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  #1  
Old 2nd May 2016, 05:26 PM
deepdown130 deepdown130 is offline
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Default KEL 84 Strange Noises

Hi everyone. I have a KEL 84 that is about 9 years old. In the last couple of weeks, the left channel has started making strange noises almost as if it is picking up radio signals that are not quite tuned in correctly.

I've tried swapping the sides that the valves are on and that didn't help. I have now just replaced all of the valves and the problem is still there.

I've moved the amplifier to a different location to no avail and I've just experimented with the inputs. However, it even does it with nothing connected.

Does anyone have any suggestions where I might start looking?

EDIT: I've just checked all the voltages on the build sheet and they are all in tolerance with the exception of the following:

- aT voltages on V1 and V2 pin 1. They are both at 175v rather than 205v (they were at 169 and 186 respectively when I built it)
- gT voltages on V1 and V2 pin 9. They are at 96v and 95v respectively rather than 75v (they were at 98 and 77 when I built it)

Many thanks,

Gavin

Last edited by deepdown130; 2nd May 2016 at 07:02 PM.
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  #2  
Old 2nd May 2016, 07:12 PM
Richard Richard is offline
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Default Re: KEL 84 Strange Noises

Hi Gavin,

The pentode ECF80 input will be very good at amplifying RF. I would try 100pf film cap (polystyrene or polypropylene) from pin 2 (g1 grid) of ECF80 to earth. This will filter RF on the input without affecting audio frequencies. Pentodes have lower effective input capacitance than triodes (which suffer Miller capacitance) and amplify RF that would otherwise be lost in a triode.

A change in local RF pollution or a small change of impedance in the signal or power supply circuit may have set it off. At 9 years old the electrolytics may benefit from changing. These typically lose capacity and increase their esr with age. Another problem might be caused by your source so try swapping it and the interconnects. Likewise a change in selector switch contact resistance or a dry joint could be the culprit. Try shorting the inputs to see if it stops.
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Old 2nd May 2016, 09:28 PM
deepdown130 deepdown130 is offline
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Default Re: KEL 84 Strange Noises

Richard,

Many thanks for your quick reply.

I guess, shorting the input is the simplest thing I could try - just waiting for the noise to start up now and I'll see if that stops it.

Then I'll change the electrolytics and add the film cap as you suggest (one at a time so I know which one solved the problem). I'm pretty sure it's not the source or the interconnects because it still does it when they're disconnected.

Thanks again - I'll report back when I get it solved.
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  #4  
Old 3rd May 2016, 02:49 PM
deepdown130 deepdown130 is offline
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Default Re: KEL 84 Strange Noises

Just realised that I have posted this in the wrong section - it should be in the WAD problems section.

Could a moderator move it if they think its necessary? Many thanks.
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Old 3rd May 2016, 03:12 PM
Richard Richard is offline
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Default Re: KEL 84 Strange Noises

Quote:
Originally Posted by deepdown130 View Post
Just realised that I have posted this in the wrong section - it should be in the WAD problems section.

Could a moderator move it if they think its necessary? Many thanks.
No problem Gavin, KEL84 is WAD's (now WD's) longest running amp from 2000 and is still available in updated form
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Old 5th May 2016, 02:02 PM
deepdown130 deepdown130 is offline
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Default Re: KEL 84 Strange Noises

Thanks for that Richard.

So the plot thickens - here we go from the beginning:

1) Built kit with supplied Edicron ECF80s and JJ EL34s (about 9 years ago)
2) Recently (a couple of months ago) I have changed the ECF80s for a pair of Pinnacle ECF80s
3) Even more recently I started to experience the problems I described and went through first swapping sides of one valve at a time to see if the problem switched sides
4) No change noticed so changed the ECF80s for a matched pair of Mazda branded valves and changed the EL34s for some Russian made valves
5) Still no change so asked for help on here.
6) Richard kindly offered some ideas. None of which solved the problem sadly
7) yesterday in desperation I refitted the Edicron ECF80s and the problem has gone!

I now have a nice silent KEL84 again :-)

But...why??
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  #7  
Old 5th May 2016, 04:58 PM
bob orbell bob orbell is offline
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Default Re: KEL 84 Strange Noises

To correct your last post,( and JJ EL 34s,) should read EL 84s, glad I got that off my chest , so if you now refit the Pinnacle ECF 80s, although you may not want to, does the problem return?, if it does not, then I would suspect dirty or lose valve sockets, if it does, then it must be a valve. BOB
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  #8  
Old 5th May 2016, 05:06 PM
deepdown130 deepdown130 is offline
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Default Re: KEL 84 Strange Noises

Thanks Bob - the clue is in the name of the amp! I'll put it down to my age😀

Yes - if I refit the Pinnacles or the Mazdas the problem returns (but stays on the left side if I swap the valves over!).


I can't believe I've got 4 faulty valves?

Maybe I'll try mix and match just to see what happens (although I'm very tempted to leave it alone now it seems good again).
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  #9  
Old 5th May 2016, 09:02 PM
bob orbell bob orbell is offline
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Default Re: KEL 84 Strange Noises

Well that's got me as well, maybe our man John can come up with something.
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  #10  
Old 5th May 2016, 09:10 PM
deepdown130 deepdown130 is offline
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Default Re: KEL 84 Strange Noises

It's strange indeed. I'm going to see if I can get some Edicron ECF80s for when these finally wear out - it seems my amp only likes these!
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