Ian Thompson-Bell:

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).

At a wild guess the noise must track a similar periodic
variation in gain or current somewhere. Only way I can think
of that happening is if the amp is oscillating.

First thing I would do is measure the electrode voltages of
all the valves, looking for some periodic change that the
noise tracks.

Same circuit you showed us before?

Any help appreciated.

I'd rather discuss.

cheers, Ian