Could a 10 watt resister reduce Heat vs a 5 Watt?

[TerryD]

I had a post about my friend's Hot Rod Deluxe...which is really not a bad sounding amp.  They had 2- 5 watt resisters close to the board that literally burned up everything around them.  I had to do point to point in there.  I lifted some new 5 watt resisters off the board and beefed up with a little JB Weld at the base.  My friend is now going to keep the amp.





In any case one person mentioned about putting the "right values" in there? 


2 questions:


1.  If I swapped out the 5 watt resisters for 10 watt resisters would that reduce the heat and make things safer for sure?  Any problems from doing this??


2.  PRR you said "You support a hot resistor by its leads and away from low-temperature stuff. In this case I would look to mount a terminal strip off-board where the heat does less harm, and run wires back. I dunno if that amp has space. "   Can you tell me a little bit on how to do this as you would explain to a layman?   


Thanks, Terry
 

[bnwitt]

If the resistance value remains the same the voltage drop remains the same and therefore the wattage and heat are the same.  The trick is to get the heat to dissipate without damaging surrounding components.  This means raising the resistor off the board away from adjacent components or moving it completely off the board somewhere it can't harm other components.  A third method is to attach a heatsink (like one for a cpu) to the component to increase the air surface.  Attached is a fix I did for the Crate palomino which has a similar issue with the on board cathode resistor.

[TerryD]

very interesting

[TheKT88KilledJFK]

Thermal energy expended is the same but it's over the greater surface area of the larger resistor, reducing intensity.  If you're trying to reduce temps inside an amp, then this doesn't work.  If you're trying to reduce local PCB temps, then this may work.  I will try to find some sort of coefficient or other quantitative source on this because it may be that doubling resistor internal volume may not have enough of an impact, but the effect is there.  From a simple theory point of view, the actual internal geometry of the resistor has impact on electron drift and interaction.  If you change the volume over which this interaction happens, then you will change thermal density while maintaining power output. 

[bnwitt]

The purpose of the Palomino mod I showed was to relocate the heat source to a place in the amp where it could radiate without damaging other components.  Using two resistors mounted to the chassis redirected the heat away from the control pots and rendered it harmless in the amplifier.   To my knowledge this mod has been made to numerous Paliminos with good results.  This could also work with the amp mentioned by the original poster. The goal is not to reduce the temperature of the amplifier internally but to take that high heat source and move it away from adjacent components that can be damaged by close proximity. 


As I mentioned in the posted JPEG the way to reduce the internal chassis heat is to change the value of that cathode resistor and lower the overall power dissipation of the power tubes but that of course would change the tone.  I have also heard from individuals with that Palomino amp that have taken those two resistors and moved them outside of the chassis itself and into the amplifier cabinet in order to further reduce the internal chassis temperature.   When dealing with an issue like this you have to be creative. Theory is great but practical service experience is very valuable as well.  Every amp issue is different.

[jojokeo]

1.  If I swapped out the 5 watt resisters for 10 watt resisters would that reduce the heat and make things safer for sure?  Any problems from doing this??


2.  PRR you said "You support a hot resistor by its leads and away from low-temperature stuff. In this case I would look to mount a terminal strip off-board where the heat does less harm, and run wires back. I dunno if that amp has space. "   Can you tell me a little bit on how to do this as you would explain to a layman?   


Thanks, Terry

Easy answers to your questions:
1) YES, swapping 10watt resistors will lower your heat issue greatly. No problems - unless fitting them in place is an issue? Then mount them elsewhere (see below)
2) If you have a "hot resistor" anywhere then it's wattage value is close to or is underrated. Go up to the next wattage available (or more). If space is an issue, simply be creative. See below attachment for examples or use BNwitt's idea for the higher wattage types (usually 20watt ratings or more).

Easy/simple calculation for determining the appropriate resistor wattage:
measure voltage drop across resistor, take calculator and multiply this by itself (squaring the number). Divide by the value of the resistor - this equals the current flowing through it. Then double this number for safety. You want the wattage of the resistor to be higher than this number.

If the resistor is hot to the touch and you're concerned at all by it or it's near an electrolytic capacitor for example - either increase the wattage of the resistor or move it away (maybe only a 1/4" to 1/2"?).