CF vs MF plate resistors.

[Steve Conner]

No, but I find the thought quite tantalising.

I also have one of those big wire wound resistors: it doesn't work as well as a THD Hotplate, but it was a heck of a lot cheaper.

Re Lowell's comment, I've seen a few amp problems that could be cured by replacing the owner.

[R.G.]

I was once told by a guy who had the history to know the fact that theater lights were dimmed at one time by raising and lowering metal plates inside barrels of what amounted to pickle juice - brine. The advantages are obvious - the power rating was huge, and the temperature was limited by water's phase change between liquid and solid. The spare parts were easy to find, too.

@leadfootdriver: The carbon-comp versus other resistor moho mania is fairly well understood now, from the voltage coefficient of resistance of CC versus other kinds of resistor. For CF versus MF, the differences are much more subtle, as CF doesn't have CC's big VCR. I have not been able to find a clear difference between the two film types in any listening test. A fair and representative listening test would be hard to set up and probably expensive.

Notice that listing to two resistors and crowning one of the resistor *types* king based on that test would not make sense.

[Austin]

Here is a nice thread over at diyaudio about it, probably one of many I am sure.. Looks like carbon comps for the win though. http://www.diyaudio.com/forums/tubes...etal-film.html

[Alan0354]

Metal film resistor don't handle current surge nearly as good as carbon comp. We use carbon comp for current limiting. There is a big big difference between steady state current draw and instantaneous current surge. Metal film and CC both can handle steady state just as good. BUT for instantaneous surge, metal film tend to blow.

The reason is because metal film is thin film and cut into circular strip or something around the resistor body. Basically it is a very long thin strip. When you have current surge, current concentrate through the cross section of that strip, current density is very high, heat is generate. At the surge of high current and short duration, heat do not get transfer out that fast and that small strip melt.

I never know this until we use high wattage resistor to limit the current during a high voltage arc. The metal film burned. When a high voltage arc to ground, even a 10' coax has like 300pF. If you charge the cable up to 10KV so the total charge is A=CV=300EE-12X10000= 3EE-6 coulomb. But if you discharge in 1nS, I=Q/t= 3000A during discharge for 1nS. We burned so many MF resistors. We finally change to CC and it never fail again. CC has a big cross section in the resistor path. No local heat build up at all.

[lowell]

That's wild because I feel it's general practice for people (myself included) to use MF in high voltage locations because they don't ignite like a CC would.

[Alan0354]

Well, you supposed to use resistors that can handle the average power so it does not catch on fire. I am talking about instantaneous power, they burn cold. There will be no sign of burning like you think, it just open up. We had many of those failure, we learned by mistake on this one. The example I gave, if you take the average over 1 second, it is very low power. 300EE-6 coulomb discharge in 1 second is only 300uA!!! That's peanut current. But if you discharge in 1nS, that's an avalanche, nothing gets hot but burn.

High voltage discharge is very serious, we had big HV relay contact fused by just discharge from the capacitance of a coax cable. I just shorted the 110V AC to the chassis accidentally a week ago and chu off a chunk of my needle nose and didn't even burn the fuse. Now it has a dent on the side!!!!

[Alan0354]

Dare i chime in? haha... liked reading about saline resistors though.

Once all other "tonal" factors are considered I find resistor types (noise aside) quite far down the totem pole. After musician, pickups, speaker, circuitry, tube types and on and on... then resistor types... maybe. IMO.

Can't say it any better. The most important factor of sound is the musician. I never buy into the CC resistors. Yes, there is voltage coef. But that only generate even harmonics be it +ve or -ve coef. You get so much more even harmonics by tilting the loadline. Refer to RDH4 cpt 13, it talked extensive on distortion and how to adjust it. The carbon thing is bull in my book.

BTW, I don't buy into the orange drop capacitor either. If someone has a sound theory, I am all ears. If it is loss tangent, you parallel different value caps just like modern electronics does, parallel 0.1uF with 10uF. If you are picky, one more of 0.001uF, you cover from low to 200MHz, don't give me that it is not good enough.

[defaced]

Probably prone to thermal noise (johnson noise?) Guys here have been using stove top elements for dummy loads. I haven't checked for myself but have read that they're about 8 ohms. How much different when hot??? Dunno. But one solution has been to stick them in a tub of water. A pair like this would be a cheap 3000W dummy load! Nice if you need to test things like giant PA systems and giant transmitter amps.

EDIT: Why not Google "saline resistor" and see what you find. Info is out there. Speculating here without doing ANY preliminary research is about as relevant as openly wondering if you left the iron on at home or how much an average human head weighs.

The new stove elements I've measured (yes I took my meter into Lowe's just for this) measured no where near 8R. Since there is a variety across different elements (cross section and length), results will vary, but there's a list of one place not to look for them.

Per the Wiki article, increase in resistance from RT to 600F is at max about 5%. Figure we wouldn't make it near 600F and that number comes down, probably significantly. Nichrome - Wikipedia, the free encyclopedia

[Austin]

The new stove elements I've measured (yes I took my meter into Lowe's just for this) measured no where near 8R. Since there is a variety across different elements (cross section and length), results will vary, but there's a list of one place not to look for them.

Per the Wiki article, increase in resistance from RT to 600F is at max about 5%. Figure we wouldn't make it near 600F and that number comes down, probably significantly. Nichrome - Wikipedia, the free encyclopedia

A small hotplate that draws 1200 watts and runs off of 120 volts would be 12 ohms, so not that far off there but the 220v version on the stove that is 1200 watts would be 40.3 ohms according to my calculator so maybe the ticket is to get the smaller 120v version that plugs into a regular outlet if you wanted to try to use this for a speaker load. Also the small 120v burner thing I had was two burners so if you turned them both on in parallel it would further drop the resistance to 6 ohms, getting even closer. I think I got it from a dept store for about 25 bucks.

I don't really know how much power a burner uses but 1200 watts seems a reasonble ballpark figure.

[defaced]

I don't recall if the burners had power ratings marked on them. I hadn't thought about an actual hot plate, I'll give some a look. It'd be really great if the little George Forman grills worked out well. Those things are super easy to find used.

[Austin]

I don't recall if the burners had power ratings marked on them. I hadn't thought about an actual hot plate, I'll give some a look. It'd be really great if the little George Forman grills worked out well. Those things are super easy to find used.

Haha, I can imagine connecting a Marshall Major or Ampeg SVT at full volume to the george foreman grill and frying an egg on there... Seriously though I have a foreman grill I will hook the ohm meter up to it later and report back. Would be interesting to get the reading cold and then plug it back into the wall and let it heat all the way up and then take another ohm reading to see the differance in resistance. If it behaves anything like a lightbulb fillament it will have higher resistance when hot.

[g1]

I don't think you'll find a 120V appliance at 8 ohms. That would be 1800W which is where our 15A breakers are supposed to blow.
When the early Carver amps came out (PM1.5?), the sales reps talked about them being able to drive an electric kettle and boil water. I never saw it done but that was the story.
I use an element from an old baseboard heater. It is a wirewound type and measures about 8 ohms. Because it is wirewound I can tap off anywhere for 4 ohms, 2 ohms, or whatever. It will handle any kind of power I can give it.

[Austin]

I don't think you'll find a 120V appliance at 8 ohms. That would be 1800W which is where our 15A breakers are supposed to blow.
When the early Carver amps came out (PM1.5?), the sales reps talked about them being able to drive an electric kettle and boil water. I never saw it done but that was the story.
I use an element from an old baseboard heater. It is a wirewound type and measures about 8 ohms. Because it is wirewound I can tap off anywhere for 4 ohms, 2 ohms, or whatever. It will handle any kind of power I can give it.

The george forman grill measures 13.9 ohms cold or hot, I have a two burner auxillery range that runs from 120v that measures 15 ohms on one burner and 22 ohms on the other, dunno why the dif but with both is 10.5 about and is adjustable with the knobs...

Seems like knobs that can handle all that current would have all sorts of uses.. Like miniature variacs...

[R.G.]

I'm thinking that if you have resistors in a tube amp where the max power versus transient burst power makes a difference in the resistors living or not, you are probably doing something wrong.

[Alan0354]

I am not talking about just in tube amp, the thread is about the difference, I mention one major difference.

Regarding to tube amp, I question the validity of using CC that cost so much more to get a little bit of volt coef that you can get by playing with the second harmonics using the load line.