Here’s an interest pentode to consider for phono stages. It was used extensively in instrumentation up until 1980s. There are a few examples around and you can check the Brimar data sheet to see what I’m taking about.
I checked a quartet of the military spec CV5086 which they measured bang on 100%. Here are some curves and Spice model which I’m sure you will find useful. In triode, it does give a mu of about 20.
And here is the Spice model:
Ale, Have you considered the 6Ж9П pentode. The noise spectrum of the 6Ж9П in triode connection is very good: The 1/f-noise is remarkably low and this noise characteristic is preserved in pentode mode..
Hi Tim, good shout. I only used the 6J9P as driver in the past. Would be interesting to check the noise performance as you suggest.