24 Jul
2005
24 Jul
'05
6:41 p.m.
Subject: Re: local demolition of SAGE building
From: shoppa_classiccmp at trailing-edge.com (Tim Shoppa)
Date: Sun, 24 Jul 2005 13:11:41 -0400
To: cctech at classiccmp.org, cctalk at classiccmp.org
This is very true.
The tube machines were not friendly to power cycling. Due to the amount of
power invloved and loading effects. The preference was to kill B+ (high volts)
to the affected section and keep the filaments hot. Further like all thermally
affected things temperature cycleing ws not only a filament problem but tube
seals (glass to metal) and element movement is problematic. Many high gain
tubes have remarkably close spacing of the elments particulary cathode to
grid1. Another phenomenum was tubes have materials to grab gasses and ions
in them that only work well when hot. Temperature cycling was (and still is)
a reliability issue. It was just much more easily noticed when you have
more than ten thousand tubes and the differential temps from cold to hot
(both filament and cabinet) are so large. The smaller mini and submini tubes
were only somewhat better but smaller means closer in smaller space and
prescreening weeded out infant mortality at the mechanical level.
There were even special tubes developed that would better withstand being
at cutoff for extended periods without losing emission. I have a few
of these, they are pin compatable with 12AX7 duo triode but have slight
differences as they are for extended operation at cutoff.
Outside of computers I have years to commercial radio where tubes have
had long standing in the transmitters especially at high power. Power
cycling was and still is avoided. When the power has been removed such
as for maintenance or expecially retubing (done based on time in service)
the initial bring up is done slowly to warm up the "getter" (degassing)
and to avoid thermal shocking parts. It's also not uncommon to see step
starting built into the power supplied to avoid the harsh effect of
surge loading the power lines or trying to bring 500uf of capacitor up to
voltage in a half cycle (kills rectifiers!).
The last bastion of widespread tube use was television and over time even
tube sets (particulary those with series string heaters) adopted
"instant on" circuits which ran the filaments at 50-70% when powered off
and removed the operating DC. This reduced warmup time to nil and also
improved the operating life of the tubes and the reciever greatly.
Treated right tubes(valves) offer excellent reliability and lifetimes
mistreaded and those can be drastically shortend.
Allison
The minis and subminis of the 1950s, made for
computer usage,
were really not effected by power cycling. Filament burnouts on these
types of tubes are exceeding rare.
But inter-element shorts and leakage become more common, and my
experience (backed up by many years of leaky/shorted tubes) is that
these are related to power-on cycles. (i.e. "it worked last time I
used it, but then I turned it on and all it does is hum").
Maybe reduced sizes make physical tolerances between the elements more
critical in the smaller tubes. Even though a lot of the leakage
I've seen is between adjacent pins, not between adjacent elements.
Tim.