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John Robertson

On 2019/05/20 1:19 p.m., 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 :-#)#