KEL 84 Strange Noises

[deepdown130]

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

[Richard]

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.

[deepdown130]

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.

[deepdown130]

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.

[Richard]

Originally Posted by deepdown130

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

[deepdown130]

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??