21-10-2023, 08:10 AM
(This post was last modified: 21-10-2023, 08:22 AM by Mike Watterson.)
Sure they knew about inherent NF of resistances vs resistance when valves were still common.
If I want low noise audio I'd use one of the specialist low noise audio IC op-amps. However it's only mag pickups and some sorts of microphones that need that at input stage. Line level audio isn't so critical. But the only valve audio amps I've designed or built since 1970s was the rod pentode one for an FM radio I was building a few years ago. Repaired plenty of them for bands and restoring radios, record-players and tape-recorders. Only the better tape-recorders used an EF86 (often having ECC8x too). All the '50s record players ceramic or crystal replaced by ceramic pickups and those have a high output and don't need RIAA compensation.
Certainly real Pentodes are inherently noisier than Triodes, but the Rod Pentode is just electrostatic rod focusing, no spiral wire grids, so is much quieter.
Edit:
I remember the BC109 was sold as "low noise", but I don't think it was inherently lower noise than a BC108, just higher gain. I'm not sure that BC107, BC108, and BC109 and graded parts a, b, c for gain were not even all the same silicon originally but graded on test. I do remember that at the start it was challenging designing a low noise transistor amp even for guitar pickups and dynamic mics, but better than a 741 could be achieved.
If I want low noise audio I'd use one of the specialist low noise audio IC op-amps. However it's only mag pickups and some sorts of microphones that need that at input stage. Line level audio isn't so critical. But the only valve audio amps I've designed or built since 1970s was the rod pentode one for an FM radio I was building a few years ago. Repaired plenty of them for bands and restoring radios, record-players and tape-recorders. Only the better tape-recorders used an EF86 (often having ECC8x too). All the '50s record players ceramic or crystal replaced by ceramic pickups and those have a high output and don't need RIAA compensation.
Certainly real Pentodes are inherently noisier than Triodes, but the Rod Pentode is just electrostatic rod focusing, no spiral wire grids, so is much quieter.
Edit:
I remember the BC109 was sold as "low noise", but I don't think it was inherently lower noise than a BC108, just higher gain. I'm not sure that BC107, BC108, and BC109 and graded parts a, b, c for gain were not even all the same silicon originally but graded on test. I do remember that at the start it was challenging designing a low noise transistor amp even for guitar pickups and dynamic mics, but better than a 741 could be achieved.
