Univox U-305R Power Resistor Rating

[Deric]

I'm working on a Univox U-305R.  Amp mostly matches the schematic with a couple of exceptions.  The biggest being a 20K resistor that is parallel with second filter cap.  The resistor is mounted to the chassis and connected directly to the power tube screens and ground.  The 5K 10W resistor after the first filter cap was cooked (along with a lot of other issues).  Amp is up and running and sounds fine but the new 5K 10W resistor is getting VERY hot.  At idle it's dropping 155v across the 5k resistor so around 5W.  My infrared temp gun measures the temp ~210*F.  The resistor is mounted up off the PCB to let it breath.


Is this ok or should I up the resistor to 25W and bolt it to the chassis?


https://el34world.com/charts/Schematics/files/Univox/UniVox-U-305R.gif


Thanks!

[sluckey]

The 5K/10W resistor is operating at half its rating. Give it plenty of breathing room and it should be fine.

That big ole 20K is not parallel to the second cap. It is between the second and third cap. Look closely.

[Deric]

Awesome - thank you!


The resistor mounted to the chassis is connected on one end to the power tube screens (red wire) and on the other end to ground (black wire is connected to filter cap ground). The green 22k on the circuit board is between the second and third cap.   

[sluckey]

Ahh yes. I see it now.

[PRR]

> temp ~210*F.

Electronics all runs on degrees C. Your number is like 99C. Which is not amazingly hot. I've worked transistors hotter than that (not for long). It won't actually boil spit, a traditional test of WAY TOO HOT.

If you look-up the resistor datasheets you will see that it is well inside ratings, assuming a 1,000 hour life. In fact it seems to extrapolate to 32,000 hours. Which means it may out-live 10 or 20 power tubes or several preamp tubes (not counting death by vibration or hasty assembly).

So this IS a perfectly reasonable, even generous, rating for a commercial product.

It is 77 years in 8hrs/week home use.

It is 25 years in 24hrs/week nightclub work.

It is "only" 4 years in 24/7 operation. (And note that the spec wants 25% time-off.)