Hi Lewm,
Thanks, you have raised some important issues. It is not true that high gain tubes are necessarily noisier than low gain tubes. Tubes, like transistors, are characterized by their input-referred voltage noise (EIN) and input-referred current noise (IIN). The test circuits that I have used have a low input impedance, similar to the impedance of a moving magnet phonograph cartridge, and in such circuits only EIN is important. The high gain 12AX7 has a lower EIN than the low gain 12AU7 and therefore will have a higher signal-to-noise ratio, as an example. Since a lower gain tube amplifies both the signal and the noise less, sometimes people perceive the lower gain tube to have less noise, sort of like turning down the volume control. But the tube with lower EIN will actually produce a better signal-to-noise ratio, regardless of its gain.
The circuit does influence EIN to some extent, but in my test circuits, as in most low noise circuits, the circuit is designed such that the tube EIN dominates the noise performance. The test circuits I’ve used are designed to provide the optimum bias current for lowest tube EIN, and are also designed to be representative of the way the tubes are used in low-noise circuits. Therefore, my measurement results should reflect the way signal-to-noise ratios in audio equipment will be affected by the tubes under test.
Please note that these measurements only characterize the intrinsic noise performance of the tubes (hiss), not their susceptibility to “hum” when AC is used on the tube heaters. In most high-performance audio equipment, tube heaters use filtered (and sometimes regulated DC), so their susceptibility to hum when AC is used on the heaters is not a concern. But since many musical instrument amplifiers do still use AC on tube heaters, I do plan to characterize this “hum susceptibility” in the future – it will be a separate measurement and a separate table in my listings.
I hope this is helpful.
Best regards,
Scott
Thanks, you have raised some important issues. It is not true that high gain tubes are necessarily noisier than low gain tubes. Tubes, like transistors, are characterized by their input-referred voltage noise (EIN) and input-referred current noise (IIN). The test circuits that I have used have a low input impedance, similar to the impedance of a moving magnet phonograph cartridge, and in such circuits only EIN is important. The high gain 12AX7 has a lower EIN than the low gain 12AU7 and therefore will have a higher signal-to-noise ratio, as an example. Since a lower gain tube amplifies both the signal and the noise less, sometimes people perceive the lower gain tube to have less noise, sort of like turning down the volume control. But the tube with lower EIN will actually produce a better signal-to-noise ratio, regardless of its gain.
The circuit does influence EIN to some extent, but in my test circuits, as in most low noise circuits, the circuit is designed such that the tube EIN dominates the noise performance. The test circuits I’ve used are designed to provide the optimum bias current for lowest tube EIN, and are also designed to be representative of the way the tubes are used in low-noise circuits. Therefore, my measurement results should reflect the way signal-to-noise ratios in audio equipment will be affected by the tubes under test.
Please note that these measurements only characterize the intrinsic noise performance of the tubes (hiss), not their susceptibility to “hum” when AC is used on the tube heaters. In most high-performance audio equipment, tube heaters use filtered (and sometimes regulated DC), so their susceptibility to hum when AC is used on the heaters is not a concern. But since many musical instrument amplifiers do still use AC on tube heaters, I do plan to characterize this “hum susceptibility” in the future – it will be a separate measurement and a separate table in my listings.
I hope this is helpful.
Best regards,
Scott