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

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.

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

--
Nick

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

Ian Thompson-Bell wrote:
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?


There is a nice article in one of the old Sound Practice issues, that
describes using tuned parallel LCR blocks in series with the TX out to
the bridge (both leads), and the a series LCR across the output of the
bridge. In both cases tuned to the TX's resonant frequency. I guess you
can work out the details



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.


Ok, I ask, as I have had cases where B+ switching pulses have been
coupled back via the heater supply.

--
Nick

Nick Gorham wrote:
Ian Thompson-Bell wrote:
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?


There is a nice article in one of the old Sound Practice issues, that
describes using tuned parallel LCR blocks in series with the TX out to
the bridge (both leads), and the a series LCR across the output of the
bridge. In both cases tuned to the TX's resonant frequency. I guess you
can work out the details


Is it available on line?

Looking at the pulses on a scope they do look like a decaying sinusoid
with aperiod of about 20uS which I guess implies the TX resonant
frequency is around 50kHz.


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.


Ok, I ask, as I have had cases where B+ switching pulses have been
coupled back via the heater supply.


That's a good point. Although the heaters are dc their source impedance
might not be very low - I'll try some decoupling to see if that helps -
that said if you turn off the HT the spikes go and there's enough energy
in the 470uF HT caps to operate the circuit for over a minute.

Thanks for the input.

Ian

Ian Thompson-Bell wrote:

Looking at the pulses on a scope they do look like a decaying sinusoid
with aperiod of about 20uS which I guess implies the TX resonant
frequency is around 50kHz.



I just did a Google search for 'tube power supply design' and one of the
first articles I found mentioned a simple 0.1uF across the transformer
secondary to snub rectifier spikes. I attached a 100nF 500V AC cap
across the secondary and it works like a charm - spikes completely gone.

Cheers

ian

Ian Thompson-Bell wrote:

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?

I tried the series L and it made no difference.

Patrick Turner.

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

"Ian Thompson-Bell" wrote in message
...
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

I would suspect the spikes are from switching transients from those fast
recovery diodes (are they hex-freds?). Try a snubbing cap across each diode
and I'll bet you a dollar to a cup of coffee that you will see a difference.
Keeping my fingers crossed for you.

west

West wrote:
"Ian Thompson-Bell" wrote in message
...
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

I would suspect the spikes are from switching transients from those fast
recovery diodes (are they hex-freds?). Try a snubbing cap across each diode
and I'll bet you a dollar to a cup of coffee that you will see a difference.
Keeping my fingers crossed for you.

west



I tried a 0.1uF straight across the HT transformer secondary and that
did the trick.

Ian

"Ian Thompson-Bell" wrote in message ...
West wrote:
"Ian Thompson-Bell" wrote in message
...
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

I would suspect the spikes are from switching transients from those fast
recovery diodes (are they hex-freds?). Try a snubbing cap across each diode
and I'll bet you a dollar to a cup of coffee that you will see a difference.
Keeping my fingers crossed for you.

west



I tried a 0.1uF straight across the HT transformer secondary and that did the trick.

Ian

A few years ago someone here asked if bypassing fast recovery rectifiers like
hexfreds was necessary. Nelson Pass surprised everyone when he dropped in
and responded with the advice to snub them with a .05 cap in series with a 100
ohm 1/2W resistor. It just makes things even quieter, in a way that can only be
appreciated when heard.

In a mic preamp, every possible source of noise needs to be attacked. That's a
big part of what differentiates the $1000 per channel professional mic pre's from
everything else. They don't always measure all that much better, but you can
hear the difference in a heartbeat.

Fred