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QUESTION:Has anyone found a good sub for the brown or green muck used as base
cement on most tubes.
In the past I have used Bondo where the muck completely fell out and
super glue if it was just loose.
I was considering stove cement, the dry kind so I can store it forever,
but $14.00 for a box of it.
I have gotten portland cement splashed on a window once and forgot to
wash it off. It stuck so good I had to scrape it off. I suspect that
Portland cement is the base of the stove cement.
I prefer not to use stuff like liquid nails or epoxy, as some of the
tubes I want to re-set are power outputs and get quite hot.
ANSWER: It may not be the best idea to use a super glue to fix loose bases.
Hardened super glue has a high coefficent of thermal expansion and at
least one person reported a cracked bulb of a ST-12 size tube operated
at working temperature. If you have to use the super glue then at
least do not use it a continuous bead but leave gaps.
The best medium to fix loose bases is shellac - it was actually used
by RCA in the Harrison plant for that purpose. Use a syringe or a
squirter, apply shellac around the entire base, wrap multiple rubber
bands around th base and bulb in order to apply pressure, and give it
a day or two. Make sure the rubber bands press the bulb uniformly and
that the axes of symmetry for both bulb and base coincide.
The best cement for re-basing and large repairs is a epoxy rosin that
is not brittle when hardened. The best choice is Loctite 480.
Lud Sibley once found the following 1948 RCA recipe for the C6 basing
cement used by RCA for large receiving and broadcasting tubes. The
recipe below gives 200 lbs, an amount sufficient to re-base 23,500
Type 50 tubes:
D3 durite (durite resin #275 from Stokes & Smith in Philidelphia) -
7.5 lbs
S15 shellac 19.5 lbs
R3 rosin 3.25 lbs
M3 marble flour 170 lbs
A4 alcohol 9 lbs
M19 malachite green 10 g
The ingredients were mixed for at least 24 h in a high shear mixer and
alcohol was added as needed in order to compensate for the evaporation
loss. The final mixture was a green paste. A blob was placed inside
the base, the leads were soldered, and the cement was baked at 150
deg. C (blistering temperature of bakelite is 190 deg. C). Heating
schedules:
CRT's: heat 10 min, cool 1.5 min.
Type 813: hear 3 min, cool 1.3 min.
Type 8025: heat 5 min, cool 1 min.
The alcohol boils out during baking, bubbles expand the cement and the
expansion providdes a good contact between the base and bulb.
Malacite green is a temperature indicator - it turns from green to
light brown if a correct temperature was used. Overheating turns the
cement reddish brown and the adhesive stenght is impaired.
Very large tubes (Type 207 etc.) with purely cylindrical seals used a
low temperature cement in order to precent cracking the seals. RCA
used BR-51 Resinoid plus acetone and curing at 50 deg. C. The cement
for bulb caps contained a coarsed marble flour and more rosin.
Some other basing cement recipes also use small amounts of tung oil as
a viscosity modifier, other kinds of rosins (calophony, litophone),
and other solvents (hexamine, methanol).
Similar cements are used in todays light bulbs but the curing
temperatures are higher, about 200 deg. C and the heat is applied
directly by flame..Sylvania is currently making cement BC-1631 cement
for fluorescent lights and BC-1652 for incandescent light bulbs. Ge
is also making its own cement. The European manufacturer I am aware
of is Glassbond NW in the UK (grades P66/MP for 180 deg. C andK14/D
for 210 deg.
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