Bottlehead Forum

General Category => Technical topics => Topic started by: vetmed on January 17, 2014, 09:20:17 AM

Title: Resistor voltage ratings
Post by: vetmed on January 17, 2014, 09:20:17 AM

I have a resistor rated 350 volts and would like to use it as a plate resistor where the B+ is ~375 volts. This would yield a plate voltage of 130 volts for a drop of 245 volts. Am I OK or should I look for a resistor with a higher voltage rating? TIA

Robert Lees

Title: Re: Resistor voltage ratings
Post by: Grainger49 on January 17, 2014, 11:30:54 AM

When you start you will see more than 375V to ground.  But... the resistor will not have any current through it.  So it will not have any voltage across it.

The key is when the tube starts to conduct.  Does the resistor ever see in excess of 350V across it. 

And I doubt it.  Give it a try.  Worst case is you could waste a resistor.

Title: Re: Resistor voltage ratings
Post by: vetmed on January 17, 2014, 11:45:31 AM

Thanks Grainger, that's what I thought but I wasn't 100 % sure :) BTW this design uses a 5AR4 rectifier so the B+ comes up slowly and will never see excessive voltage so no need to worry about that case.

Regards
   Robert Lees

Title: Re: Resistor voltage ratings
Post by: Paul Joppa on January 17, 2014, 02:14:40 PM

We've had instances where resistors have failed near their rated voltage. Some sources say that they can get nonlinear above 70% of their rated voltage. The simplest thing is to use two resistors in series.

Remember, the plate resistor will see nearly all the B+ when the tube is driven to clipping by transients.