Resistors question yet again.

[dragonbat13]

After some struggle and reading data sheet after data sheet, I've come to the conclusion that I need some help from you guys.

I'm gonna start stocking on resistors.

For Metal Oxide 2 and 3 watt I like the PO2 and PO3 Vishay drailect (spelling?).

I need to know what everyone is using for the smaller ones. Seem, especially with metal film, that the 1/2 watters are just so small. I can't get myself to use them.

So, for metal film and carbon film, for the places that the old schematics call for 1/2 watt, what's everyone using for each? What wattage?What brand? Where to get them? Not talking custom mojo nonsense NOS five dollar ones. Just good quality, well established brands.

Thanks.

[R.G.]

There are a few things to think about with resistors (well, also anything that gets hot) and physical size. I find these to be some of the more useful factoids that remain from my college courses. This will sound confusing for a minute, but bear with me.
1. Temperature is not heat. Temperature is the "pressure" exerted by the heat to move around. In that way, temperature is like voltage, and heat is like charge. If you keep stuffing charge into a capacitor, the voltage goes up. If no charge gets out, the voltage rises without limit.
2. If you keep stuffing heat into an object and it can't get rid of the heat fast enough, the temperature will rise until the outgoing heat equals the incoming heat. This is really unlimited. A 1/8W grain of wheat incandescent bulb has an internal temperature of 2550C/4600F. At this temperature it glows white hot and the heat is conveyed out as IR and visible light.
3. Things only get rid of heat by radiation, conduction, and convection of the air around them. (N.B. I'm simplifying here). So a resistor can only get rid of the heat inside it by radiating (IR, usually), conducting it out to a film of air or conducting it out the leads, or having air flow around it. For SMDs, there's essentially no surface area, so most of the heat is conducted out by the solder and pads. For leaded resistors, the body is generally bigger and the leads longer, so although some heat goes out through the leads, almost all of it leaves from the surface of the resistor.
4. Here's some of what you're looking for. If you have two resistors, one physically larger than than the other and each is having 1W of heat stuffed into it, which one gets hottest? The little one. Exactly how much hotter is determined by the surface materials of the two resistors and the surface area. The difference between the too-small half-watt resistors and the old standard sized ones is that the smaller resistors just get hotter at the same power.

That may or may not be a problem. If the resistance materials can stand the heat without degrading, it doesn't matter much which one gets hotter. We've been conditioned by the universal use of carbon composition resistors to think of there being a fixed "too-high" temperature. There's really not if you change the internal materials, like going to wire-wound or MOX, or some other kind of resistance. If the power sent into the resistors is the same, it makes no difference to the amp's insides and workings how hot any one resistor gets. With the same amount of power inside the amp chassis, the chassis heats until the power input is balanced by the power flowing out, no matter what the temperature of any resistor inside it is.

So the crux of the problem is what temperature rise does to any one resistor, and how that temperature rise affects reliability. Everything else being the same, it's better if things don't get too hot. The rule of thumb is: know how much power is going into the resistor, and select a resistor rated for at least twice that amount of power.

This is a sneaky way of setting the temperature rise. For the common materials used in resistors - the body, the resistance materials, the coatings, the lead attachment methods - they are designed to withstand about 150 to 200C before degrading. Using them only at half their rated power will mean that their temperature will be kept below 75 to 100C, and they'll last a long time.

Resistor makers have used this set of information to give us smaller, but higher power rated resistors. Carbon composition material is temperature limited to surface temps of about the 70-100C range. Carbon film and metal film are good for a little hotter. MOX (metal oxide) resistors are good to even higher temperatures, up to a few hundred C. Wirewound resistors are made of metals like nichrome that doesn't degrade up to about 1100C (!), so the limits in wirewound resistors are all in the body, coating, and attachment. So it really is possible to make a resistor that is reliable and physically smaller than amp techs and makers normally think of doing.

EDIT: forgot to add this - it's a good background. https://eepower.com/resistor-guide/resistor-materials/

[LOUDthud]

A couple of issues to watch out for:

G10 or FR4 circuit boards will turn brown and possibly catch on fire under super hot resistors. Mount resistors up in the air off the PCB. I'm not sure about the black eyelet board material used by Fender.

Tin/Lead solder can melt and the resistors can fall out and/or possibly short to chassis. Lead free solder goes to a higher temperature. If you used turrets, wrap the leads around the turret, don't just stick the lead in the hole.

[R.G.]

Good advice, L.T.

[Guy77]

After some struggle and reading data sheet after data sheet, I've come to the conclusion that I need some help from you guys.

I'm gonna start stocking on resistors.

For Metal Oxide 2 and 3 watt I like the PO2 and PO3 Vishay drailect (spelling?).

I need to know what everyone is using for the smaller ones. Seem, especially with metal film, that the 1/2 watters are just so small. I can't get myself to use them.

So, for metal film and carbon film, for the places that the old schematics call for 1/2 watt, what's everyone using for each? What wattage?What brand? Where to get them? Not talking custom mojo nonsense NOS five dollar ones. Just good quality, well established brands.

Thanks.

For the smaller 1/2 watt resistors I see some high end boutique company's using the KOA Speer 1/2 watt carbon film resistors. They are much less than $5.

Merry Christmas /Happy Holidays!

Guy

[dragonbat13]

Yes Guy, I have looked into KOA Speer. I had not, for what reason I don't know, looked at this website

https://www.koaspeer.com/products/leaded-resistors/

I have a PCB ruler coming so I will be able to use it to help with sizing if needed.

I can understand what the data sheets say regarding size.

[Guy77]

I missed ready that website as well! It has some great information. I found this page that gives a ton of information about the materials used in manufacturing and the terminology used for describing their characteristics.

https://www.koaspeer.com/fundamentals-of-a-resistor/

Cheers!

Guy