I finished my amp (as can be seen elsewhere in the forum ) and was working on adding two more points to my B+
Now I see I ordered 2W metal film resistors for the preamp dropping string based on the B+ resistors in one of the Dumble BOMs floating around on this forum. Will that cause problems down the line? I don't want to have to take apart the dropping string as I have the amp perfect and the voltages dead on. I do have higher rated resistors for screen grid node.
The power rating of the resistor depends on the voltage dropped across it and the resistance in question. (Watts = Voltage x Amps*, often re-written as P = (E^2)/R )
*i.e.: Voltage/Resistance = Amps
Rule of Thumb is to have the resistor rated at 2 x what the power dissipated by the resistor is, in order to assure longevity. Most times 1W resistors are okay in the power rail, but the lower the resistance is, the more likely it is going to need to be higher than 1W rating. In short - you need to know the voltage dropped and the resistance in order to work it out.
The power rating of the resistor depends on the voltage dropped across it and the resistance in question. (Watts = Voltage x Amps*, often re-written as P = (E^2)/R )
*i.e.: Voltage/Resistance = Amps
Rule of Thumb is to have the resistor rated at 2 x what the power dissipated by the resistor is, in order to assure longevity. Most times 1W resistors are okay in the power rail, but the lower the resistance is, the more likely it is going to need to be higher than 1W rating. In short - you need to know the voltage dropped and the resistance in order to work it out.
The power rating of the resistor depends on the voltage dropped across it and the resistance in question. (Watts = Voltage x Amps*, often re-written as P = (E^2)/R )
*i.e.: Voltage/Resistance = Amps
Rule of Thumb is to have the resistor rated at 2 x what the power dissipated by the resistor is, in order to assure longevity. Most times 1W resistors are okay in the power rail, but the lower the resistance is, the more likely it is going to need to be higher than 1W rating. In short - you need to know the voltage dropped and the resistance in order to work it out.
You got that last part backwards I think. The higher the resistance, the higher the wattage needs to be (for a given power level). This statement is hard to justify if you're talking generally but for the same current, obviously the wattage needs to be higher.
Unless I read wrong, for the preamp rail Rs, 2W should be fine.
Example, three 12ax7s (two for preamp and a PI) will draw what, around 5mA? (~2mA each)
If you drop 50V at the PI, you're still only at 50*0.005 = 0.25W so 2W is 8x that.
So add up the current draw (I) for all the tubes "downstream" of the dropping resistor, add it up. Take that number, square it, then multiply by the ohm value of the resistor:
Pwr in Watts = R * I * I
edit, geez man, that's you're first build!!!????!!! That build looks like it was blue-printed!
The power rating of the resistor depends on the voltage dropped across it and the resistance in question. (Watts = Voltage x Amps*, often re-written as P = (E^2)/R )
*i.e.: Voltage/Resistance = Amps
Rule of Thumb is to have the resistor rated at 2 x what the power dissipated by the resistor is, in order to assure longevity. Most times 1W resistors are okay in the power rail, but the lower the resistance is, the more likely it is going to need to be higher than 1W rating. In short - you need to know the voltage dropped and the resistance in order to work it out.
You got that last part backwards I think. The higher the resistance, the higher the wattage needs to be (for a given power level). This statement is hard to justify if you're talking generally but for the same current, obviously the wattage needs to be higher.
Okay lets check it out with some sums using a 50V drop as an example:
50V across 27k = 50 x 50 / 27000 = .092W dissipation
50V across 2k7 = 50 x 50 / 2700 = .92W dissipation
50V across 270R = 50 x 50 / 270 = 9.2W dissipation
Its a function of the voltage and the current that produces the heat. In a higher resistance, all other things being equal, less current will be present.
Yeah I was thinking about this earlier over dinner and realized I was backwards. It's just weird seeing it in writing as opposed to actually staring at the math. I originally thought that you meant for the same current level higher resistance means more dissipation. It just depends on how you look at things I guess .
On commercial amps they also would have to consider if a flame proof type resistor is needed in the power rail., i.e. if there was a short right after the resistor would it burst into flames. This is obviously not a commercial amp but it’s something to bear in mind as a safety issue.
) and was working on adding two more points to my B+
.