Oscillation in tube amps - Page 2 - 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 28th November 2014, 02:51 PM   #11
hego is offline hego  Europe
diyAudio Member
Join Date: Jan 2008
Location: Tarraferma in Catalonia
Good try: it works

" Cats (max 64khz) or dogs (40-60khz) act weird around the amp, when they don't around others or playing similar music/sounds"

  Reply With Quote
Old 30th November 2014, 03:01 AM   #12
JamesG is offline JamesG  United States
diyAudio Member
Join Date: Feb 2006
Location: Somewhere North of the Equator
In my experience, oscillation is usually a function of layout and proper placement of grid resistors, which should always be soldered directly to the tube socket. For similar reasons, I have never used tag board construction for high gain guitar amps. I am a proponent of point to point construction. Of course, I don't build 10,000 amps. I build just one.
  Reply With Quote
Old 30th November 2014, 10:59 AM   #13
diyAudio Member
bayermar's Avatar
Join Date: Oct 2007
Location: Tuebingen
Could this become a thread for the sticky section?
As George said: A lot of information about oscillation is usually scattered across a lot of different threads, some of th em quite long, therefore hard to find.

  Reply With Quote
Old 30th November 2014, 03:50 PM   #14
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
Originally Posted by robertaudio
oscillation can happen if the Amp has ALL the following conditions;
1. Gain too high
2. Positive feedback path.

Eliminate any of the conditions and oscillations will go away.
OK, provided that you remember that these criteria apply to each stage separately as well as the whole amp. (And any pair of stages in a three stage amp).
  Reply With Quote
Old 30th November 2014, 06:23 PM   #15
Celsius is offline Celsius  Spain
diyAudio Member
Celsius's Avatar
Join Date: May 2011
Location: Flying to the moon
Same time is not only one cause.
Im my case I have change all rectifier bridge by HER 207 diodes and ferrite on the anode of the preamp valve, now the oscillation have been reduced a lot, but follow with a little, tomorrow will mount a snubber in the 211 filament line, I think 99% that the noise residue that have is provide by the switching power supply, maybe the snubber circuit help to reduce in other case will change by coil transformer.
  Reply With Quote
Old 2nd December 2014, 04:20 AM   #16
wa2ise is offline wa2ise  United States
diyAudio Member
wa2ise's Avatar
Join Date: Apr 2005
Location: NJ
Originally Posted by JamesG View Post
In my experience, oscillation is usually a function of layout and proper placement of grid resistors, which should always be soldered directly to the tube socket. ...
That's called a "grid stopper". Idea being that the resistor combines with the stray capacitance inside the tube to create a low pass filter that kills the tube's ability to go into supersonic oscillation. I've fixed strange hum problems that turned out to be byproducts of supersonic oscillations.
  Reply With Quote
Old 2nd December 2014, 11:03 AM   #17
diyAudio Member
Join Date: Nov 2009
Location: London, UK
In small signal pentode circuits, frequencies at which grid stoppers become effective is at high rf because miller C is so low. When such circuits do oscillate, it can be at very high frequencies indeed. As in super high GBW op-amps, it's worth intentionally restricting stage bandwidth by design to avoid self-oscillation. Small signal pentodes seem to have a reputation for this, but really it's generally the circuit design and layout which often sets up the opportune circumstances for pentodes to do what they do best, having terrific GBW by nature. Just my 2p worth.
  Reply With Quote
Old 2nd December 2014, 06:14 PM   #18
kevinkr is offline kevinkr  United States
diyAudio Moderator
kevinkr's Avatar
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Made the thread a sticky since it seemed like a good idea and was also suggested by another member.
"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 8th December 2014, 12:06 PM   #19
nl3prc is offline nl3prc  Netherlands
diyAudio Member
nl3prc's Avatar
Join Date: Mar 2012
Location: Den Helder The Netherlands
Originally Posted by celsius235 View Post
is possible that in the first 20 minutes after switch on the tube donīt oscillate and later begin, I rebuilt my amp and now in the first 20 minutes the listen in perfect but later begin as Tubelab said, high piano notes begin to be horrible.
Is it maybe possible that in the beginning he oscilates above the 20000 Hz and you can't hear it
maybe its better to look at this with a scoop

salutions look at the feedback your using and also look at your OPT so that the input is shielded from it
  Reply With Quote
Old 16th December 2014, 08:11 PM   #20
diyAudio Member
Miles Prower's Avatar
Join Date: May 2005
Location: USA
Blog Entries: 7
Originally Posted by luckythedog View Post
In small signal pentode circuits, frequencies at which grid stoppers become effective is at high rf because miller C is so low. When such circuits do oscillate, it can be at very high frequencies indeed.
This isn't the case. Grid stoppers don't prevent oscillation by acting as LPFs. Stoppers act by loading down parasitic resonant circuits to drop their Q-factors below the point where they can sustain oscillation. Your best bet for grid stoppers are C-comp resistors. 4K7 for control grids; 1K for screen grids (if using pents) are good. For power pentodes, screen stoppers from 470R -- 1K5 are good for stopping/preventing snivets. Some types are worse than others for this. 807s and other 6L6-oids like to make snivets, while others (6V6-oids, 6BQ6GA don't seem to have this problem) These values are high enough to de-Q parasitic LC tuners, low enough so's not to excessively roll off treble frequencies.

When doing high gain circuits (small signal pents, cascodes, BJTs and MOSFETs) it's always a good idea to treat 'em like RF circuits: keep all leads short as possible, and include stoppers. Especially with pents/cascodes, look for sockets with a central pin, and connect it to the circuit ground. Screen bypasses should be installed last, and installed so's they span the socket between the plate and control grid, with the "outside foil" connected to the ground plane. That way, it does double duty as a bypass and electrostatic shield. even if you need a big electrolyric to handle low audio frequencies here, you can always parallel it with a smaller capacitor as a high frequency bypass.

That'll keep the oscillation gremlins at bay.
There are no foxes in atheistholes
  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
For Sale: Tube Amps, Class-A Amps, BG Ribbons, Chassis and Other Goodies! opc Swap Meet 23 2nd November 2009 05:04 PM
Help Please with tube amps!!!!! bobcat1979 Instruments and Amps 1 28th June 2009 11:29 AM
Tube amps & MP3's jim Tubes / Valves 18 20th February 2008 04:15 AM
Tube Oscillation or Microphonic Question Trout Tubes / Valves 6 29th December 2005 04:18 AM

New To Site? Need Help?

All times are GMT. The time now is 10:30 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright Đ1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2