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previous: Steve Ahola     Is there an... -- 6/18/2000 9:23 AM View Thread

Re: 3 Watt screen resistors?

6/20/2000 8:52 PM
Re: 3 Watt screen resistors?
Carbon comps can sometimes take surges as well as overcurrent with less damage than other types of the same rating.  
Most likely you are looking at 3watt metal oxide which can take the HEAT better than the carbon (or especially metal film), maybe not an extreme voltage surge or continued overcurrent though . . . I used a 2watt 1K metal oxide on the screen of an individual 6BQ5 and it doesn't even break a sweat. Since it is soldered right to the tube socket with a fairly short lead, most of the heat comes conducted & radiated from the tube, but voltage drops across the resistor only add to that. It is easy to underestimate the wattage if you only consider the DC voltage drop at idle, DC must be checked at maximum volume, even then the AC voltage drop needs to be measured and added to the DC behavior to calculate theoretical heating watts. I have found it easier to just leave the slide rule in my pocket protector, and jam extensively at maximum volume before testing the resistor temperature using the human hand ;)  
Incidentally, when it comes to heat, remember a 1watt 1K will dissipate the same calories as a 10watt 1K when operated at the same voltages & currents. The higher wattage units just handle it better and dissipate it without rising to as high a temperature themself. The amount ot heat released is exactly the same though, so when enclosing a resistor results in the chassis becoming too hot, replacing it with a higher wattage one may help the resistor survive but will not reduce the temperature reached by the chassis.  
I think you would be fine with either a single 3W on each screen, or just one 3W to isolate both from the screen supply, with the latter arrangement I might also add another 100R to each screen to isolate them from each other a bit. I used a 2W carbon comp on each tube in a 6L6 amp and it takes a while of jamming before they heat to a comfortably warm touch, I do turn the amp off and bleed the caps right before I touch them to avoid all the high voltage danger points.  
Hope this helps,  

Steve Ahola 1k vs 1k5 vs 2k7 vs ??? for EL84's -- 6/24/2000 6:54 PM