[David G4EBT]

One of the key considerations mentioned at the outset and in other posts is the ability of the adhesive to withstand 'high temperatures', notably in rectifiers and output valves. However, 'high temperatures' is a highly subjective and undefined term, so it would make sense to check the temperatures of such valves. As far as Araldite is concerned, standard Araldite has a maximum operating temperature of 65C, and 'Rapid' is 80C. Thus if the valve temperature doesn't exceed that, maybe it would be fine. It's worth noting that Araldite isn't recommended for re-fixing rear view mirrors to windscreens in cars. (Glass adhesive requires UV to cure).

The original shellac/wood flour adhesive set rock hard which seems not to have been a problem in terms of risks of cracking the glass, so as I see it, any modern adhesive which sets hard and can withstand the operating temperature ought not to pose a risk . Over time, in many instances the original adhesive has been shown to have poor adhesion to glass, but in fairness, it's taken decades for the bond to break and many - perhaps most - bases (and top caps) are still firmly fixed. I my experience, it takes an overnight soaking in meths to dissolve the shellac if it's desired to remove say a cracked base to fit a new one.

Just a note of caution for anyone wishing to remove a base to replace it:

Don't just unsolder the pins and pull the base off because the wires will almost certainly spring out of place and apart from the heaters, you wont know which wire is which. They emerge from the base of the glassware in as flat 'pinch' - not in a circle. Before unsoldering the pins, I always use a Dremel with a diamond disc to carefully cut a ring about 1cm wide around the base then remove the ring so that the wires can be seen. Then I label each wire with the pin number, de-solder the pins and remove the the base. I slip a short length of coloured sleeving over each wire, using the resistor colour code to denote the pin number.