Value drift over time

[John Robertson]

On 2019/05/18 11:22 a.m., Cursitor Doom wrote:

On Fri, 17 May 2019 21:19:07 +0100, Mike Coon wrote:

I can hardly wait!

Then I shall keep you in suspense no longer.

Here's what I found from a random selection of old components I snipped
out. Firstly, pretty much *all* the capacitors were fine. The 350VDC
Hunts capacitors could easily have been new. An Erie plate ceramic of
0.01uF likewise. A Dubilier type SM22 50pf cap, however, had gone up to
62pF. That one was one of the ones used for tuning. The biggest changes
were as expected in the carbon resistors, all of which aged to higher
values like so:

27k became 38.6k (bad)

another 27k ----> 29k (within 10%)

100k ----> 107k (within 10%)

10 ----> 10.7 (ditto)

3.3k ----> 4.2k (bad)

4.1k ----> 5.2k (bad)

15k ----> 20.7k (bad)

220k ----> 246k (barely bad - just above 10%)

8.2k ----> 9.9k (bad)

400k ----> 509k (bad)


These were all marked with a silver tolerance band, so clearly Taylor
back then at least not *that* bothered about accuracy.

No, those resistors have drifted since their original construction.
People did have ohm-meters back then and would verify values on
resistors particularly if they were colour deficient or colour blind as
I did, testing Rs when building kits - red/green deficient vision. I
still check almost all resistors I come across with a meter as I don't
trust my green, dark red, and brown differentiation.

If they are in power circuits - plate or cathode or voltage dropping
then those do drift as they were only compressed carbon and heat/cooling
cycles would cause them to change value - usually upwards as you have
discovered. I consider your values typical of tube gear that is 40 or
more years old.

1920s resistors were a chunk of carbon rod with a wire wrapped around
each end then dipped in a sealant - how long do you think that value
would last within X%? Wire wound resistors external connections were
pressure bonded to the resistance wire, not uncommon for that joint to
fail over time...

Like I said earlier you HAVE to verify all the resistors as they wander
over time. Modern Rs are much more reliable when operated under their
rated wattage.

John :-#)#

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On Monday, 20 May 2019 13:40:53 UTC+1, John Robertson wrote:

1920s resistors were a chunk of carbon rod with a wire wrapped around
each end then dipped in a sealant - how long do you think that value
would last within X%? Wire wound resistors external connections were
pressure bonded to the resistance wire, not uncommon for that joint to
fail over time...

I presume you mean carbon composition rod. Pure carbon would give too few ohms to have much use in a valve radio.

Some Rs used metal caps instead of wires, and were mounted in a clip-in holder. I guess they needed to be replaced sometimes as different valves sometimes needed different grid leak values.

Like I said earlier you HAVE to verify all the resistors as they wander
over time. Modern Rs are much more reliable when operated under their
rated wattage.

John :-#)#

as long as it's well under. Rated life for power Rs operated at specced power can be terrible.


NT

 

[Cursitor Doom]

On Mon, 20 May 2019 08:11:31 +0100, Mike Coon wrote:

In article <rvOdnSbLyqpFZHzBnZ2dnUU7-TmdnZ2d@giganews.com>,
jdangus@att.net says...

Hydrogen atoms are really really small.
Trying to keep hydrogen in or out is always problematic.

In practice you get molecules on the two-fer principle which are much
bigger. Helium is bad too because then the atoms come at you singly...

May seem counter-intuitive, but hydrogen is not actually the smallest
atom. Heliums come singly alright, which is one of the reasons, along
with small size, they're used for this kind of leak testing.



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[whit3rd]

On Monday, May 20, 2019 at 3:46:16 PM UTC-7, Cursitor Doom wrote:

On Mon, 20 May 2019 08:11:31 +0100, Mike Coon wrote:

In article <rvOdnSbLyqpFZHzBnZ2dnUU7-TmdnZ2d@giganews.com>,
jdangus@att.net says...

Hydrogen atoms are really really small.
Trying to keep hydrogen in or out is always problematic.

In practice you get molecules on the two-fer principle which are much
bigger. Helium is bad too because then the atoms come at you singly...

May seem counter-intuitive, but hydrogen is not actually the smallest
atom. Heliums come singly alright, which is one of the reasons, along
with small size, they're used for this kind of leak testing.

Good point; also, the H2 molecule has two 1S-like electrons, which is
very similar to a Helium atom, and just about as slippery.
H2 atoms are lighter than He, on average.

 

[whit3rd]

On Monday, May 20, 2019 at 3:46:16 PM UTC-7, Cursitor Doom wrote:

On Mon, 20 May 2019 08:11:31 +0100, Mike Coon wrote:

In article <rvOdnSbLyqpFZHzBnZ2dnUU7-TmdnZ2d@giganews.com>,
jdangus@att.net says...

Hydrogen atoms are really really small.
Trying to keep hydrogen in or out is always problematic.

In practice you get molecules on the two-fer principle which are much
bigger. Helium is bad too because then the atoms come at you singly...

May seem counter-intuitive, but hydrogen is not actually the smallest
atom. Heliums come singly alright, which is one of the reasons, along
with small size, they're used for this kind of leak testing.

Good point; also, the H2 molecule has two 1S-like electrons, which is
very similar to a Helium atom, and just about as slippery.
H2 molecules are lighter than He atoms, on average.

 

[John Robertson]

On 2019/05/20 1:19 p.m., tabbypurr@gmail.com wrote:

On Monday, 20 May 2019 13:40:53 UTC+1, John Robertson wrote:

1920s resistors were a chunk of carbon rod with a wire wrapped around
each end then dipped in a sealant - how long do you think that value
would last within X%? Wire wound resistors external connections were
pressure bonded to the resistance wire, not uncommon for that joint to
fail over time...

I presume you mean carbon composition rod. Pure carbon would give too few ohms to have much use in a valve radio.

Thanks, I did mean composite.

Some Rs used metal caps instead of wires, and were mounted in a clip-in holder. I guess they needed to be replaced sometimes as different valves sometimes needed different grid leak values.

Ads in the early radio magazines are handy for that sort of data. And
yes, you did keep a drawer of grid resistors handy if you wanted best
performance out of your rig.

Like I said earlier you HAVE to verify all the resistors as they wander
over time. Modern Rs are much more reliable when operated under their
rated wattage.

John :-#)#

as long as it's well under. Rated life for power Rs operated at specced power can be terrible.


NT

Power derated curve curves can be surprising for almost any component.

Heck Molex pin connectors are rated at 25 insertions if I recall
correctly, and they are heavily used in arcade games...

Spec sheets do need to be read after all.

John :-#)#

 

[Ralph Mowery]

In article <1c3fb9af-1eba-4642-92da-dc860731b861@googlegroups.com>,
whit3rd@gmail.com says...

May seem counter-intuitive, but hydrogen is not actually the smallest
atom. Heliums come singly alright, which is one of the reasons, along
with small size, they're used for this kind of leak testing.


Good point; also, the H2 molecule has two 1S-like electrons, which is
very similar to a Helium atom, and just about as slippery.
H2 atoms are lighter than He, on average.

Hydrogen is also explosive or will burn. That is another reason not to
just spray it out for leak detection.

I think that hydrogen may be the smallest atom, but they often join in
pairs to make up a larger molicule. Some other atoms that are normally
gas do the same thing.

 

[Cursitor Doom]

On Tue, 21 May 2019 10:20:02 -0400, Ralph Mowery wrote:

Hydrogen is also explosive or will burn. That is another reason not to
just spray it out for leak detection.

I think that hydrogen may be the smallest atom, but they often join in
pairs to make up a larger molicule. Some other atoms that are normally
gas do the same thing.

Oh boy. Where do I start?



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