cracky glue on NOS (1976) 6SN7 / 6H8C

[JochenH]

Hi,

i bought 2 NOS Reflector 6H8C / 6SN7 tubes for my planned eiclones. On one of them, the glue seemed to have cracked so that the glas is a bit loose in the black base but you cannot twist them against each other.
Does this affect the performance / function / vacuum of a tube or is it usually not a problem? Can this be fixed with new glue (i.e. epoxy based) and what temperature must the glue withstand?

Thank you for your answers.

Jochen

[erichayes]

Hi All,

Octal tubes in loose bases have more tendency to be microphonic than those firmly attached to their bases. Reflektor 6SN7s lean toward microphony anyway (I had one that was so microphonic it picked up my voice from over 1/2 meter away), so I'd suggest using some fairly stiff adhesive that can withstand a reasonable amount of heat (60°C). I don't know what's available in Europe, so I won't suggest anything more specific than that.

[Shannon Parks]

Can this be fixed with new glue (i.e. epoxy based) and what temperature must the glue withstand?

Jochen,

I read a similar query on Audio Asylum or rec.audio.tubes. You might try a search of those archives and see what resulted.

Shannon

[mesherm]

Most epoxies will take up to 120 DegC without a problem.
Select a slower curing general purpose two-part epoxy (not the 5 minute kind). It should be clear or straw colored and definitely NOT aluminum or metal filled. Warming the tube and base up a little with a hair dryer will help the epoxy flow into small spaces. You can warm the epoxy a little too but be warned that heat will make it cure quicker so be ready to pour. 10-20cc would be more than enough. Sometimes you can buy epoxy in small two-part packages which will work fine. Just don't use the quick cure kind. You want the epoxy to flow into all the nooks and crannies. I would mask off the outside of the tube socket to make any spillover easier to remove after the epoxy has hardened. If the wife doesn't object you can place the epoxied tube, supported right side up somehow in a warm oven <70 deg C for an hour or two to cure.
Even if you cure it at room ambient, I would recommend the oven treatment anyway to give the epoxy a post cure.