Cap Questions

[Liquids]

Can anyone out there answer a few basic questions about capacitors:
1.) I am aware that electrolytics have a limited lifespan. What about epoxy encapsulated caps (orange drops, xicons, silver mica, etc.)? Do these (in theory) last forever? Same questions about axial caps such as Mallory 150's or similar?
2.) My '76 distortion V4 has a few caps that do not seem to be easily sourced, specifically a couple .33uf 400v caps in front of the power tubes and a couple of .68uf caps used with the midrange control. The originals are the ERO green rectangular PC mount caps. Reuse or replace with what?
3.) I'm finding several old and cracked "tropical fish" caps in some locations, I'm trying to determine why they were selected by the OEM. Do these have some vintage mojo for tone, or was it because they are relatively small for their values, or are they more heat resistant than other caps, or something I haven't even considered?
4.) I know this last topic has been beat to death many times over, but I've got to ask: Bearing in mind I'm aiming to retain the original Ampeg tone, are Mallory 150's a decent choice to replace coupling caps in the signal path? What about locations that require a smaller footprint?
Thanks all for any assistance.

I'm REALLY not the guy to reply to questions about vintage cap mojo...but no one else has chimed in. I'll do my best--a guy I've done work with before is big on cap mojo.

1) No caps last forever. Electrolytics have shorter lifespan, generally(?) than non-electrolitic caps. But they all wear, drift, and have lifespans. Even modern ones--datasheets will list lifespan hours--at a given temperature--if the manufacturer feels confident that they can meet the published spec, or publish a spec they can meet. Some may outlast that, some such caps in old fenders are still working--though component value drift and ESR may become significant. Depends on what meets one's standard for 'working' and how one views "original" ideologically and tonally.

2) caps with those values are out there. Check tubesandmore.com for example..at least last time I checked, those places had them. If they do, whether or not they'll fit your personal requirements, and if their shape/size will make them drop in or not are more the questions. But especially with the coupling caps between PI and PTs--if these caps leak, even intermittently, you do get real problems. Some say the values are overkill, and can be reduced, but that's a personal call.

3/4) I don't think anyone is under the impression that Ampeg was choosing any of their caps (say, tropical fish) for their tonal value. If one thinks that cap type is tone-critical in one spot or everywhere...one would want to find equivalent replacements as needed, sure. So You could do a lot of a good swapping where needed--between the Mallorys and Orange drops, if it's worth your time to swap and audio test. I think smallbearelec (no affiliation) also stock tropical fish caps, but not clear on values. As for why those caps are there--originally, probably mass quantity for price, size, and availability, in some order. Chosen for "tone?" At that time especially--no. Just spec (mentioned, maybe others like ESR or lifespan, but doubtful). Cap size issues--many take available caps that are larger, and tastefully make them work when needed. The aesthetic values of that are personal as well.

Last note--personally, I have gone through my Ampeg and replaced virtually all the caps, and done so primarily with the brownish-red caps that tubesandmore/weber tend to have available as their 'budget' high voltage caps-- or at least, had available at one time. I've got really picky ears and don't want any issues with my amp either, hence the "shotgun" replacement, to make a longer story short. I'm quite content. The biggest tonal change, for me, happened when I replaced the tone stack caps with different types (moving away from ceramic) and values (changing where the frequency points affected by the bass/mid/treble controls), after calculating, trial by error, etc. My amp sounds great--at least, it does when I play it, and no one else says otherwise...