No subject

Sun May 13 10:46:53 EDT 2007

73, Mark W1EOF

From: Lud Sibley, 10JAN2000:  BASE CEMENT AT RCA
(More'n you ever wanted to know!)

The usual base cement was "No. 6," AKA "C6."  The official recipe for this

D3 Durite 7.5 lb.
S15 Shellac    19.5 lb.
R3 Rosin  3.25 lb.
M3 Marble flour     170.0 lb.
A4 Alcohol     9.0 l.
M19 Malachite green 10.0 g.

Makes 200 lb., or enough to base 23,500 Type 50s at 0.0085 lb. each.  Basic
recipe is in Standardizing Notice 34C-C-6 ("Confidential") of 12-29-48.  No.
6 was used in transmitting tubes too:  814s, 860s, etc., and CRTs (7BP7,
etc.).  The bills of material for "modern" tubes (post-'30s) didn't specify
the type of cement, but I've seen no sign of a change up to the end.

The "Durite" was Durite Resin #275, from Stokes & Smith of Philadelphia, not
further defined, bought in 50-lb. pails.

The curing process required a cement temperature of about 150øC.  (This, vs.
the blistering temperature of bakelite at about 190øC.)  The heating
schedule varied:

CRTs:  heat 10 min. cool 1.5 min.
813s:  heat 3-« min., cool 1-1/3 min.
8025s:  heat 5-« min., cool 1-« min.

The color change in the malachite was considered only a rough indicator of
temperature; it was backed-up with a thermocouple thermometer.

There was a slightly modified cement for top caps, with the same ingredients
but using fine and coarse marble flour.

For use in very large tubes like 207s, where excess heat might crack the
seals, there was a low-temperature cement.  It was made of "BR-51 Resinoid"
(bought commercially), acetone, and marble flour.  It was for only purely
cylindrical seals, and cured at 50øC.

There's been some talk in restorer circles as to the cured cement being
soluble in toluene, acetone, etc.  I haven't had any luck.  In the old days,
if the RCA warehouse discovered loose bases, they just fixed them with


Magers' book on WECo tubes says they bought their base cement premixed,
adding the malachite and some alcohol.  That's consistent with WE's smaller

> -----Original Message-----
> From: Carl Ratner	<artdeco at>
> Sent: Monday, March 24, 2003 12:48 PM
> To: glowbugs at
> Subject: GB> Vacuum Tube Repair - Glue Warning
> Quite often we find old tubes with loose bases, and then we
> use glue of one sort or another to re-attach the base to
> the glass. In the January 2003 issue of Antique Radio
> Classified there are some warnings about this practice in
> the Radio Miscellanea column.
> One writer stated that cyanoacrylate glues (Crazy Glue)
> should never be used to repair power tubes, rectifers, or
> other tubes that become very hot. The reason is that these
> glues have a very different coefficient of expansion than
> glass, which will cause the glue to act like a glass
> cutter. According to the writer, the heated glue will cause
> the glass envelope to snap off cleanly at the base!
> However, the writer also said that the glue may be used on
> small signal tubes or battery tubes that don't become too hot.
> Another writer pointed out that cyanoacrylate glues are
> conductive, and may run down into the pin area where they
> can cause trouble. He suggests using the gel version of the
> glue to prevent running, or to hold the tube horizontally
> when applying glue, to keep the glue from flowing downward
> inside the base.
> Does anyone know what kind of glue was used originally by
> the tube manufacturers? Apparently it had some special
> properties such as resistance to heat, and a coefficient of
> expansion similar to that of glass. It probably was
> quick-setting too, for efficient use on assembly lines. Is
> anything similar still being made? Has anyone tried the
> high-temperature silicone sealants sold in automotive
> supply stores?
> 73,  Carl Ratner

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