For me it’s not really about chasing a holy grail, as a relative newcomer to the world of DIY audio it’s just about getting an understanding of what does what. This was my first ‘proper’ build and used the Analog Devices chassis, guide and BOM - electrolytic was just what was specced there. I tried to be as true the original specs as possible (using carbon comp resistors where possible, NOS allen Bradley If i could find them, also with the pots). This is just an extension of that really.
I just picked up another chassis from Hugo on here and was planning to go with all metal film resistors and ‘modern’ parts when I do that one. If a polypropylene or polyester cap is a possible alternative there (I’ve asked but that hasn’t been suggested so far) then I’m happy to give it a try. Like I say - just learning at the moment and, though I heard of polystyrene caps being great for audio when I built a don 250eq, I’ve no idea yet when one type of cap could / should be used in place of another.
I just figured with that first La2a build it could be used as a ‘control’ rather than trying different things and modding before I’d even heard a hardware la2a. The next build, also planning to try the 12au7 mod with different resistor values I’ve seen suggested by (I think) CJ.
All suggestions welcome - what sounds best wins in my book (as subjective as that can/will be)
Polyester or polypropylene film will work great, and modern caps can fit.
Polyester tends to be smaller than polypropylene, and metallized film tends to be smaller than film-and-foil.
The theoretical best would likely be polypropylene film-and-foil, but at 10µF this will be rather bulky--perhaps too bulky to use.
I used a Solen "Fast" metallized polypropylene in a recent LA-2A build. It fit well (though only just!) but was slightly bulkier than an old electrolytic (and much bulkier than a modern electrolytic).
You won't find a polystyrene cap in the proper value and voltage rating, and if you somehow could, it would definitely be impracticably large.
The Allen-Bradley pots are worthwhile if only because their log tapers were a bit different than tapers found on most modern parts. For the pots to behave as you'd expect an old LA-2A to behave, it's worth using those older parts (or their expensive PEC descendants). If you don't care what the knob pointer says and only care about the sound, it's likely not critical.
The electrolytic output coupling cap, the class 2 ceramic (Z5U) coupling caps, and carbon comp resistors are all about trying to mimic the distortion characteristics of the old LA-2A amplifier.
This assumes two things: 1) the distortions introduced by the cheap old parts are audible, and 2) the distortions are desirable.
That gets into the realm where things like confirmation bias and personal taste play a part, so I won't editorialize on either point.
I will say that an LA-2A is a bit of a moving target, anyway. We have three vintage ones at our place, and all sound just a bit different from one another. The one I built (with metal films, film-and-foil polypropylene coupling caps and a metallized PP output cap) isn't any exception--but it's very much in the same family, and I'd happily use any of the four in place of one another.
The transformers and tubes will probably have a much bigger impact on the sound of the amp than any of the caps and resistors, and the T4B will have a far bigger impact on the gain reduction behavior than will anything else.
