Go Back   Home > Forums > Amplifiers > Tubes / Valves

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

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 24th November 2010, 09:57 AM   #1
vlucian is offline vlucian  Romania
diyAudio Member
 
Join Date: Mar 2005
Location: bucharest
Default Strange hum in a Williamson amp

Hi there.
I have the following situation:
- an williamson amplifier with 6p3s final tubes
- initially the amp was built with 2 small output transformers without ultraliniar windings but latter I decided to replace them with bigger ones (also having UL)
- I had to change the box (the new transformers are 4 times bigger) and to rewire everything
- in the end everything was fine excepting for a low hum not very disturbing but still audible
- I tried few things including changing the wires or movement of the power transformer outside the box only to see if a wrong positioning was the cause: the hum was still there no matter what
- I must mention I used a star ground pattern
- I went to a friend who has an oscilloscope and we made some measurements: we found a very high frequency (around 1 megahertz)!! The oscilloscope was an old Tesla one so we didn't had to much measurement possibilities (like harmonics).

What do you think should I try? Maybe there are not much info, please ask if something is not clear.
thanks a lot!
  Reply With Quote
Old 24th November 2010, 03:26 PM   #2
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Oscillation can sometimes happen with UL. If not already present, try adding grid stoppers to the output control grids. Then g2 stoppers (say, 470ohms or 1K 2W wirewound). Finally, you may have to add snubber networks from each anode to g2.
  Reply With Quote
Old 24th November 2010, 04:47 PM   #3
Svein_B is offline Svein_B  Norway
diyAudio Member
 
Join Date: Oct 2005
Try disconnecting the global feedback until you are sure everything is OK without it.
If stable without GNFB you may need to tweak the feedback loop, or some other FR compensation to get it stable with feedback.

The Williamson can sometimes be tricky to get right.

Svein
  Reply With Quote
Old 24th November 2010, 05:01 PM   #4
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
1MHz oscillation is unlikely to be due to the global feedback loop. More likely to be local feedback in the output stage due to parasitics in the OPT.

However, disconnecting the global loop is a good debugging technique as it stops interactions complicating things.
  Reply With Quote
Old 25th November 2010, 10:28 PM   #5
roline is offline roline  United States
diyAudio Member
 
roline's Avatar
 
Join Date: Sep 2010
Location: East Tennessee
Default Williamson Amp Stability

I'd look for possible ground loops but star ground really should prevent it. I had to increase my power supply cap ( double the size) to rid me of a very low level hum to a totally inaudible level.
If it is a circuit instability, try to jumper 1,000 pf to the input pins of the tubes and see if the hum still exists. Move to the next input etc... This should force a dominant pole into the k-hz range and stop high frequency oscillation. If it works, you know the area of the circuit to investigate. (without the global feed back) Just thinking, could you have connected the output feed back inverted?. Does it get louder of quieter with the global feedback?
In the attached schematic R7, C5 create the dominate pole for Amp's stability. The capacitor can be increased for stability but excessively large with decrease the power bandwidth.
Attached Images
File Type: jpg williamson amp.jpg (301.3 KB, 216 views)
  Reply With Quote
Old 27th November 2010, 07:13 PM   #6
vlucian is offline vlucian  Romania
diyAudio Member
 
Join Date: Mar 2005
Location: bucharest
Thank you all for your ideas and suggestions. Next week I'll go to my friend again (I let the amp there) to try some things especially with the feedback. I'll let you know the results.
thanks again
  Reply With Quote
Old 2nd December 2010, 09:03 PM   #7
vlucian is offline vlucian  Romania
diyAudio Member
 
Join Date: Mar 2005
Location: bucharest
So, we did the following:
- changed the resistor in the NFB from 10k to 20K => the hum was louder
- changed the resistor with a potentiometer and try to obtain minimum audible hum => the new value is 2k5
- view on the scope: the now hardly audible hum still has high frequency component with about 200mV amplitude; rotating potentiometer changed the noise level but also cahnged the frequency
-disconnecting the global feedback only bring more noise
- we didn't changed the feedback capacitor value
I wonder there is no way to cut completly that noise. What else sgould I do?
thanks
  Reply With Quote
Old 3rd December 2010, 10:20 AM   #8
roline is offline roline  United States
diyAudio Member
 
roline's Avatar
 
Join Date: Sep 2010
Location: East Tennessee
Disconnect the coupling caps from the driver stage and jumper them to ground. Does the output still have the instability? If it does, then it is a combination of the bias and output tubes. Also disconnect the GNFB and look at the output of the driver section.
With the input grounded, it should be quiet.
The GNFB was trying to overcome it, just need to find the source.
  Reply With Quote
Old 3rd December 2010, 12:37 PM   #9
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
The feedback resistor reduces the hum simply by reducing the gain. It doesn't solve the problem, just hides it.

Can you sketch an exact diagram of your star grounding? It could be charging pulses getting into the audio. This only needs one wire connected to the wrong place, which could be very close to the right place!
  Reply With Quote
Old 3rd December 2010, 06:49 PM   #10
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Fix your RF oscillation problem first, the hum problem may then go away. (From hard experience)
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote

Reply


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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
My KT88 Williamson Amp Build Loren42 Tubes / Valves 101 14th March 2014 11:59 PM
Strange transformer hum Atilla Power Supplies 12 1st January 2010 01:12 AM
Strange circuit - heater hum opt feedback? m6tt Tubes / Valves 3 23rd April 2008 06:13 PM
Strange Hum pftrvlr Tubes / Valves 4 20th February 2008 12:02 PM
strange hum problem zeus_threat Solid State 8 18th December 2007 02:55 PM


New To Site? Need Help?

All times are GMT. The time now is 10:24 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2