MF Resistor Question

[Jim Donelson]

Some clarification on the meaning of RN65.

First RN65 is a military specification. Don't go looking for RN65 1 watt MF resistors, because RN65 is a 1/2 w resistor. RN65 resistors could be made by anyone - in theory they should be inter changeable. RN70 is a 1 watt mil spec resistor.



The RN65 specification has a 350 Max. Working voltage, but as we have already discussed this is fine. The dielectric VAC rating is 900 for RM65.

Vishay/Dale model number CMF65 is and RN65 speced part.
Note that RN60 (CMF60) is a slightly derated part in terms of voltage handling the max working voltage is 300 Max. still fine in most cases. Also note that not all Vishay/Dale 1/2 w resistors on mouser are mil spec. They have a CMF Industrial line, and I'm not competely sure what the difference is. If it has RN65 or RL20 in the part number it is Mil spec.

RL20 (CMF20) is a slightly derated RN65 - the working voltage is still 350, but the Dielectric VAC is 700 (still fine).

mouser in 1/2 w has all three of those types in vishay/dale.

Now for the important part. An acceptable voltage rating depends on the value of the resistor.

V = sqrt( P * R)
(note that this is just a refactoring of ohms law) is the maximum voltage than can be across the resistor and not dissipate more than 1/2 watt. In the case of 1/2 resistors P = .5.

So take a 3K0 1/2 W resistor. Plugging it into the equation, we get about 39 (38.72) Volts - so for this resistor, we need not worry about the voltage rating.

In fact, rearranging the equation to solve for the value of R that could exceed the voltage rating R= V^2 * P for a 350V 1/2 watt resistor, this come out to 245K0. This means if you have determined that a 1/2 W resistor will do the job, then you don't need even consider the voltage rating if it's resistance value is too low to ever be able to drop enough voltage across the resistor.
<b>
Now, what's the point of all this ? Just because a resistor is rated to 350WV doesn't mean you can't it use correctly in a circuit that has a 400V supply.
</b>
Note that the Xicon 1/2W resistors on Mouser are NOT mil spec and are somewhat inferior specification wise. Not surprisingly, the are about 1/3 the price. And unlike NOS parts, you are paying for better specs, not imaginary mojo.

Note however that this discussion does not address what the power rating of the resistor should be. If any one cares I'll address that later.
<b>
(comment removed, error corrected - sorry - )<b>

[ayan]

Now for the important part. An acceptable voltage rating depends on the value of the resistor.

V = sqrt( P * R)
(note that this is just a refactoring of ohms law) is the maximum voltage than can be across the resistor and not dissipate more than 1/2 watt. In the case of 1/2 resistors P = .5.

So take a 3K0 1/2 W resistor. Plugging it into the equation, we get about 29 Volts - so for this resistor, we need not worry about the voltage rating.

{snip}

If you find this "math" stuff un-comfortable, I suggest you learn ohms law and move up from "repair tech" to Engineer. If you take the time to learn ohms law, you will be able to design 99% of everything.

Eeerr.... I do find the above math uncomfortable because it is wrong, Jim. A 3K resistor will dissipate 0.5W when its voltage drop is 39V (38.7), not 29V.

I remember being an engineer right out of school exactly 20 years ago... techs have little respect for engineers because of their arrogance and lack of "street smarts," and in some cases, their attitude is warranted.

Cheers,

Gil

[Funkalicousgroove]

Jim,

What's your point? we're all friends here, you certainly don't have to prove your worth by spouting off alot if redundant info. If you say you're an engineer I'll believe you, but to assume that we aren't familliar with something as elementary as ohm's law is insulting and unnecessary.

for anyone who is unfamilliar here ya go:

[HiGain]

Great table. Thanks!

Jake

[Jim Donelson]

Actually, he is quite correct - some where in the post I changed the 3 to a 2.

However, the equation and procedure are correct. Glad to see that you guys don't shy away from that sort of thing.

I didn't say I was an engineer, but I am trying to get us thinking along those lines, (sounds like we are already) and to figure stuff out instead of just taking what we are told for granted.

Sorry if I came across as arrogant. In the future I will refrain from meta comments. I really did not mean to offend anyone, if anything I was trying to inspire.

(You'd be amazed at how many tube amp gurus don't know ohms law!)

[jelle]

Hi guys,

Does anybody of us happen to know what brand these light grey/blue-ish resistors are?
And are they metal films 100K/ 1.5K combo's?


Thanks again,
Jelle

Ps. FWIW I use the 1/2W MF resistors (plate and cathode) in a '97 clone and I like them....

[dogears]

Maybe Mepco. I have shots of a degooped Dumble and the ones in it said Mepco. Similar to the ones in this shot.

I would not fixate on these. I have even more pics of Dumbles using what look like the Dale RN65 type brown ones.

Hi guys,

Does anybody of us happen to know what brand these light grey/blue-ish resistors are?
And are they metal films 100K/ 1.5K combo's?


Thanks again,
Jelle

Ps. FWIW I use the 1/2W MF resistors (plate and cathode) in a '97 clone and I like them....

[groovtubin]

[quote="jelle"]Hi guys,

Does anybody of us happen to know what brand these light grey/blue-ish resistors are?
And are they metal films 100K/ 1.5K combo's?


Thanks again,
Jelle

[Funkalicousgroove]

I had an early Two-Rock with those Very Resistors, I didn't pay any attention to them, but The fellas over at Two-Rock are cool about most things, I'm sure they'd tell you. FWIW that was a less-than-spectacular sounding amp.

[groovtubin]

I had an early Two-Rock with those Very Resistors, I didn't pay any attention to them, but The fellas over at Two-Rock are cool about most things, I'm sure they's tell you. FWIW that was a less-than-spectacular sounding amp.

[Funkalicousgroove]

yeah, I used a Rodenberg Flexloop and it opened it up quite a bit. The amp was just too stiff and too Fendery.The guy who bought it swears by it, and so did I until I heard Dumble!!

[groovtubin]

yeah, I used a Rodenberg Flexloop and it opened it up quite a bit. The amp was just too stiff and too Fendery.The guy who bought it swears by it, and so did I until I heard Dumble!!

[groovtubin]

yeah, I used a Rodenberg Flexloop and it opened it up quite a bit. The amp was just too stiff and too Fendery.The guy who bought it swears by it, and so did I until I heard Dumble!!

[groovtubin]

yeah, I used a Rodenberg Flexloop and it opened it up quite a bit. The amp was just too stiff and too Fendery.The guy who bought it swears by it, and so did I until I heard Dumble!!

Yeah, i get to compare mine shortly w/late 90`s HRM, should be interesting. jim

[Funkalicousgroove]

BEauty is in the ear of the beholder!!! People Love their Two-Rocks until they hear My clone!!! although I do like the most recent Fuchs stuff, but they're still just a tad stiff for me. I think Dog has the best one out there, but his is Cathode biased, the clips sound very chewy to me.