This has me puzzled (resistor change)

Really need to see a schematic. AFAIK the only functional difference between film and carbon is that latter are noisier.

A bulk carbon resistor has less self-inductance.

They also have a large negative voltage coefficient.

Cheers

Phil Hobbs

Also, Carbon composition resistors will take spikes and surges that will render a film resistor open circuit, and are more tolerant of short term overloads.

Regards, Tim

If I am not mistaken that is why film resistors are used where you want flame-proof behaviour.

Carbon comp let out lots of magic smoke...

John :-#)#

And when they don't, they change value in a big way when overheated.

So IIUC he changes the R value and it no longer oscillates. Presumably he's thereby changed the loop gain. Where is the surprise?

NT

I kind of thought that might be the case....

I prefer using the old carbon resistors in vintage tube gear, but they are getting hard to find and expensive. They are all NOS now. i dont think there is any place making them anymore.

Rapidonline does carbon comps. I'm sure it's far from the only supplier. There's no upside to them other than pulse power ability & for some cases looks.

NT

Carbon resistors are noisy.

We used that for making white noise and then by filtering, make sine wave or else.

With metal resistors there is not enough noise.

Could it have been possiable that the new resistor was bad, or maybe the color code was read wrong so the wrong value was put in ?

On our jukebox tube amplifiers we test every resistor as many of them have happily drifted off-spec more than their tolerance. Usually plate of cathode resistors of course because they pass the most current.

John :-#)#

he's thereby changed the loop gain. Where is the surprise?

Prob not "bad", but maybe just way off value (or read incorrectly as you su rmised). I've been given tons of old parts including resistor assortments. You can't believe how often the wrong resistor (usually multiplier) was in the wrong drawer.

When sorting new old stock carbon carbon comp resistors, I don't sort them by printed value, but actual measured resistance since so many of them are closer to the next value up or down than they are to their own printed valu e. I only keep them to put in old tube radios for authenticity's sake.

I was going to say they also have a negative temperature coef. But I'd be wrong. A 1.5 kOhm CC in hot air gun and resistance rises. (I know we use to use these as low T (LN2/ liquid He) temp sensors with a resistance that rises with lower temp. (Kinda like it was a thermally activated semi-conductor.))

So I stuck the same 1.5 k resistor into LN2... R increased to 2.1 k!

George H.

Very high value film resistors are made with some composition sprayed on a ceramic tube, and then an abrasive jet cuts a spiral groove in the film element. That can add inductance, and the higher the resistance value they are aiming for, the more inductance. In tube circuits, it is not unusual to have megohm resistors, so I can see how this could be a problem.

Jon

Where does this s**te come from ??

ONLY with *low value* resistors can the TINY inducatance created by spiralling become significant and then only at RF frequencies.

Jumps straight from the rediculous to the absurd.

Go away fool.

.... Phil

As a lad, scratching through my dad's old box of mixed resistors, looking for a near-enough value to one I needed, I used to get a strong urge to choose any old value and re-paint the colours...

Mike.

I have not done much repairing electronics in a while and had a bunch of

Found out when I ordered an assortment of modern reisitors that they had an extra band where there were 3 digits and the multiplier instead of the 2 digits and multiplier that I was use to.

I am well aware that the older carbon resistors often go up in value as they age even if new. I always measure the resistors before I use them. I do the same with the electrolytic capacitors.

I find it amazing how I can spot a certain color code in a box of mixed up resistors.

I'm looking for yellow purple red.

Ah there's one.