Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RoadShow on April 21, 2025, 11:32:09 am

Title: TAD KT66 with loose base
Post by: RoadShow on April 21, 2025, 11:32:09 am

Shame on me for not checking when I bought these a couple of years back, but when I opened the boxes yesterday one of the tubes had a loose base.  I guess you learn when you get bitten.

Any recommendations for securing it back into place?  Is there a particular glue or adhesive that will work and stand up to the heat?

Thanks

Title: Re: TAD KT66 with loose base
Post by: acheld on April 21, 2025, 04:12:37 pm

My go-to adhesive is Permatex RTV sealant.   Used to be called gasket sealer, IIRC.   Since it can take the heat of an engine block, it should take the heat of a tube easy.   And it is somewhat flexible, so will provide so tube damping . . .

That said, I have not used it for this purpose . . .

Great stuff!   But it can be messy if you're not careful.

Title: Re: TAD KT66 with loose base
Post by: tubeswell on April 21, 2025, 06:12:23 pm

I’ve used a thin bead of 5min epoxy to fix loose octal bases before. Don’t splash too much on.

Title: Re: TAD KT66 with loose base
Post by: dogburn on April 21, 2025, 06:35:04 pm

I used liquid super glue once for this. Got all over the place and made a bit of a mess. But seemed to work.

Title: Re: TAD KT66 with loose base
Post by: scstill on April 22, 2025, 09:18:53 am

this stuff and a toothpick between glass and base wipe off excess
wonder if the heat of the tube might deteriorate or melt non-heat resistant glues
 

Title: Re: TAD KT66 with loose base
Post by: scstill on April 22, 2025, 09:45:46 am

just discovered that this 500deg permatex epoxy is not available anymore
I looked up and Google says tube glass can get up 280degC (~500F)
Regular epoxy and super glue can withstand 150F
Seems that a hi temp adhesive should be used.

Only hi Temp epoxy I can find is a gray JB Weld which might be harder to keep tube looking original

Title: Re: TAD KT66 with loose base
Post by: RoadShow on April 22, 2025, 09:52:25 am

Thanks for the replies and suggestions.

On other spots on the web I found suggestions that also included high temp epoxy (JB Weld) and high temp RTV.  Some mentioned alcohol and super glue, but I'm not confident about going that route.  There was also tube base glue from Emission Labs, but that looks like it's no longer available, and was $36 when it was.

I'll pick one and reply back after the repair and heat soak.

Thanks again!!

Title: Re: TAD KT66 with loose base
Post by: mresistor on April 22, 2025, 09:53:16 am

Toyota Fix in Place Gasket (FIPG) can withstand heat up to 482 degrees but I don't know how well it would work for this use. It sticks and seals really well.


Have you guys watched this video? This person uses shellac for loose base repair.

Title: Re: TAD KT66 with loose base
Post by: scstill on April 22, 2025, 10:47:21 am

I could not find in the video where he addressed the operational heat at the base and that shellac max temp is much lower at about 200F, in fact he melts the shellac with heat gun. Or does Shellac have a higher op temp when hard? I'm thinking the base glass is not much lower temp than the body glass (same glass). Or maybe shellac/glue softens in operation and then hardens again when cool. We would never know without a test. Seems that the adhesive will eventually fail if going thru this kind of cycling.

I have also used shellac to seal old transformer plates to get rid of vibration hum, but I'm thinking xfmr are lower temp than tube.

Title: Re: TAD KT66 with loose base
Post by: RoadShow on April 22, 2025, 11:25:39 am

I'm gonna give JB Weld Hi Temp a shot, it's good up to 550.

Plus, I'll be able to use it for other stuff.  I have the regular clear JB weld now, but it's not temperature rated.