I spend almost all my tinkering time with 1970s era logic circuits, which use almost all original 7400-series TTL. I do a fair bit of repair and am now starting to design additional logic to work with old circuits, and I am a little fuzzy on the risks/rewards of using (say) 74LS series parts in these designs, either as substitute/replacements or in new parts of a circuit.

The chart on this page indicates that the 74yy00 variants largely vary across propagation delay and power consumption:

My sense is that the LS variants are generally appropriate to substitute for the basic versions, with the general caveats that:

1. An existing circuit design might implicitly rely on certain propagation delays and that using parts with faster timing could alter the behavior undesirably.
2. I guess you could use more power than expected if you swap in just the S variant instead of LS?

Are there other, more functional or more concerning reasons to beware of intermixing, or specific other things to watch out for?

I want to not be foolish about these variants in old circuits but at the same time if a 74LS04 works just as well as a 7404, they're certainly cheaper and easier to find (and some of the logic chips never existed in the basic version, only in LS, etc.) AS and ALS (per the chart) look even better to me if I'm not sensitive to propagation delay, but again, I don't want to be blind about it.

Thanks.

I spend almost all my tinkering time with 1970s era logic circuits, which use almost all original 7400-series TTL. I do a fair bit of repair and am now starting to design additional logic to work with old circuits, and I am a little fuzzy on the risks/rewards of using (say) 74LS series parts in these designs, either as substitute/replacements or in new parts of a circuit.

The chart on this page indicates that the 74yy00 variants largely vary across propagation delay and power consumption:

My sense is that the LS variants are generally appropriate to substitute for the basic versions, with the general caveats that:

1. An existing circuit design might implicitly rely on certain propagation delays and that using parts with faster timing could alter the behavior undesirably.
2. I guess you could use more power than expected if you swap in just the S variant instead of LS?

Are there other, more functional or more concerning reasons to beware of intermixing, or specific other things to watch out for?

I want to not be foolish about these variants in old circuits but at the same time if a 74LS04 works just as well as a 7404, they're certainly cheaper and easier to find (and some of the logic chips never existed in the basic version, only in LS, etc.) AS and ALS (per the chart) look even better to me if I'm not sensitive to propagation delay, but again, I don't want to be blind about it.