Submini tubes

[SGillespie]

Hi Guys

Steve: Quarter-watt resistors can be used in most positions in a tube amp but there are two parameters to check for certain positions.

In a tube circuit the plate resistor should have a power rating sufficient to withstand the power resulting from the voltage across the part during normal operation. Say Ra=100k and the voltage across it is 150V. Ohm's law and the Power equations suggest a dissipation of 225mW. The instantaneous power and voltage will be higher and lower than the quiescent values, and we are already too close to the maximum for a 250mW part, so go to 400-600mW.

At 100Va the voltage across RA may be only 50V or less, so a 250mW resistor may be ample. You can figure out the R value where you would have to switch to a higher-rated component by dividing the safe idle power into the voltage-squared. For example,  (50x50) / 200mW = 12k5. Pick any power rating that seems reasonabe for the component in question.

All of the other positions in a tube preamp circuit can be 250mW quite easily as the voltage across these is less than a few volts.

The other parameter to check with your resistor purchase is the voltage rating. Yes, resistors have voltage ratings and hardly anyone notices unless they read TUTs

Also, remember that electrolytic cap ratings are WORKING voltage, which means they can be operated reliably and nearly forever at that value. Doing so actually keeps them healthier than derating them.

Have fun

Hey,
Thannks for taking the trouble to answer my questions. If anything it's taught me that sticking to JFETs and BJTs are probably my best bet, unless I can find someone who knows what they're doing to hire for the design work 
Cheers
S