why do Ortofon SPU C from the 50ies sound so good?

I mean the old Mono Ortofons, not the Stereo Pick-Ups - Uups

Probably the equipment you're hearing it through? They probably sounded very good back then, through a decent amp and speaker. A nice Quad valve/tube control unit and one of their push pull KT66 power anps driving a loverly Quad electrostatic, aaagh and your Ortofon in a SME Mk III and a Garrard or Goldring Lenco TT. `High End' systems arn't a new idea!.

Thuchan.
I also wonder it my self . I see they all seem went to Japan .
what arm you have it on ?

Martinhall & Ldvalve,
I am using the old Mono carts with an Ortofon RMA 309 running on an EMT R80. Maybe it is a well matching combination and the carts are also in a pretty nice condition. Nevertheless I was wondering because most people regard that old vintage stuff not as HiFi or even High End which is not true - You are right.

Best & Fun Only - Thuchan

Dear Thuchan: +++++ " Nevertheless I was wondering because most people regard that old vintage stuff not as HiFi or even High End which is not true - " +++++

that's why exist that long MM/MI thread that confirm your statement with cartridges in the 40+ years old range ( not 60 as your cartridge. )

Now, I own old Ortofon LOMC cartridges and today IMHO are competitive, I own the MC2000, MC30 and MC20MK2 and all them are better that any one could imagine.

Not only Ortofon but other vintage ( by 1980. ) cartridges are really god like Goldbug Ms. Brier or Pioneer PC-70MC or Satin M21P or Victor MCL1000.

So it is no surprise your Ortofon mono sample. As Martinhall point out several vintage cartridges sound good in those old times and with our today systems it still sounds good.
This fact IMHO is a testimony of its great design, build quality and very good execution design.

Regards and enjoy the music,
Raul.