Nick Gorham wrote:
Ian Thompson-Bell wrote:
As I have mentioned I am designing a 6AU6 based mic preamp. Thanks to
Patrick et al I now have hum free HT rails. However, I still find some
50Hz hum and 10mS period PSU spikes in the output. By shorting the
grid of the first stage these both disappear so that is where they are
getting in. By turning off the HT they also both disappear so it seems
the source is the PSU. So the question is what should I do to stop it.
I tried grounding the mains transformer metal clamp and this helped a
bit but there is no connection of this transformer for a screen of any
kind. I am also using fast recovery diodes in the HT supply because I
have read they produce less spike interference. Any help appreciated.

For a not very good picture of the waveform at the preamp output look
he

http://s103.photobucket.com/albums/m...6CFopnoise.jpg


Cheers

Ian

Maybe some snubbers or LCR filtering on the secondaries.


Yes, I was thinking maybe small series inductors for each diode - is
that the sort of thing?

Are you using DC heaters? you may find the spikes are from the heater
supply


Definitely not heaters - at present they are supplied by a battery.

Ian