R(k) listening report

Magnet wire resistors, enough to make any engineer scream.
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DowdyLama
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R(k) listening report

Post by DowdyLama »

Dr. Dave was kind enough to provide a prototype pair of 910 ohm resistors to me - for use as a very low wattage R(k). Tube was a ni choke-loaded 3C24; B+ was approx 335 v; 120 mfd oil bypass capacitor.

Regardless of what theory may dictate, this was a very audible improvement vs a standard NIWW resistor [Mills]. The closest sonic analogy I can make would be to the act of removing an unnecessary grid stopper. Considerably more clarity and focus!
dave slagle
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Post by dave slagle »

lets start looking at the three main issues i see with the copper resistors.

Temperature co-efficient of copper: copper changes resistance with temperature. a 30 degree change in ambient temperature is roughly equal to a 5% change in resistance. Since most of these things are placed near tubes... you know they are going to get warm. Now add to this the addiitonal heating from the copper losses and its clear getting a 1000 ohm resistor that carries 60ma in a hot amp chassis might require a resistor that measures 750 ohms before you solder it into the circuit. Luckily, i suspect that the temperature of an amp after an hour of operation becomes very stable so it just requires a little planning. If the resistor is the cathode resistor of a tube, you just have ot be aware that it will operate a bit hard until it stabilizes. Fortunately this is a self correcting behavior, since the extra current will cause the resistance to go up more quickly. We will need a few of you suckers... um i mean testers to report back to get a grasp of this.

L: these things are inductive, the ones i did for jim were bifilar and when wired to cancel inductance they were not too alarmingly inductive. they can be rewired to be very inductive (aircored) and that is always a listening option.

C: the sheer amount of wire (look at a wire table and resistance per thousand feet) makes these things capacitive.

Now lets assume these beasts are going to be bypassed by a capacitor anyways. From the simple model, that means they are out of the signal path and shouldn't make a difference. So I guess we need to dig a bit deeper. I have been looking at current loops lately and what if we look at it this way: Assume at a given frequency the impedance of the resistor is 100X the impedance of the capacitor. This suggests 99% of your audio current will travel through the cap and 1% of it through the resistor. If we hear a difference with that 1% of current through various different resistors, (remember 1% is only 40dB) maybe we want an inductive cathode resistor to make that ratio 1000:1 (60dB) or greater. Another approach could be to just get multiply the value of the cap by 10 :-) or finally maybe just using a better conductor with a built in bypass cap might just sound better even though it creates a third current loop :-)

i dunno.... theory is easy and predicts a circuits function and assures adequate sound. I want to start with the idea of awesome sound and then find the theory that matches.

dave
Johnny
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Post by Johnny »

Hi Dave,


What kind of inductance are you getting with these? With a cap in parallel you have a "tank" circuit with a oscilation frequency of roughly the square root of 1/LC.

Johnny
dave slagle
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Post by dave slagle »

Johnny wrote:Hi Dave,


What kind of inductance are you getting with these? With a cap in parallel you have a "tank" circuit with a oscilation frequency of roughly the square root of 1/LC.

Johnny
interesting... this is a two fold problem, first I am not confidant on the inductance measurements that my meter spits out (B&K 885) and second dowdy has the resistors :-)

when i do the next ones, i'll tell you what the meter says. iirc from the ones i sent jim, in the inductance cancelling position it was like 15 uhy and in the adding hookup it was around 100 mhy. jim mentioned a distinct difference in sound between the two hookups and the position of the tank may give us some insight as to why.

the next round of them will be designed here so we will see. BTW, anybody with some nice test equipment willing to measure, please chime in, i'll do some extras.

dave
DowdyLama
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that's it!

Post by DowdyLama »

Thanks Johnny - I've been scratching my head wondering why there was a pronounced flabbiness in the non-inductance canceling mode...the mini-tank circuit is the answer!
dave slagle
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Re: that's it!

Post by dave slagle »

DowdyLama wrote:Thanks Johnny - I've been scratching my head wondering why there was a pronounced flabbiness in the non-inductance canceling mode...the mini-tank circuit is the answer!
whoa there... can we at least wait to see some numbers before we pronounce the patient dead :-)

i would love to know where the tank circuit happened, remember each hookup has a tank circuit, so the frequency where it happens would be a nice help so the next time we hear some flabby bass, we can look for a tank circuit around that frequency.

i propose the next pair i document the measurements prior to sending them and if the results are the same, we see where the tank circuit is.

i really think if we predispose ourself with measurements prior to listening it can have an influence on what we hear. (or want to hear)

dave
DowdyLama
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yes...

Post by DowdyLama »

Sorry if my enthusiasm got the best of me - what I intended to communicate was that the tank circuit was the first possible explanation that made sense to me.
dave slagle
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Post by dave slagle »

hey jim,

had a chance to play with your resistors tonight... trying to get heating (both ambient and copper loss) under control... or at least predictable.

i played with the 260 ohm ones you requested, but i need to know two things... what is the current through them, and where will it ultimately be placed. Both of these lead to heating concerns and i need to know what room temp DCR to set them at so they hit your desired value during operation.

i'm not exactly sure how to determine the operational chassis temperature and i believe this will be the biggest factor heat rise factor.

dave
DowdyLama
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260 ohm resistors

Post by DowdyLama »

R(k) for a 75TH, current draw is about 25 mA...placed under the chassis.

I don't anticipate under-chassis temperature will ever exceed 90 degrees F.
dave slagle
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Post by dave slagle »

so i'll do them to be 260 ohms at 90 degrees with 25ma of current.

but i honestly think it will be warmer than 90 degrees, ambient temp with the amps off can be that in the summer without AC. maybe 120 degrees is a better target temperature for the 260 ohms???

let me know.

dave
DowdyLama
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Post by DowdyLama »

Gosh Dave, I really don't know what the temperature will be. So your best guess will be good enough for me.

How much variance will there be? Meaning, if the resistor is 260 ohms at 90 degrees F, what will it be at 120 degrees F? [Or vice-versa?]
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Post by dave slagle »

DowdyLama wrote: How much variance will there be? Meaning, if the resistor is 260 ohms at 90 degrees F, what will it be at 120 degrees F? [Or vice-versa?]
a resistor wound to be 248 ohms at room temp (68 degrees) will be 260 ohms ay 90 degrees and 277 ohms at 120 degrees.

I think once the amp stabilizes, the temp inside will be relatively consistant, but finding out what that temperature actually is will take some experimenting. Simply measuring the hot resistance will tell us the temperature.

then we have to add the heating from the power dissipated, but luckily the first experiment with your resistor showed only an increase form 265 to 287 ohms with 100ma of current. (2.6W) and you will be at a fraction of that power so i suspect it can be ignored. The difference in heating from current with #36 and #40 wire rinning 100ma was quite small which tells me they are quite good at dissipating heat.

with the #40 wire it turns out to be a single layer so removing turns to lower resistance is relatively easy so lets start with a target resistance of 260 ohms at 90 degrees. I suspect you will be removing turns, but at .44827 ohms per turn, it shouldn't be to difficult to adjust :-)

once we establish a pattern for the best sounding connection adding a tap or two shouldn't be too hard. Do the whole resistance at 60, then add taps at 90, 120, and 150

we need to first establish (and confirm) if the inductance cancelling or adding sounds best and if wire guage makes a difference.
Guest

Post by Guest »

The real difference between 260 ohms and 277 ohms in the cathode of the 75TH is minimal. And by the way, I was only drawing 24 mA [I initially said 25 mA, but that was incorrect].

L cancelling mode was the clear winner in the first experiment [910 ohms in the 3C24's cathode], we'll see if that holds true this time around.
DowdyLama
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Post by DowdyLama »

The 260 ohm [nominal] resistors have been installed.
Excellent sonic results once again, quite similar to previous experiments.
The L-cancelling mode still sounded best, however the difference between L-cancelling and non-L-cancelling was much less than with the 910 ohm resistors. Dr. Dave assures me that is simply the function of less layers of wire.
After running for several hours, measured resistance was 266 ohms...sorry, but it did not occur to me to attempt an under-chassis temperature measurement.
FWIW, final current draw is approx 21 mA.
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