Bob, are you sure "Sleeping sickness" also occurs when B+ is applied
without "filament" power? I never heard of that before, although that
clearly doesn't mean it doesn't happen. <snip>
Yes, it does! I witnessed this happen one time which resulted in an
supervisor earning a little unpaid vacation time.
Old multiplex equipment associated with Types J, K and L carrier
systems used by AT&T's various companies built in the '40s and '50s
used basically two tubes...the 311B triode, and the 310A sharp cutoff
pentode, essentially a five prong, 5 V filament 6C6, for anything
below the mastergroup MUX level. Above that, the 404A (basically a 5V
6AK5) and the 417A single triode were used for mastergroup gain and
stacking. One day, a migration to IC-based equipment on another floor
occurred in our office, the largest carrier office in the US,
rendering an entire floor's worth of antique channel modems, group
demods, supergroup demods and all associated equipment such as carrier
supplies to be relegated to "spare" status. A transmission man
working that floor, trying to earn a few "brownie" points,
disconnected all the -24V filament battery at the BDFB to all this
gear. Laziness and timidity precluded him from removing the +130 and
+315 plate supplies. Thus, over 750 311B and 310A tubes were left in
situ with their usual B+ on the plates and cold filaments.
About three months later, a surge in traffic demand prompted the
circuit provision bureau to reassign new multiplex facilities to this
equipment, and within a short lead time. When such work happens, the
"circuit order" worker tests the gear both directions, sets levels as
appropriate and checks for basic transmission impediments. In this
case, the equipment didn't pass tone anywhere and wouldn't mod or
demod anything at all, and a trip to the BDFB found boxes of 1 1/3 amp
grasshopper fuses all placed neatly on the floor in front of the fuse
bay. After replacing all the filament supply fuses, the equipment
still failed, but some of it would pass modulated/demodulated signal,
but at bad levels and with not nearly enough gain to meet
specifications. After some checking, they called me down to try to
figure what happened.
Western Electric gear from that era used an "in service" tube test
regimen that looked basically at plate and filament current and
"filament activity" (an old term that really meant "cathode activity"
in anything other than direct heated tubes.) The in service tests
showed acceptable filament current, but the plate current was either
gone or very weak. In cases where there was at least some plate
current, dropping the filament current 10% wouldn't cause a dip in the
plate current...odd. A trip to the Hickok Cardmatic (KS version, of
course) showed all the tubes on the entire floor to be "dead" for Gm.
That's when the "brownie" said, "Oh...well, I took all the filament
fuses out of everything to save power. I reported it to my boss, and
he put an attaboy in my folder." A little investigation proved this
to be true, and the supervisor was given some time off for being an
idiot. A look at the Bell System Practices relating to vacuum tubes
specifically stated that at no time should any tube of any
configuration, except for cold cathode tubes, be allowed to stand with
B+ on any element without the filament being hot.
Some further investigation with the folks at the Littleton, CO WECO
tube plant confirmed that running any tube with the plates energized
and no filament will cause the same, or worse, symptoms as "sleeping
sickness" generally attributed to having a tube run in cutoff for long
periods of time. In short, what happens in either case of "sleeping
sickness" is that the plate winds up acting as a getter, thus becoming
unreceptive to electron reception from the cathode after being plated
with contaminents within the envelope. That explained immediately why
the tubes, while testing bad for Gm, tested good for cathode activity.
This was further confirmed by the fact that newer tubes were still at
least conducting something, while tubes that were some 30-40 years old
were completely dead on test, although the records showed their last
"in service" current test to be well within specs. Conversations with
retiring engineers at the tube plant confirmed that no "real life"
vacuum tube had a very good vacuum in it, and even if it had one, it
would be partially destroyed during the initial aging process by
gasification of the tungsten on the filament and thorium from the hot
plates. That's why tubes have getters in them, after all. As the
fellow told me, "You cut off electron flow, and that plate makes a
really attractive getter...the higher the B+, the more it "gets!" Add
to this that the cathode, grids and filaments are all at or near
ground potential, and you see how this can happen to the plates.
In the final tally for this goof, over 350 310A tubes, at $150 a pop,
and 200 some odd 311Bs, at $75 a pop, had to be replaced on an
emergency basis. At the time, Western Electric was getting out of
tube manufacturing altogether, and the assembly and aging lines for
the old ST envelope tubes were out of commission while the equipment
was being sold to Richardson Electronics. As it turned out, a canvass
of toll offices across the country had to be done to mine every
available 310A and 311B, even old "pulls" from retired equipment, to
get the MUX gear back into service. As it was, the due date for the
facility additions was jeopardized by over two months, and the carrier
group responsible for the gear (ours) had to buy all new Richardson
tubes for the offices which gave up their spares. Total cost of the
fiasco: over $130,000. There was little solace in the fact that the
removal of the filament battery saved about $500 in power costs. To
add insult to injury, the equipment only carried the service for
another six months before being finally retired and scrapped.
"Audiophools" worrying about "cathode stripping" has nothing whatever
to do with "sleeping sickness." I've yet to see any "audiophool" who
actually knows how a tube works, anyway. You have to expect this from
people who refer to audio phenomina as "air," "stage," "detail,"
"crispness" and other assorted laughable terms.
dB
Sidebar: On that particular floor resided many old pieces of gear
from the 1930s, including bays of voice order wire equipment
associated with long gone J and K carrier systems. In them were rows
of bayonet based 101D triodes and 201As, most dating from the 1930s,
some from the '40s. All tested good when pulled after 45+ years of
continuous service. I shudder to think what these old things would've
brought today on fraudBay. The secret to long tube life at the phone
company? Running filaments 10% below rated voltage and excellent
quality elements. The Richardson replacements which came later were
nowhere near the quality of any old WECO tube, and WECO tubes made in
the early '80s were almost as bad.
. If Ludwell Sibley's publications were available
