World Audio Design KEL34

[willbewill]

I hope both you and Nigel will be glad to hear that the Kerr McCosh’s are still performing perfectly thanks to your fine workmanship.

Looks really good that amp and I’m glad you’ve got it back working again.

Thanks David, great to know they are still being enjoyed and for the encouraging words on the KEL34.

[willbewill]

I'm still having a small issue with the KEL34.

It only affects the right channel and displays itself as intermittent low level crackle through speaker. It is worse on start up and slowly settles down over the course of about half an hour, it is much less frequent and obtrusive after this and only really noticeable with no music playing (I have left amp on with no music for a prolonged period to ascertain this). After an hour it's pretty much silent.

I've done the 'normal' easy things - swapped valves around left to right and cleaned all valve pins with IPA and inserted/removed them multiple times to try to clean sockets.

So question is where to I go looking next?

[Chris65]

Since you have eliminated the tubes, a possible culprit is a dry/cold solder joint. As the noise reduces as the amp warms up, it indicates that the joint is expanding and making better contact.

Carefully inspect all the joints in that channel & reflow any that look suspect.
After that, if needed, try the ‘chopstick test’, pushing on joints with a chopstick to check for noise - obviously this has to be done with the amplifier on, so observe all safety precautions  .

And this assumes you’ve checked the other obvious things first - cleaned volume pot & input selector, checked cable connections, etc

[willbewill]

Thanks Chris, you confirmed what I was thinking about the joints, a pain to check on this as the circuit board will need to come out. But a good point on checking and cleaning all connections and controls as I'd overlooked that.

Also one thing I've noticed, with hindsight, is that the right hand 6au6 socket feels looser than the left and as 6au6 pins are also tarnished maybe this is the problem? Should I try to tighten them a little? Obviously switched off and with the capacitors safely discharged.

I'm thinking longer term, especially if board needs to come out, to reflow all joints, maybe upgrade some components, renew pot and change the selector for a 4 way including doing away with the tape switch.

[willbewill]

OK I think the problem may have been the selector switch so I've bought a new one to fit, but...

Now a new problem has appeared - getting a crazy buzz through the speakers at about 1/2 to 3/4 volume, both channels, with or without inputs connected, on all sources. It's not hum but a definite buzz which appears around that volume and disappears again after it. I don't normally play above 1/2 volume so it may have been there a little while. I've tried different speakers but same result.

[willbewill]

Update: I backtracked over what I'd recently done - cleaned switches with contact cleaner, cleaned pins of valves.

So tried swapping in a pair of spare EL34, trying different combinations - no change!

I don't have any spare ECC82 or 6AU6 to swap in but swapped those in place left to right and right to left...

BINGO! must have been a bad contact with one or more of them?

[willbewill]

And back again when amp left on for a while - I get the feeling it must be the 6AU6 tubes and/or their sockets

[Chris65]

I’d get some spare 6AU6, not expensive, e.g from £5 at Langrex.

[willbewill]

I’d get some spare 6AU6, not expensive, e.g from £5 at Langrex.

Thanks Chris - just ordered a couple and a couple of ECC82.

[willbewill]

New 6AU6s and ECC82s fitted, much tighter fit than originals, but problem is still there.

Hoping for more suggestions where I should look? It now makes all sorts of other weird noises above half volume - oscillations?

It's nothing to do with the selector switch as I can bypass that by using the tape monitor toggle.

[analogadikt]

Are the noises emanating from both the channels?

Regards,

Anwesh

[willbewill]

Are the noises emanating from both the channels?

Regards,

Anwesh

Yes

[analogadikt]

Yes

Maybe it is time to check the power supply components that are common to both channels?

Regards,

Anwesh

[Chris65]

Troubleshooting a problem needs a methodical & careful process to establish where the fault lies. As Anwesh suggests, if the fault is common to both channels, the power supply is a good place to start.
Faults are most commonly due to faulty wiring/connections or a faulty component. Without suitable test equipment it is a bit harder but can still be done.
There are a few videos in YouTube land about troubleshooting tube amps, it may help to watch some .

[niclaspa]

It now makes all sorts of other weird noises above half volume - oscillations?

Weird noises can be a symptom of an oscillating amplifier.  The oscillations usually occur at very high frequencies.  The amplifier will act like a small radio transmitter.  I guess that you don't have an oscilloscope, but there are some other ways to check.  If the tubes have become very microphonic then you probably have an oscillating amplifier.  Tap lightly on the tubes to check.  You can also get an AM radio and put it next to the amplifier.  Oscillations can often be picked up in that band.  If you have a TV nearby, it is also likely to pick up the oscillations that will show up like distortions in the picture.

Weird noises can also be caused by radio frequency interference (RFI). 

Since this is a commercial amplifier, I am sure that it doesn't normally oscillate or pick up RFI.  You mention that you have cleaned the tube pins, but how about the sockets?  Dirty tube sockets can cause weird problems.  I am using CRC oxide clean.  I spray it into the tube sockets and insert an old tube and pull it out to rub the dirt off.  Then I spray CRC electronic cleaner into the sockets and do the in-and-out with the old tube in again.