Broskie regulator oscillates - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs 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

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
Thread Tools Search this Thread
Old 18th July 2009, 08:01 PM   #1
rman is offline rman  Canada
diyAudio Member
rman's Avatar
Join Date: Oct 2007
Default Broskie regulator oscillates


I built this regulator from the gassware webite.Broskie reg

It oscillates at about 3MHz and 0.3v pp. I was just wondering what causes this. I know that an oscillator needs a resonant circuit and positive feedback to work. I just don't quite see how it happens here.

Here are some observations I made about it on another thread.
The load for this test is a big wire wound pot set to 12.5k (20ma. @250v). It was intended for a phonostage, the regulator that is. Interestingly, when the load is adjusted to about 12.1k, the oscillation stops. The Mosfet is an IRF BC40 (found on shop floor at work). swapped in an IRF 540 and the amplitude of the oscillation rises. I really don't' know enough to account for all this. I can only conclude that layout is critical, and that mine is not great.
  Reply With Quote
Old 18th July 2009, 08:06 PM   #2
Colt45 is offline Colt45  Serbia
diyAudio Member
Join Date: Feb 2002
Location: Canada
Send a message via ICQ to Colt45
Throw a cap on the output...?
  Reply With Quote
Old 18th July 2009, 08:16 PM   #3
rman is offline rman  Canada
diyAudio Member
rman's Avatar
Join Date: Oct 2007
I forgot to mention that I tried that as well. Some small valued caps. the amplitude of the oscillation actually went up! I don't really need this unit right now and am not just looking for a quick fix, just trying to understand how it happens in the first place
  Reply With Quote
Old 18th July 2009, 09:40 PM   #4
athos56 is offline athos56  United States
diyAudio Member
athos56's Avatar
Join Date: May 2006
Location: Yakima, Washington
Did you try stopper resistors? Everything I've made with mosfets needed to be encased in stopper resistors.
  Reply With Quote
Old 18th July 2009, 09:51 PM   #5
rman is offline rman  Canada
diyAudio Member
rman's Avatar
Join Date: Oct 2007
Hi Athos.

The original design show a 200r gate stopper. When I saw the oscillation on my scope, I upped it to 1k and moved it as close to the mosfet's gate as I could get it. The oscillation waveform only became distorted.

When you say encased in stoppers, do you mean something more Than just a resistor on the gate?
  Reply With Quote
Old 18th July 2009, 09:56 PM   #6
Gordy is offline Gordy  United Kingdom
diyAudio Member
Join Date: Nov 2006
I think this guy did the same design, and has documented his experiences:
  Reply With Quote
Old 18th July 2009, 10:04 PM   #7
iko is offline iko  Canada
diyAudio Moderator
iko's Avatar
Join Date: May 2008
Location: Toronto
Try a generic 470uF capacitor on the output. Something not with a low ESR.

Could you please give a few more details? What opamp are you using? You might want to try a few different opamps. When I played with various opamp based regulators I found that some did not oscillate so readily. Another thing to try is a 200pF to 10nF capacitor on the G-S pins of the mosfet. I would not use that in the long term, just temporary to troubleshoot the circuit.
  Reply With Quote
Old 18th July 2009, 10:54 PM   #8
pointy is offline pointy  United Kingdom
diyAudio Member
pointy's Avatar
Join Date: Mar 2009
as I am very rusty and maybe a bit mad........................OK ?

I think the feedback is the diode loop just to the left of the marking +295 in red.

the resistance is ESR of the wire.

and if I remember right you can move the oscillation by putting in a resistor or something.

PS..... I have been awake for more than 24 hours or so
  Reply With Quote
Old 19th July 2009, 02:20 AM   #9
rman is offline rman  Canada
diyAudio Member
rman's Avatar
Join Date: Oct 2007
Hi guys.

Thanks for all your suggestions. I hope pointy is getting some sleep. After messing around some more I still see a signal at around 1MHz. Then I notice it is still there only bigger with the power turned off! also it is pulsating. I must be looking at an AM radio station.
  Reply With Quote
Old 19th July 2009, 09:12 AM   #10
diyAudio Member
richwalters's Avatar
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
This is an old problem. Most constructors layouts are designed without any RF consideration. The mosfet is oscillating parasitically at will without one knowing about it and so high that most scopes won't show it. The decoy is the scope probe often stops it.
Fit small ferrite beads in both gate and source leads. I fit in both, some may find the gate is enough to dampen. Make sure they aren't touching anything else and that should keep the device quiet.
Try it !

Attached Images
File Type: jpg mosfet ferr.jpg (51.2 KB, 263 views)
  Reply With Quote


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
marshall g-80 oscillates unclejed613 Instruments and Amps 1 9th October 2007 04:24 PM
marshall g-80 oscillates unclejed613 Instruments and Amps 0 21st September 2007 12:46 AM
Amp oscillates with new speakers hajame Solid State 27 9th May 2007 12:01 PM
broskie voltage regulator dave dove Tubes / Valves 2 14th January 2007 09:50 PM
Designed an amp, it oscillates!!!!! Duo Solid State 7 15th September 2003 06:53 PM

New To Site? Need Help?

All times are GMT. The time now is 12:59 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2