Moderm "high watt" resistors seem too small

[doozerdave]

Originally Posted by Richard Ellis

A "tube down" chassis? Am I to assume you have a PCB with nine tubes 'upside down' in your chassis? If so, you've got heat rising up toward the PCB....Yeah, some forced air cooling would definately be in order.
When we build these things we are spending hundreds of dollars on TXs....what's fifty dollars or so upping the ratings...as high as we dare?...Stuffing big wirewounds should not be a spatial problem inside differing chassis'......Too often when one designs.....component available space is not given enough priority. We tend to rush to build....only when fingers get burned consistently during wire-up do we tend to realize our chassis' are too small. But by that time...it's too late, & a full chassis re-do is not "preferred" One tends to NOT want to start all over......."It'll be OK!" is the refrain.
Perhaps a rule of thumb should be implemented.....If you calculate your chassis 'should' be X size.........double it.....you won't be sorry.

Yes, tubes down, it's a typical guitar "combo" amp chassis where the amp would be in the top and the speakers in the bottom. I have lots of room, so larger resistors are certainly possible, but there are only the two cathode resistors (push pull parallel, 4 tubes, shared cathode bias) that are in question... and as this discussion has progressed, the over rating factor of 2.7 sounds sufficient. My HT resistors are WAY over rated, so I'm not concerned about them at all.

[doozerdave]

Originally Posted by scitizen17

I've been using these and overrate x3. Very stable.

http://www.vishay.com/docs/30204/rsns.pdf

Those are exactly what I have

[doozerdave]

Originally Posted by ElectricMan

Hi
I would do some paper and pen math, dump the CAD program
I use my meter, and my calculator
150 MA through a 250 ohm resistor = 37.5 volts = 5.625 watts
125 MA though a 250 ohm resistor = 31.25 volts = 3.9 watts
ETC
What is the volts across????
Stop guessing and pretend you are in High School!

Who's guessing? I know it's 1.764W, that's why I said 1.8W in my first post. It's 21V across 250R, (21/250)*21=1.764W (and before someone tells me that I can do P=V^2 / R, I do it the other way because I always want to know the current anyway). I was just concerned about these relatively small, relatively hot-running resistors. I won't pretend I'm in high school, I'll just pretend that I've got an electronics engineering diploma and 15 years field experience.

[tubelab.com]

http://www.vishay.com/docs/30204/rsns.pdf.......Those are exactly what I have

There is one of those 5 Watt Dale / Vishay resistors on the PC board in this amp which is enclosed in a closed Lexan box with little airflow. It is dissipating about 3 watts. That amp has been working just fine for about 7 years now. The output tubes were purchased new in 1929, not by me of course.

[doozerdave]

Originally Posted by tubelab.com

There is one of those 5 Watt Dale / Vishay resistors on the PC board in this amp which is enclosed in a closed Lexan box with little airflow. It is dissipating about 3 watts. That amp has been working just fine for about 7 years now. The output tubes were purchased new in 1929, not by me of course.

That's very good to hear, thanks for that. I shouldn't worry about mine then, especially after I add air flow.