The Ta caps in old HP gear should last virtually forever, especially
if they have already lasted for decades. What you are referring to as
"wet slug" Ta caps are mostly dry solid ones in hermetically sealed
cans. There can be some actual wet slug types, but only in certain
spots where their unique characteristics are necessary. These can
often be identified by the end seals - they are not hermetic, but
elastomer sealed with rubbery material, and the anode lead is a solid
Ta wire that's butt-welded to a steel wire that's solderable. You can
see the weld where it goes from the bluish or brownish Ta color to
the tinned lead, except in types that put a glob of epoxy over the
end. As I recall, a lot of the ones used by HP are marked "109D" type
after the original Sprague (or Mallory?) product line.
The dry types have a true hermetic glass to steel seal that's
soldered to the can and the anode lead, which is steel or copper
alloy all the way. This is the best kind of electrolytic cap for
lifetime and durability, in my opinion.
The wet slug ones can leak after many years, and the sulfuric acid
electrolyte can damage things nearby. Whether they can be replaced
readily depends on the application. Wet slug types have the lowest DC
current leakage and highest temperature range of all electrolytic
caps, very low ESR, and wide voltage range. If it's a low-leakage
circuit requirement, and fairly low C (like up to a few tens of uF),
the best bet may be to use plastic caps, even several paralleled to
get the right amount - but the physical size will be much larger. For
larger values, this is impractical, so the next closest thing is dry
Ta caps, with more leakage, but may be OK for bench use at room
temperature. One very common use in HP gear is to have a wet Ta
switched across a YTO coil whenever the oscillator is in CW or
narrowband mode, for extra noise filtering. Any DC leakage would
cause errors in the tuning current.
Don't use regular Al or OSCON caps for low leakage circuits.
Ed
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