The 6AU6 based mic pre I am designing has an interesting quirk. I am
testing various valves for low noise with the first stage input grid
grounded. As expected different valves exhibit different noise levels
and susceptibility to hum. To eliminate hum from the readings my
millivoltmeter has an LF cut switch. When this filter is used I noticed
a small but regular variation in the indicated noise level. It varies by
about 10% over a regular 2 second period. We are talking very small
values here as the indicated noise is only a tenth of a millivolt. But
it is very regular and I have tried half a dozen different valves all
with the same result. So the question is what is it? Both stages of the
preamp are decoupled with 470uF caps via 10K resistors (time constant
4.7 seconds).

Any help appreciated.

Cheers

Ian