is 30 watts enough for a sag resistor?

[daz]

My 2xEL34 cathode biased amp has just under 400v coming off the SS rectifier. I have a 270R/30 watt sag resistor on the B= and it gets awful hot. I haven't yet made it permanent so it's not heat sunk to the chassis. But still, it gets awful hot and i want to know whether i should just go ahead and mount it or get a couple at twice the R value and parallel them to be safe. I would just go and do that but theres not much room to mount 2 of them.

[tubeswell]

Hi Daz

Measure the DC voltage across the resistor to get the voltage dropped by the resistor.

Then divide that result by the actual DC resistance of the resistor to get the current.

Then multiply the current by the voltage to get the power dissipated by the resistor.

If the dissipation is around about 15W or less you should be fine.

It could be that the resistor might be getting hot if it is one of those bolt-on types that is bolted to the chassis in a situation where the chassis gets real hot anyway just through normal operation. In which case maybe try mounting say 4 x 1k 10W cement resistors in parallel suspended away from the chassis instead?

[daz]

I'm sorry...i seem to be doing this wrong because it's giving me 49 and that can't be right can it? Heres exactly what i'm doing...

the resistor measures exactly what it say on it...180R. The voltage across it is 22.5vdc. So i enter 22.5 in the calculator and divide that by 180 and i get .125

Now i clear the calc and enter .125 and multiply that by the B+ of 395vdc and you get 49.374. What am i doing wrong ?

[tubeswell]

22.5V x 0.125A = 2.81W

A 10W 180R resistor should be plenty in my humble opinion (allowing for peak voltage under operation).

Is the resistor picking up chassis heat? (in which case try mounting a 10W resistor away from the chassis)

B-b-b-b-b-ut I've been wrong about lots of things

[daz]

Actually the amp was dead cold when i tried it and the resistor go too hot to touch aside from a quick jab. So no, there wasn't any heat contribution from the chassis or anything. By the way, just to curb potential confusion after reading the original post and my later ones, i used a 270R at first but then went to a 180. Thanks tubeswell. I think i'll bolt it up. Gonna use a switch too because i'm not sure i'll want it there all the time.In the past i preferred no sag R, but now i'm thinking it's a nice alternative in those situations where the room sounds/a bit hard.

[octal]

If you're really concerned, it might be best to measure the power dissipation in the resistor with the amp at max output. If you have a sine wave generator and a dummy load....

That being said, cathode bias amps don't seem to have as much spread between idle current draw and "full honk" draw. tubeswell is right... it's doubtful that if it's dissipating under 3w at idle it's going to exceed 6-7 watts running flat out.

[daz]

No, i'm not concerned now. I just wasn't sure how to determine whether it's within a safe zone but i'll just bolt it up. On a side note tho, how do you all go about determining what R value to use? I didn't seem to notice any difference between 270 and 180. Is there a point of diminishing returns?

[daz]

I bolted it on. Wow...i had no idea the chassis would draw away that much heat ! Now it's just a bit warm vs being too hot to touch ! And even the chassis in the mounting area remains quite cool. Thats all. Youse guys is da best.