Understanding suitable transistor and opamp substitutions

[Studioalethea]

Hey all,

currently in the process of getting back into DIY electronics after a bit of a hiatus and attempting to get back up to speed. I'm trying to understand how I can go about picking suitable transistors and opamps when I'm subbing in and out parts in a circuit, specifically what would I be checking for?

Transistors I get that it's primarily going to be about hfe, and in my current situation building a CGS panel with elby pcbs, pinout is going to be a consideration as well. Beyond that I guess that vbe values, noise and power rating maybe? but what else might differ?
For a concrete question/example, the CGS VCA specifies bc558b transistors which are apparently obsolete, and I'm wondering if I could just use 2n3906 instead or if there's something I've missed?

Op-amps feels a bit tricker, as beyond the obvious it's not actually clear to me what the functional difference is between say an lm358 and a tl072? Is it mainly that the latter has jfet inputs so a higher input impedance?

Sorry if this is a bit obvious, but searching back through these forums I found a fair bit of info on substitutions, but not much explaining what the logic/reasoning is behind them.

cheers for any help

Unpopular opinion here.

I really enjoy the thrill of the hunt in finding vintage and obsolete parts. I do my best to find original parts through surplus or harvest from used gear.

Why do I do this? Well, I enjoy it. Also, while I fully understand that modern parts are superior, maybe what I am looking for is something other than perfection. I like replicas. It’s sorta like building a sailboat when a speedboat gets you there faster. Speedboats are loud and there are a lot of fumes from that motor.

Additionally, if I have some original parts in the original circuit design, I can then make the substitutes myself to experiment and have tangible evidence for those substitutions. Sure, I could model it in LTSpice and get mathematical precision. This is again something I don’t want to do.

So, my process is to find the part where I can (there are only a select few that are impossible to find, UA726HC for example) and breadboard or socket a pcb. I can make the measurements of the original component, and then put in my modern replacement and measure/listen again.

All of this is to say that we all enjoy this hobby for different reasons. Don’t let others tell you how to be happy.

[KSS]

Starting from the original parts is pretty important if you're going after a replica. But as long as you have those, and start there; you can almost always get to new parts giving the *exact same* result.

@Studioalethea
Why am I not surprised you're a sailor? Agree about the powerboat thing too. Sailing teaches you that while you *can* get there from here, it's not always -or often- a straight line! And the wonderful life lessons gained when a broad reach feels like you're flying -and you're not- while running with a spinnaker set seems like you're not moving -but you now *are* 'flying'. The gut feeling and the actual fact is not the same.

Which also applies directly to the goal of this thread's title.

[Studioalethea]

Starting from the original parts is pretty important if you're going after a replica. But as long as you have those, and start there; you can almost always get to new parts giving the *exact same* result.

@Studioalethea
Why am I not surprised you're a sailor? Agree about the powerboat thing too. Sailing teaches you that while you *can* get there from here, it's not always -or often- a straight line! And the wonderful life lessons gained when a broad reach feels like you're flying -and you're not- while running with a spinnaker set seems like you're not moving -but you now *are* 'flying'. The gut feeling and the actual fact is not the same.

Which also applies directly to the goal of this thread's title.

I mean, I would have more money to learn to sail if I didn’t spend all of my money on vintage parts!

[emmaker]

Hijack.

It’s sorta like building a sailboat when a speedboat gets you there faster. Speedboats are loud and there are a lot of fumes from that motor.

Sounds like you've never been cruising on a SV and had to run the diesel 4-5 days at a time. The smell and vibration get old fast.

Why am I not surprised you're a sailor? Agree about the powerboat thing too. Sailing teaches you that while you *can* get there from here, it's not always -or often- a straight line! And the wonderful life lessons gained when a broad reach feels like you're flying -and you're not- while running with a spinnaker set seems like you're not moving -but you now *are* 'flying'. The gut feeling and the actual fact is not the same.

I'd rather wing it (wing on wing) if I can. I've gotten in trouble with a whisker pole a couple of times.

Still rather be on an SV than a MV.

Jay S.