Finished EL34 tube amp... but oscilates weird.. - diyAudio
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 3rd May 2011, 04:38 AM   #1
diyAudio Member
 
Join Date: Feb 2010
Default Finished EL34 tube amp... but oscilates weird..

Ive finished my tube amp and it plays music !!

But there occurs a weird oscillation. See my youtube vid.
YouTube - El34 Diy

The moment i touch something with the screwdriver was the cathode resistor to ground from a EL34 tube.

Any one has ideas to kill that oscilation sucker ? This evening im going to try to remove the feedback.
Attached Images
File Type: gif El34 oscillate.gif (85.7 KB, 234 views)
  Reply With Quote
Old 3rd May 2011, 06:51 AM   #2
Itsmee is offline Itsmee  England
diyAudio Member
 
Itsmee's Avatar
 
Join Date: Feb 2009
Pull the ECC82, see if it still oscillates, if it does, you know the problem is in the output stage
  Reply With Quote
Old 3rd May 2011, 08:00 AM   #3
izozaya is offline izozaya  Poland
diyAudio Member
 
Join Date: Jan 2011
Also try to disconnect the feedback loop. If oscillation stops, it is 99% chances that you swapped the two anode wires in the OPT, thus working in positive feedback mode rather than negative.

Cheers
  Reply With Quote
Old 3rd May 2011, 08:45 AM   #4
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
That kind of squealing on power up and power down suggests marginal stability. As for why wiggling wires might stop it, check for cold joints as well.

I would also suggest bypassing that bias resistor for your floating paraphase. That design doesn't have balanced cathode currents, and it does need bypassing.

And do tell us the schemo is wrong, and you have a DC return resistor on the grid of the upper paraphase triode.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 3rd May 2011, 12:16 PM   #5
diyAudio Member
 
Join Date: May 2010
Location: Ball Ground, GA
While this is not related to the stability issue, the grid return resistors for the output stage also appear to be backwards. Usually, the lower value resistor belongs with the upper tube, while the higher value resistor belongs with the lower tube. Connected as shown, the AC balance could be compromised and should be checked as well.

Dave
  Reply With Quote
Old 3rd May 2011, 05:34 PM   #6
diyAudio Member
 
Join Date: Feb 2010
Missing grid resistor of the upper paraphase to ground is in my amp ive used 1M.

Funny thing is when i only use one channel ( put out the other two EL34 ) it works without making weird noises.

But still its missing power. Even my Sansui EL84 from the 60's can even go much louder.

And ive seen that my B+ is to low. It only stays with 4x EL34 to 330. And with 2x El34 350.
  Reply With Quote
Old 3rd May 2011, 08:47 PM   #7
diyAudio Member
 
Join Date: Feb 2010
Quote:
Originally Posted by Miles Prower View Post
I would also suggest bypassing that bias resistor for your floating paraphase. That design doesn't have balanced cathode currents, and it does need bypassing.

And do tell us the schemo is wrong, and you have a DC return resistor on the grid of the upper paraphase triode.
Hmm i don't know what you mean ? what do you mean with bypassing the bias resistor ? remove the 150k resistor ?
  Reply With Quote
Old 3rd May 2011, 08:55 PM   #8
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
missing grid to ground resistor before ecc82 ?
  Reply With Quote
Old 4th May 2011, 03:25 AM   #9
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
close to similar design
Attached Images
File Type: gif DHT EL34 guitar amp.gif (94.5 KB, 137 views)
  Reply With Quote
Old 4th May 2011, 06:37 AM   #10
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by TubeManiac View Post
Hmm i don't know what you mean ? what do you mean with bypassing the bias resistor ? remove the 150k resistor ?
No. I meant something like a 100uF capacitor in parallel with the 1K resistor between the cathodes of the ECC82 and ground. You can't remove the 150K resistor, otherwise it won't work at all.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  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
Trouble with a DIY EL34 Tube Amp Kit elleelle Tubes / Valves 32 21st February 2011 03:55 AM
question about swapping el34 for kt77 in my tube amp johnzm Tubes / Valves 12 2nd September 2010 06:11 PM
EL34 tube amp that needs work on el34ecc88 Tubes / Valves 0 17th December 2008 08:05 AM
tube amp being weird -- any ideas soundbadger Tubes / Valves 13 12th March 2008 01:08 AM
EL34 vs. 6L6GC for 1ST DIY Tube Amp? pjkunz Tubes / Valves 4 17th December 2001 02:51 AM


New To Site? Need Help?

All times are GMT. The time now is 09:43 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