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In Reply to: RE: Resistor dissipation Question posted by timp on September 15, 2012 at 20:26:39
If the Dale is enclosed in an aluminum casing, with two tabs on each end that allow you to mount it, and has some cooling fins, it is ONLY rated 50 Watts when properly heat sinked to a chassis or sufficient material to "sink" the excess heat into. Without the proper mounting, it is no wheres near 50 Watts rated. Most likely it is 20 Watts maximum when not heat sinked.There is a white paste you use, (thermal grease or heatsink compound are common names), applied between the resistor and the chassis contact area, to more easily transfer heat from the device to the heat sink area.
Often, the type resistor I describe is called a "chassis mount" power resistor.
If this resistor is reducing B+ to the output tubes, there are far better ways to accomplish this, but that is another story for another day!
Jeff Medwin
Edits: 09/15/12 09/15/12Follow Ups:
Jeff
Thanks for the reply! Yes, it is housed in aluminum enclosure that has two screw holes. I will definitely try to find a heatsink and mount it.
I did not know that 8 watt dissipation would heat up, let's say a 20 watt resistor. Is there a set ration between wattage/dissipation in resistor?
Thanks again.
This is just Ballparking-- but to add clarity-- that resistor that was designed to have a heat sink-- the 50 watter as the example-- I would rate it at 1 1/2 to 2 watts without the heat sink.Chassis make poor heat sinks for a power resistor that is generaing a magnetic field. Why bolt this distortion generator onto/into your chassis?
The easy way is to use air-cooled power resistors that are already rated well above the wattage needed, and mount these floating-- under the chassis on stand-offs-- keeping them away from anything else.
Holes should be drilled into the chassis-- both below and above the resistors to allow air-cooling vertically. Mount the resistor either horizontally or at a slight angle.
--Dennis---
Edits: 09/16/12
Dennis hi,
Would you mind explaining how said resistor, when bolted to a chassis becomes a distortion generator?
If so, what type of distortion/harmonics,etc., would that be?
Cheers,
W
My only experience with these has been with Dale aluminum-housed, heat-sink types.
These generate a lot of heat in free air with very little power thru them. It amazes me how little it takes if there is no heat-sink.
If you're using a chassis as a heat sink, you are heating that part of the chassis more than other areas. This heat-created area is different in density and temp from other areas.
The heat differences aren't conducive to max. performance of the chassis as a neutral device.
Magnetic fields also differ in the heat-sinked and non-heat-sinked areas.
I do not know all of the possibilities for mischief from this-- but common sense tells me some of them. The difference in sound quality between a uniformly made and operated chassis, and one that has different anomalies in either its construction or in its activation induced from various influences on it-- heat and magnetism in the wrong places being only two of them-- the difference in performance of the circuit is obvious when you're comparing amp A- to, say, amp B-- one with a proper-operated chassis and the other with one or more of these problems-- is evident on listening.
Perhaps someone here would care to elaborate on this-- I'm afraid I have only partially answered your question.
Rest assured that this anomaly DOES EXIST if you allow it.
I have used chassis-cooling for FWB's in low-volt power supplies with very few problems-- In these cases, I used Wakefield aluminum coolers bolted onto a chassis bottom that was also aluminum-- the chassis bottom had been cleaned and as much as possible, the aluminum heat sink bonded with it.
As voltages get higher (as in tube plate supplies, etc.), I would tend to avoid letting any part of a circuit get anywhere near a metal chassis.
This attention to detail is far more important in a S.E. amp than it is in a Push-Pull circuit.
---Dennis---
I see the merit of mounting the resistor with good clearance (on standoffs) outside of the chassis based on heat dissipation reasons, but I am at a loss understanding how the unevenly heated chassis contributes to distorsion...
Certainly, any AC circuit is better kept away (cleared, shielded, etc.) from any other circuit, especially signal carrying ones. I see the merit in doing this. Particularly when common mode rejection is small or zero.
I would appreciate if other inmates with similar experiences would pitch in and explain, especially the heated chassis and distorsion part...
Thanks,
Radu.
I'm really confused; the heat is generated based on I^2R. Or, of course if the Vmax of the resistor is exceeded, but that's a different story line.
A low voltage heater or cathode supply (both high current) has the ability to cook the resistor at much higher temperatures than a plate supply.
We're referring to, AL housed resistors and "commonly" affixed to an AL chassis. I'm not seeing the magnetic field?
What part of the circuit is more sensitive, albiet SE vs. PP?
FWIW, I use the RH for heaters sized to NOT cook and bolted to the chassis.
Cathodes get Mills (Huntington) NI 50w bolted to the chassis. When switching from MRA's in series to a single 50w bolt style, I couldn't not hear any difference. Ample convection of air movement around them is something I do address.
If were talking an extreme audible difference, an albino monkey coffin in these positions, will surely make your ears bleed.
I learned so much going through this thread. It is my experience too that RH resistors run pretty hot. I have them in my pre and phono stage. In any case, I am thinking that the resistor is not dissipating more than 11-13 watt. So,just for the sake of it, I bought a 50 watt resistor. I am going to solder it and see how hot it gets.
If you are dissipating 13 watts, wether you use a resistor rated for 25 watts or one rated for 50 watts you are going to create 13 watts of heat. The difference is in the area used to dissipate that heat, correct? In the case of a larger resistor, the heat is spread over a larger area.
I'm guessing that if no air flow is provided, given enough time the 50 watt resistor will get as hot as the 25 watt resistor (asuuming the same power).
I've been amazed at the small size of modern Kiwame 2 watt carbon film resistors, as compared to the carbon resistors of 30 years ago with the same power rating. But these little buggers get HOT!! No touchy.
Same with mills mra. Especially NI resistors due to the construction.
cook an egg.
Without calculating the ambient temp. into the actual heat dissipated, they can and will get very hot. Not had one fail yet though.
FWIW, I kinda got away from the finger tests for heat.
My preferred method is a contact probe.:-)) It doesn't blister.
Love your sense of humor!
S.E. amps have no common-mode rejection, so any distortions introduced into their simple circuit gets amplified.
The reason a good one sounds so good is that the builder has learned to minimize adding distortions into the circuit, and the fact that since S.E. can't reject common-mode, it also can't reject parts of music.
Aluminum on aluminum isn't so bad-- that's OK, the resistance will generate heat and some magnetism when current flows thru the resistive element.
It's better to isolate this as best you can.
---Dennis---
The reason a good one sounds so good is that the builder has learned to minimize adding distortions into the circuit, and the fact that since S.E. can't reject common-mode, it also can't reject parts of music.
....Would the "part of the music it can't reject" stated above be mains ripple or mains x2 ripple depending upon the rectifier used when NOT utilizing critical L?
What has the builder implemented that minimizes the added distortions?
Proper tube topology > finals? Avoiding Miller C, A/C filament supplies away - far and away....
Keeping in line with a good sense of humor, analogously, I sound better in the AM because of 8oz's of Prune Juice at bedtime.
Cheers,
W
"the fact that since S.E. can't reject common-mode, it also can't reject parts of music."
Can you explain what this means? Failure to reject common mode power supply noise is no guarantee of amplifier fidelity.
--------------------------
Buy Chinese. Bury freedom.
Power equals Voltage times Current (in Amperes), Ohm's Law.I always make sure there is 2 to 1, rated versus dissipated, and I am MOST HAPPY when it is 4 to 1, rated versus dissipated, for long term reliability and fail-safe operation.
I ventilate my chassis on DIY builds, above and below power resistor bodies that dissipate a lot. I drill 3/8ths inch vent holes on 9/16ths inch centers, three in a row, for Mills MR-12 power resistors, looks "uptown".
Jeff Medwin
Edits: 09/15/12
...... "Power equals Voltage times Current (in Amperes), Ohm's Law" is not Ohms law. Ohm's law: I=V/R more or less.
it's a combination of Joule's first law and Ohm's law, but all of those relationships between I, V, R, and P are commonly referred to as "based on Ohm's law..."
I ^2 R will give you the actual Pdiss in watts across the resistor.
or
Ohm's law: I=V/R , as Steve O pointed out.
Then x3 or x4 that result, for the resistor rating in watts for good sleep factor and longevity.
NI wirewounds run much hotter than inductive. Mills used to post the curves on their website prior to being bought out by Huntington.
The Dale RH AL. housed resistor(or any AL housed resistor) should have a "W" rating for mounted and not mounted. See how low the wattage rating drops when not properly heat sinked to a metal housing. Some x-fer grease helps.
Cheers,
W
Useful reference, on my wall....
Thanks Jeff!
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