As Bricksnbeatles & peccary mentioned, the Beatles bass sound starts with the strings and how you play them. I've had good luck with both flatwound & tapewound strings, a bridge mute, and a pick. I think the flat/tapewound sound played with a pick is the core part of Paul McCartney's technique (
see this TalkBass thread):
Strings:
- Pyramid flatwound - original manufacturer for Hofner strings, again in 2010
- Rotosound flatwound - mid-‘60s, again in the 80s/90s
- Rotosound Tru Bass black nylonwound - During the Let it Be sessions.
Picks:
I absolutely
LOVE the sound of felt picks, and would highly recommend them to bass & guitar players alike. They present a much rounder sound, and can also prevent you from resorting to a more unwieldy bridge mute.
If you want to try the bridge mute method, here are a million ways to approach it. I like the sound of felt, rather than foam, mutes because I find it easier to get more subtle muted sounds more easily. Carol Kaye has some
great tips on her site, including
this one on her bridge mute setup:
"The way I mute the strings is by folding over a piece of felt muting (buy at the sewing section) so it's doubled to a width of about 1-1/2". Take it & tape it (I use masking tape) to on top of the bridge area, but laying slightly ahead of the bridges. It won't be too loose but you will have to re-tape it tighter from time to time. Thus, it lays on top of the strings and kills the over- & under-tones, making your bass sounds more defined.
- You use a doubled up piece of felt *on top* of the strings when you play *only* with a pick.
- If you play with fingers (or even with fingers sometime and then pick sometime), then get a piece of foam about the same width, but fit *underneath* the strings, barely touching the strings. This takes some doing. You don't want the foam to mute the strings so much it gives off a "plunk" sound, yet you need it to touch all the strings relatively the same amount.
The strings in all instances should ring almost as much as if there wasn't any muting at all. In recording, it's a must."