Re: electrical deaths

On 2024-12-01 21:42, Liz Tuddenham wrote:

john larkin <JL@gct.com> wrote:
 

On Sat, 30 Nov 2024 15:12:47 +0100, "Carlos E.R."
<robin_listas@es.invalid> wrote:
>

On 2024-11-30 06:29, Don Y wrote:

On 11/29/2024 7:33 PM, Edward Rawde wrote:

I don't think the back of our tube-based TV was ever on.
While it was under warranty, service people seemed to arrive monthly.

>
It was a routine task for me to be charged with pulling ALL the
tubes and walking up to the corner drugstore (where they had a
tube tester) to check them.  Invariably, one would be "weak",
replaced and then the TV returned to "new" condition.  A fair
bit cheaper than a replacement TV.

>
What meant "weak"? Perhaps they had lost a bit of vacuum? Or a problem
with the cathode?

>
Usually bad cathode emission, sometimes gas, rarely a short.
>
CRT cathodes would get weak, and one could add a filament voltage
booster thing to get a bit more lifetime out of a tube.

  One of the problems was identified as an insulating layer building up
between the bulk of the cathode coating and the surface layer.  If I
remember correctly it was caused by molecules of occluded gas (mainly
oxygen) being released from the electrodes.  Better de-gassing methods,
which included induction heating the electrodes while the valve was on
the pump, coupled with improved getters, improved valve life - but these
improvements only came at the end of the valve era and didn't benefit
1950s televisions.
 The problem was made worse by the set designers trying to screw the
maximum performance out of each valve and component  ...and then some.
Over-run screen grid resistors changed value, which overheated the
screen grid and released gasses, which caused ionisation, increased the
anode cureent and overheated the anode - which released more gas etc.

I did not study valves in college, nor saw them in the field. It was the transistor and IC era. But I looked at valves with some interest.
--
Cheers, Carlos.