Alex wrote:
If there is no negative feedback circuit in the amplifier, then there is no
reason to suspect coupling via power supply as a cause of the phenomenon.


Nothing global but there is an unbypassed cathode resistor in the first
stage and of course the CF has 100% feedback.

I do not know how you are measuring the noise, but if you are using a
multimeter, it is possible to have some beat frequency between the 60Hz grid
and the harmonics of the switching cycle of the dual slope ADC which the
multimeter is based on. If you have a residual 60Hz ( I am assuming you are
in the USA) on the preamp output and if the ADC in the multimeter is not
absolutely synchronised with the grid (which is not), then you will have
sort of "product detector" delivering the beat frequency.


Good idea but not the cause I think. I am using a purely analogue meter
(Ferrograph RTS2).

And by the way, I am in the UK.

Cheers

Ian