 An amplifier with a 0 dB noise figure would be perfect.The noise properties of tubes at audio frequencies can be modeled well by a perfect noiseless tube having a source of voltage noise in series with the grid.For the EF86 tube, for example, this voltage noise is specified (see e.g., the Valvo, Telefunken or Philips data sheets) as 2 microvolts integrated over a frequency range of approximately 25 Hz to 10 kHz.