Cathode resistor heat

[EFK]

I just finished a cathode bias 2X6L6 amp. I have it set up w/ 2 diff. cathode resistors on a switch, both being the 25W aluminum housed resistors. (They're switchable so I can go 6L6GC or 6L6WGB without having to pull the chassis and physically change it out.) My question is, how hot should these resistors be getting when the amp is operating? I have them bolted to an aluminum chassis. After abouut 1/2 hour of play, the aluminum wall to which they are bolted is just about too hot to keep your finger on. Is this normal? They're set up for about 80% dissipation at idle. If this is not indicative of a problem, I think I might pick up a little finned heat sink of some kind and attach it to the outside of the chassis on the other side of the resistors.

[Amp Kat]

Obviously the resistor will get hotter the louder you play as it dissipates more power. The calculation of Cathode Voltage / Cathode Resistor will give you the Cathode current then multiply by the resistor to get the wattage you shouldn't exceed. The plate dissipation in watts = Plate voltage - Cathode Voltage X plate current which includes the little bit of screen current here that is very little I assume. Dropping the Cathode resistor will give you better tone at the expense of you're tubes and running even hotter but it is normal for the Cathode resistor to get that hot, just don't exceed it's wattage rating. You could heat sink it if you like and if you have a Cathode cap be sure to keep it as far away as possible from that resistor to preserve it a bit.

[EFK]

Thanks for the information - I'm pretty sure everything is normal, I just like to try to reduce heat as much as possible. Bypass cap is on the board so it's removed a fair degree.

[Enzo]

That 80% figure is for the tubes, not the resistor. AS KB mentions, you know the resistance, you know the voltage across the resistor, calculate the power IT is dissipating, not the tubes. We see figures like 10 watts and think small, because a 10 watt light bulbg is small. But 10 watts is a lot of heat, so is 5 watts.

Cover the wires on the ends and mount the whole darn aluminum resistor outside the chassis. Even 50 watt aluminum resistors are not all that large. No matter how large the resistor wattage gets, the thing will always dissipate the same wattage. It may be spread out a little bit so the heat wonlt be quite as concentrated in one tiny spot, but it will heat the chassis the same either way.

[1ampman]

Cathode biased amps run hot! Make a bracket and mount a fan on the back. Everything will last longer. With small fans in everthing these days you can find them in the trash.