Low Noise Operating Points
Date: Sun, 13 Aug 2000 00:10:23 -0600
Subject: [JN] Operating points (silly question)
Paul Joppa wrote:
> Christian asked about an operating point for a phono first stage. Here’s
> what I have thought, I hope others will contribute more and/or
> different.
>
> First, you want the tube to run “hot”, that is at enough current
that it
> is well out of the nonlinear region and none of the electron paths are
> cut off – that maximizes the active cathode area, which minimizes noise.
> Usually this meant that Va/Ia is no more than 4 times rp (plate voltage,
> plate current, and plate resistance respectively).
>
> Second, grid bias should be enough that there is no possibility of grid
> current. This varies with tubes, depending on the metals used and the
> temperatures – high transconductance tubes especially have grids very
> close to the hot cathode. Buddha used to use 2 volts as an estimate, if
> that helps.
>
> Third, the plate voltage should be high enough to draw lots of current
> so the transconductance is high (makes low noise) but not too much
> voltage, because that will excite some other noise mechanisms. This last
> is just something I have heard about, I’m not too clear on what those
> mechanisms are. 50 to 70 volts has been suggested, sorry but I don’t
> remember where.
These “noises” are due to the residual-gas ionization effects that
start
about 25eV and secondary emission from the plate . . . Grid emission is
another source of noise that is greater with greater Va . .
Doubtless there are others . . .
> Those criteria are, alas, sometimes mutually exclusive – especially if
> mu is high.
>
> -Paul Joppa
Bill – PEARL, Inc.
