World-Designs-Forum  

Go Back   World-Designs-Forum > World Designs > WD Amplifiers
Register FAQ Members List Calendar Gallery Today's Posts Search Today's Posts Mark Forums Read

WD Amplifiers For discusson of World Designs Amplifiers

Reply
 
Thread Tools Display Modes
  #1  
Old 8th August 2017, 08:51 AM
snodge snodge is offline
Junior Member
 
Join Date: Oct 2012
Location: Bristol
Posts: 26
Default KEL 84 parasitic osscilation

Hi a mate of mine has a kel 84 it been tested and has occasional parasitic osscilation. Does anyone know what causes this? Regards David
Reply With Quote
  #2  
Old 8th August 2017, 10:44 AM
andrew ivimey's Avatar
andrew ivimey andrew ivimey is offline
Senior Member
 
Join Date: Dec 2005
Location: Bedford
Posts: 2,026
Default Re: KEL 84 parasitic osscilation

parasitic oscillation is very unusual for this amp imho ....

- what approx frequency are we talking about?
- how does it affect the amp's performance?
- is it a real KEL84 using the PCB with nothing hanging off? (I say this because I built a KEL84 but hardwired it all; I didn't scope it and it sounded cheerful enough, so I moved on.)
Reply With Quote
  #3  
Old 8th August 2017, 11:26 AM
snodge snodge is offline
Junior Member
 
Join Date: Oct 2012
Location: Bristol
Posts: 26
Default Re: KEL 84 parasitic osscilation

Yes it's a real kel kit with circuit boards. It wasn't scope tested and the amp buzzes or screeches when the volume pot is between 8&10. It's around 10 years old and a new set of valves (Watford valves) were recently fitted regards david
Reply With Quote
  #4  
Old 8th August 2017, 02:08 PM
Richard Richard is offline
Super Moderator
 
Join Date: Dec 2005
Location: Notts
Posts: 4,500
Default Re: KEL 84 parasitic osscilation

Hi David,
Yes similar has been reported on 2 or 3 Kel84s and I've had it on Kit88 and Kit6550.
Parasitic oscillation might also be confused with RF or mains borne interference but the various cures are related to either blocking the interference or lowering the gain of the amp at the frequency causing the problem.
Here are a few things to try;
Increase grid stopper values to 10K on ECF80 and EL84
Put a 100pf cap between ECF80 pin 2 and ground
Increase the hf feedback compensation cap from 3n9 to 4n7 or 5n6
If you think mains noise may be an issue try the filter components shown in the FAQ, http://www.world-designs.co.uk/forum...read.php?t=149
and it wouldn't hurt to bypass C1,2,3,22 with ceramic disc caps such as 2kV 10nF RS 721 5076
__________________
'
Reply With Quote
  #5  
Old 8th August 2017, 03:16 PM
snodge snodge is offline
Junior Member
 
Join Date: Oct 2012
Location: Bristol
Posts: 26
Default Re: KEL 84 parasitic osscilation

Hi Richard many thanks I will pass these comments on regards David
Reply With Quote
  #6  
Old 8th August 2017, 04:37 PM
andrew ivimey's Avatar
andrew ivimey andrew ivimey is offline
Senior Member
 
Join Date: Dec 2005
Location: Bedford
Posts: 2,026
Default Re: KEL 84 parasitic osscilation

Yes that would damp things down a bit! But, I wonder what started this off in the first place.
Reply With Quote
  #7  
Old 8th August 2017, 09:18 PM
bikerhifinut bikerhifinut is offline
Senior Member
 
Join Date: Jan 2006
Location: Penrith, Cumbria
Posts: 828
Default Re: KEL 84 parasitic osscilation

I had a bit of an issue with HF oscillation with my WD88 when i first built it, it was as if a capacitor was charging up and creating the circumstances for a steadily rising low volume oscillation. The cure was in the end very simple, i simply replaced the original JJECC83. The strange thing is it was just fine in my then EAR834P in the cathode follower but in the preamp stage of the WD 88 the circuit conditions must have been just right to oscillate.
So I'd look first at the ECF80 i had a KEL84 and have to say i got through a couple of sets of ECF80 before getting a pair that i was happy with.
I'd also go with Richards suggestions on grid stoppers up to 10k is fine and you wont hear any difference at all.
For what its worth.

A
Reply With Quote
  #8  
Old 9th August 2017, 10:30 AM
pre65's Avatar
pre65 pre65 is offline
Senior Member
 
Join Date: Jan 2006
Location: Ashen- Essex/Suffolk bord
Posts: 4,344
Default Re: KEL 84 parasitic osscilation

Quote:
Originally Posted by Richard View Post
Hi David,

Here are a few things to try;
Increase grid stopper values to 10K on ECF80 and EL84
Is it still best practice to use carbon resistors for grid stopper use ?

I use ferrite beads as well myself.
__________________
Philip.

Everything in this post is my honest opinion based on what i thought I knew at that very moment in time.
Reply With Quote
  #9  
Old 10th December 2017, 10:13 AM
snodge snodge is offline
Junior Member
 
Join Date: Oct 2012
Location: Bristol
Posts: 26
Default Re: KEL 84 parasitic osscilation

Hi I have borrowed this amp and it just doesn't sound right at all. The sound is rather like a cheap transistor radio on AM. I had a KEL 84 a few years ago and it sounded superb. I have tried it Klipsch, AVI and Tannoy speakers and several interconnect and speaker lead combinations.

Any advice would be gratefully received.

Regards David
Reply With Quote
  #10  
Old 10th December 2017, 10:18 AM
bob orbell bob orbell is online now
Senior Member
 
Join Date: Nov 2009
Location: kettering northants.
Posts: 1,451
Default Re: KEL 84 parasitic osscilation

Just a simple question, what are you using as a input source ?. BOB
Reply With Quote
Reply

Thread Tools
Display Modes

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


All times are GMT +1. The time now is 09:20 PM.


Powered by vBulletin® Version 3.7.3
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
Copyright World Designs