This has me puzzled (resistor change)

[tub...@myshop.com]

There is a video on youtube, where a guy re-caps and repairs and tests
out a late 1940's Precision Apparatus E-200 signal generator.

Video name:
"Repair of a late-'40's Precision E-200-C RF signal generator"

This sig gen has a built in audio tone modulator (oscillator). He
replaced a resistor in that circuit, which was out of tolerance. He
replaced it with a modern film type resistor. The audio modulation did
not work afterwards. After testing everything, he went and replaced that
resistor (again) with an old style carbon resisstor, and after that, the
modulation worked.

What the heck?????

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????

[Cursitor Doom]

On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:

> I was always under the impression that film resistors are identical
> (electrically), and their only difference is appearance. Now I learn
> that is not true.....
>
> WHY?????

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





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[Sjouke Burry]

On 20-1-2019 23:50, Cursitor Doom wrote:
> On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:
>
>> I was always under the impression that film resistors are identical
>> (electrically), and their only difference is appearance. Now I learn
>> that is not true.....
>>
>> WHY?????
>
> 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.

[Phil Hobbs]

On 1/20/19 5:50 PM, Cursitor Doom wrote:
> On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:
>
>> I was always under the impression that film resistors are identical
>> (electrically), and their only difference is appearance. Now I learn
>> that is not true.....
>>
>> WHY?????
>
> Really need to see a schematic. AFAIK the only functional difference
> between film and carbon is that latter are noisier.

They also have a large negative voltage coefficient.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

[Tim Schwartz]

On 1/20/2019 6:24 PM, Phil Hobbs wrote:
> On 1/20/19 5:50 PM, Cursitor Doom wrote:
>> On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:
>>
>>> I was always under the impression that film resistors are identical
>>> (electrically), and their only difference is appearance. Now I learn
>>> that is not true.....
>>>
>>> WHY?????
>>
>> Really need to see a schematic. AFAIK the only functional difference
>> between film and carbon is that latter are noisier.
>
> 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

[John Robertson]

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 :-#)#

[John-Del]

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

[tabb...@gmail.com]

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

[tub...@myshop.com]

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.

[tabb...@gmail.com]

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

[Look165]

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.

[Ralph Mowery]

In article <2450f296-8d19-4c68...@googlegroups.com>,
tabb...@gmail.com says...

>
> > I was always under the impression that film resistors are identical
> > (electrically), and their only difference is appearance. Now I learn
> > that is not true.....
> >
> > WHY?????
>
> So IIUC he changes the R value and it no longer oscillates. Presumably he's thereby changed the loop gain. Where is the surprise?
>
>
>

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 ?

[John Robertson]

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 :-#)#

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[John-Del]

Prob not "bad", but maybe just way off value (or read incorrectly as you surmised). 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 value. I only keep them to put in old tube radios for authenticity's sake.

[gghe...@gmail.com]

On Sunday, January 20, 2019 at 6:25:07 PM UTC-5, Phil Hobbs wrote:
> On 1/20/19 5:50 PM, Cursitor Doom wrote:
> > On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:
> >
> >> I was always under the impression that film resistors are identical
> >> (electrically), and their only difference is appearance. Now I learn
> >> that is not true.....
> >>
> >> WHY?????
> >
> > Really need to see a schematic. AFAIK the only functional difference
> > between film and carbon is that latter are noisier.
>
> They also have a large negative voltage coefficient.

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.

[Jon Elson]

tub...@myshop.com wrote:


>
> I was always under the impression that film resistors are identical
> (electrically), and their only difference is appearance. Now I learn
> that is not true.....

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

[Phil Allison]

Jon Elson wrote:

>
>
> 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.

** In fact, film resistors of all values are made that way.

> That can add inductance, and the higher the resistance value they
> are aiming for, the more inductance.

** Absolute CRAP !!!!

Where does this shite come from ??

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

> In tube circuits, it is not unusual to
> have megohm resistors, so I can see how this could be a problem.
>

** Huh ????

Jumps straight from the rediculous to the absurd.

Go away fool.


.... Phil

[Mike Coon]

In article <MPG.36afad472...@news.east.earthlink.net>,
rmower...@earthlink.net says...

>
> 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 ?

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.

[Ralph Mowery]

In article <6ea7b3ba-ea28-4479...@googlegroups.com>,
ohg...@gmail.com says...

>
> Prob not "bad", but maybe just way off value (or read incorrectly as you surmised). 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 value. I only keep them to put in old tube radios for authenticity's sake.
>
>

I have not done much repairing electronics in a while and had a bunch of
1/2 watt carbon resistors that I had been using.

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.

[mako...@yahoo.com]

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.

m

[Phil Allison]

Ralph Mowery wrote:


>
>
>
>
> 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.
>

** If you find an electro well above its marked capacitance value - what do you conclude ??



... Phil

[Sjouke Burry]

That the oxide layer has degraded, and might be about to fail?

[John-Del]

Back when I used to carry a big box of tubes (valves) for on site repair, I could spot any tube out of hundreds in seconds. After a long time using them, we don't "read" the tube number as much as just flat out recognize it - just like when reading words.

[Phil Allison]

Sjouke Burry wrote:



> >>
>
> >
> >
> > ** If you find an electro well above its marked capacitance value
> > - what do you conclude ??
> >
> >
> >
> > ... Phil
> >
>
> That the oxide layer has degraded, and might be about to fail?
>

** How do you check for this?


..... Phil