R value (watts) for cathode resistors

[daz]

In a cathode biased EL34X2 amp in which i am running separate cathode resistors for each El34, what wattage would you say is plenty safe? It seems to me a small say 5 or 10 watter would be fine, but i always see much bigger values like 25 watts used in the commercial amps i look at. What should i use at a minimum and why do those amps use such huge resistors?

[MWJB]

Measure the voltage dropped accross the resistor, square it, then dvide by resistor value in ohms.

So let's say you drop 40v accross a 330ohm cathode resistor...

40*40 = 1600.

1600/330 = 4.8W dissipation accross the resistor, double it to be on the safe side & you see that 10W will be adequate in most cases for a pair of 6L6/5881/EL34. I see 10W used for a pair of 6L6 in high end amps.

No harm in going to 25W.

[daz]

It drops 30v across the cathode, but remember this is a amp with seperate resistors for each tube, so still the same? If so then obviously a 10 would be fine. Thats what i thought but i've seen amps with much higher so i just wanted to be safe. Thanks.

[tubeswell]

I go on twice the size of something you would use for a single tube in Class A. FWIW I used 1 x 10W for a pair of 6L6GC. (Oops I see MWJB has posted something while I've been getting my act together)

I suspect the critical aspect is somewhere above the average value of the volts/amps that the resistor sees when you are whacking out power chords.

As for production amps, bigger resistors = more insurance against warranty repair costs I guess, although it must come down to a trade-off between that and the unit cost of the resistor at some point

[MWJB]

It drops 30v across the cathode, but remember this is a amp with seperate resistors for each tube, so still the same? Yes, the formula is the same, you just need to consider each cathode seperately.

If you can get bigger resistors at a fair price use them, sometimes guys like to use big wattage heat sink resistors...which I have also done in the past. It might depend on scale...one 25W resistor may be cheaper than a 10W than comes with 9 siblings?

[Enzo]

The resistor has no idea how many tubes are making the current through it, nor does it know if there are any other resistors nearby doing similar work. You figure each resistor on the job it is doing. And as everyone has said already, add at least a factor of two wattage wise. DOn;t run a 4 watt resistor if 3.8 watts is dissipated. Use a 10.

I like those aluminum power resistors from Dale or others - check Mouser - these are hte finned extruded aluminum guys with mounting holes on the ends. Here is a 300 ohm 25w Dale from Mouser, sells for a little over $3. You can mount it on the chassis wall.

[daz]

Thats what i've been using Enzo. I get them locally at a sort of suplus electronics place cheap.(about .50 each) But i wanted to go with cement or sand ones, whatever they're called, because the finned ones obviously don't float well and i often don't have the room to bolt a couple to the chassis.

[Enzo]

They make cement ones in all sizes too. Check the dimensions, the 25w and under are not very big, not much larger than a 10-15w cement.

[pdf64]

Measure the cathode voltage when the amp is putting out max power,as it will likely rise considerably over what the no signal or small output signal value is. Use that voltage to calculate the power dissipation needed, and also the rating of a bypass cap, if used. Peter.