Re: {Collins} Centron 811A

I've used the red/orange Permatex hi-temp silicone sealer (made for auto exhaust manifolds) on the loose plate caps. I unsolder the plate cap and suck out the solder from the center hole beforehand, then resolder the connection after the silicone has cured overnight. Excess goop can be removed with a razor blade after curing. I was worried about the effects of phosphoric acid possibly released during the curing process but it doesn't seem to be an issue. Some of these tubes have been in use for over 30 years.

The problem with some of the 811A brands is that some makers did not control the orientation of the plates with respect to the base, so the long filaments can sag and come in contact with the plate when the tube is used horizontally.

Russ WQ3X

Subject: {Collins} Centron 811A

I have 3 sets of 811A from NB but never had an issue like some have. I have
fixed a few rare tubes in the past that had plate caps that came loose.
Talking with a friend that was a chemist he said that the silicon based
adhesive used in old tubes needed to be heated to set the adhesive and in
cases of some they just were not cured long enough and over decades the
adhesive just broke down. Now as many of you know when restoring old radio
equipment sometimes we have to reinvent past engineering and do it our
self's. Their is a company called Flexbar that makes several products that
bond glass and metal from -350 to 2200 deg F it is about $30.00 also there
is a half dozen venders on fleabag that sell similar products. I have never had an issue after rebounding old plate caps back on. So let me know if you have (NEW) old Centron 811A's that have loose caps and I will give you five
bucks apiece for them and even pay the shipping.

John W0XAZ

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