carbon comp resistors

[romberg]

Does it measure the whole curve (20 points) or just one point in that 1ms pulse? If it can capture all 20 data points in 1ms then that is 1/4 of a 4k triangle wave.

Mike

[martin manning]

It gets one data point at a time using multiple 1 ms pulses. In this case both channels are measuring at the same time, to very nearly the same voltage. The resistors are getting hit with 1 ms square wave pulses that progress from 10 to 350V.

[Mark]

Here is a bit more that RG Keen had to say on carbon comp resistors.

1. high voltage across the resistor is necessary, in the range of 100V on up
2. large signal swings across the resistor are needed - ideally, a large fraction of the static DC voltage so you have signal swings of 50 to 100V too.
3. only positions in the amp that have both high DC voltage and wide signal swings as in 1 and 2 will give you enough resistor distortion to benefit from; other places should be chosen for low noise and/or economy.
4. resistor power rating should be the minimum needed to work for a reasonable life in the circuit to maximize resistor distortion. Maybe a good guideline is that the dissipation should be selected to be as close to two times the average dissipation as possible.
5. as a corollary to the power guideline, we should be prepared to replace CC's every few years as the life at high temp makes them drift and get noisy(-er)

The distortion is specified to at 0.035% per volt.

In the Radiotron Designer's Handbook ( 4th edition, pg. 1345) they list the JAN-R-11 specification for CC resistors as less than 0.035% per volt for 1/4 and 1/2W resistors, and 0.02% per volt for higher power ratings. Given that the max voltages for these parts was 1/4W- 200V; 1/2W - 350V; 1W and 2W - 500V, that works out to a 7% change in resistance for a 1/4 W part used at its max voltage, a 12.3 % change for a 1/2W, and a 10% change for bigger resistors.,

But wait there is more.

More importantly, these percentages represent the maximum beyond which a resistor would have been rejected in the 1950's. Today's CC resistors are much lower distortion. From IRC's web site, we find some numbers. A typical resistor voltage coefficient can be seen at http://www.irctt.com/pdf_files/IBT.pdf - which shows carbon comp at 0.005%/volt for that company's products. Another was 0.008%/V. These are smaller than the max allowed under the JAN military spec.

I SUSPECT the carbon comp pictures is the type of resistors RG Keen was referring to.

When RG Keen put out this article, I put an 0.5 watt 82K and 100K carbon comps on the P. I. and 0.5 watt 220K resistors on the grid leak/bias stage. I listened carefully and I heard no change in timbre at all.

Thanks to all who contributed.

[martin manning]

I don't believe it is necessary to have a standing voltage across the resistor to observe the voltage coefficient of resistance. It refers only to the change in resistance with applied voltage. A pulse measurement is preferred since the temperature of the resistor stays nearly constant.

[romberg]

Maybe this spec applies to how much a resistor is allowed to drift in value as it nears the point of cooking itself to death? So, maybe the measurements should all be made at the same temperature but that temperature should be at the high extreme rather than the low? So, maybe run a 1/2 watt 100k resistor at 220Vdc for a while and warm that sucker up. Then run the uTracer and see what you get? Maybe EVH was right in wanting the sound of an amp right on the edge of destroying itself?

Mike