? If you want a voltage drop of 100 volts, then a 20K 1/2 watt resistor should be the minimum level to use. 5 mA becomes the maximum current before the 1/2 watt rating is exceeded. However if the resistance can vary by 20% and the resistor's actual resistance is only 15k, then the trouble starts. Current becomes 6.7mA, and i2R gives about 0.7 watts.
I hope my maths is right here guys - don't flame me if I'm wrong.

Wattage is a secondary consideration.

If the dialectric will not stand the voltage across the resistor , its wattage becomes irrelavant, as it will flash over.

That is the primary issue. B+ with Filament rectifiers can get to close to 500V.

A resistor with a circuit allowing it to have that much voltage across it and draw current, will be vulnerable.

Marc

Got that. Thanks for info.