Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

8mV of noise causing havoc in valve pre amp.
8mV of noise causing havoc in valve pre amp.
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 9th August 2016, 12:59 AM   #1
nigelwright7557 is offline nigelwright7557  United Kingdom
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Default 8mV of noise causing havoc in valve pre amp.

I have managed to get the noise down quite a bit in my pre amp.
I am now down to 8mV AC hum in the first stage.
The 8mV becomes 80mV after the next stage which is pretty useless.
I have changed heater supply to DC and that is rock solid.
I have to connect one side of DC heater supply to zero volts to reduce hum.

I had a transformer very close to the first stage so I have removed it and put it on flying leads so it cant affect the circuit.

I thought perhaps it was star grounding issue but I have made sure all ground are star-ed.

I have ordered a couple of different valves that hopefully should be lower noise, a 12au7 and a Ruby low noise 12ax7.

The second stage works fine if I short out its input, there is zero noise on the output.

An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.
__________________
PCBCAD51 and PCBCAD360 out now >>> https://www.murtonpikesystems.co.uk

Last edited by nigelwright7557; 9th August 2016 at 01:05 AM.
  Reply With Quote
Old 9th August 2016, 01:02 AM   #2
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Quote:
Originally Posted by nigelwright7557 View Post
The second stage works fine if I short out its input, there is zero noise on the output.
How about shorting the input, does that affect the hum? Does a tube shield help? Is it 50Hz or 100Hz?

Last edited by rayma; 9th August 2016 at 01:06 AM.
  Reply With Quote
Old 9th August 2016, 01:26 AM   #3
nigelwright7557 is offline nigelwright7557  United Kingdom
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by rayma View Post
How about shorting the input, does that affect the hum? Does a tube shield help? Is it 50Hz or 100Hz?
I have the volume control set to zero.
It is 50Hz.
Shorting out the grid stopper alters the shape of the waveform quite a bit.
Without grid-stopper it is reasonable sinusoidal.
With grid stopper it is a very badly shape sine wave.
Its about 100mV peak to peak.
An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.
__________________
PCBCAD51 and PCBCAD360 out now >>> https://www.murtonpikesystems.co.uk
  Reply With Quote
Old 9th August 2016, 01:33 AM   #4
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Quote:
Originally Posted by nigelwright7557 View Post
I have the volume control set to zero. It is 50Hz.
Shorting out the grid stopper alters the shape of the waveform quite a bit.
Without grid-stopper it is reasonable sinusoidal.
With grid stopper it is a very badly shape sine wave. Its about 100mV peak to peak.
Is the USB input disconnected? There must be some kind of ground loop.
  Reply With Quote
Old 9th August 2016, 02:03 AM   #5
nigelwright7557 is offline nigelwright7557  United Kingdom
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
The USB pot is turned off.
Its normally connected to a USB DAC but I haven't added that in yet.
__________________
PCBCAD51 and PCBCAD360 out now >>> https://www.murtonpikesystems.co.uk
  Reply With Quote
Old 9th August 2016, 02:10 AM   #6
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Quote:
Originally Posted by nigelwright7557 View Post
The USB pot is turned off.
Its normally connected to a USB DAC but I haven't added that in yet.
I would remove the coupling capacitor after the wiper and see if the noise remains.
If so, it almost has to be a ground loop.
  Reply With Quote
Old 9th August 2016, 02:16 AM   #7
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
8mV of noise causing havoc in valve pre amp.
How about some pictures of how things are laid out and are you absolutely sure your pre-amp is not oscillating?

How far is your input stage from the rectifiers and transformer.

How have you grounded things and in particular the HV winding center tap?
__________________
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
  Reply With Quote
Old 9th August 2016, 02:55 AM   #8
kmaier is offline kmaier  United States
diyAudio Member
 
kmaier's Avatar
 
Join Date: Jul 2007
Shorting the input to the second stage (if done at the pot input) doesn't actually short it... it simply takes the output noise of the first stage and creates a voltage divider via the two 100K resistors meant to mix the two inputs. The output from the voltage divider drives the input to the second stage. If identical noise is induced to both stages, the input (via the voltage divider) to the second stage is basically out of phase and the end result is likely less noise from the second stage. Without seeing a mechanical layout of the circuit, it's difficult to suggest where the problem is.

Regards, KM
__________________
... just because you're paranoid, it doesn't mean they're not after you...
  Reply With Quote
Old 9th August 2016, 10:07 AM   #9
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
What does the supply rail to the first stage look like? Is this a PSU grounding error?
  Reply With Quote
Old 9th August 2016, 12:44 PM   #10
Charlie123Maldor is offline Charlie123Maldor
diyAudio Member
 
Join Date: May 2016
Location: Ireland
I don't have much experience with tube circuits but I would try a 5-20H transformer type choke on the H2 rail making a RCLC filter after the rectifier. They can be bulky and expensive. If that did not work the next thing I would try is running both anodes off the one rail. Meaning that from HT I would further filter and decouple the voltage and supply the input side anode with that. This might reduce the hum a little as it would see more filtering. I might have to change the resistors to get the required voltage though and also use less capacitance.
  Reply With Quote

Reply


8mV of noise causing havoc in valve pre amp.Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
What is causing this noise? terranigma Solid State 27 14th October 2015 10:48 PM
LCD brick causing noise over speakers Ramie Solid State 0 17th February 2015 08:43 PM
Fan causing line noise at turn on moray james The Lounge 0 26th January 2015 04:49 PM
Amp causing popping noise? guitarguy12387 Car Audio 7 15th April 2008 02:58 AM
valve pre amp dudemyster Tubes / Valves 4 13th April 2004 05:13 PM


New To Site? Need Help?

All times are GMT. The time now is 10:21 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki