Large Channel Imbalence KEL84

[mudvin]

Hi All,

My newly completed KEL 84 has an issue where the Left Channel is much lower in volume than the right. The issue is a little strange in that the extent of the imbalance varies a lot and can be changed by quickly moving the potentiometer. Depending on the extent the issue can present as one channel being almost completely mute, or just a slight imbalance of the stereo image.

The issue remains in the same channel if I switch the valves, and if I switch the inputs (So I'm pretty sure that the problem is not in the valves or input wiring) and Voltages all check out as per the manual. Other than the channel imbalance the amp is silent, with no popping or hiss. I have attempted to poke all the joints with a chopstick and nothing presents as a bad solder joint.

So with that in mind I tried the nuclear solution, removing the main PCB and re flowing all joints (and by extension all the cables leading to it, however there was no change in the issue and this was pretty much a complete rebuild of the amp/)

So at this stage the only thing I have left is that the the pot may be faulty, and I will replace it as soon as I can. In the meantime though, I was wondering if there was anything else I can try. The was a previous post with a similar issue suggesting to check negative feedback and ground connections. However I believe I have addressed those with the rebuild, anything I've missed ?

Thanks in advance, Rob

[A Stuart]

I am not au fait with this circuit, but your comment that the gain of a channel appears related to the rate of change of position of the potentiometer must lead to the centre of the problem.
Check if there is a DC voltage across the potentiometer.
As I said, not familiar with this particular circuit, but in general I think it would be true to say there should be no DC across a pot.

[BOONDI]

Try a meter set to resistance on both channels of the pot.
Attach leads to input of pot and wiper of pot.
Then turn the pot from side to side.
This should give a smooth rise or fall from 0 to 50k.
Then do the same again on the other channel.
Again you should see a smooth rise or fall between 0 and 50k

You will need some clip leads or 3 hands to do this.

If you get anything other than a smooth rise or fall of resistance i would suspect the pot.
If you swap the channel wiring on the pot and the fault follows then it is the pot.

If the pot tests good, try swapping the speaker leads over. If the fault is still in the left channel then you have a faulty speaker. If it moves to the right then it' s in the amp. Wouldn't be the first time ive seen this either.

Let us know how it is going just in case you need further assistance.....


Alternatively, you may have the left. Channel wire directly across the pot instead of wired to the wiper. This would give same volume on left channel no matter what position yoh have the pot in, so check this carefully just in case.... Also, your pot may be 100k. It would still be the same as above.....

[A Stuart]

My thinking was that the only way the amp can "know" whether the pot is being moved is if there is a voltage across the pot and moving the wiper alters the standing bias on a grid.
It does depend exactly what Rob means by "moved quickly". If he means eg tapping the knob, then what I have said does not apply.

Otherwise, only do what Bob says with the amp off and discharged, unless you have already ruled out the presence of any voltage across the pot. Depending on the design, putting a voltage into an ohmmeter could wreck it.

[BOONDI]

Yes, power down first justin case.

That said, i dont see where there would be dc at the pot. The wiper goes direct to the grid via a 3k3 resistor with 1M to ground. The input to the pot is direct from the source inputs. Sometimes the wiper is not making good contact with the track, so turning the pot can push the wiper onto it causing it to work properly. Also, sometimes the track is not fully coated properly and again moving the pot will cause differing output. Further, if the grid is incorrectly wired to the other end of the pot then limited volume and scratchy sound when the pot is rotated. Dont laugh but i have done this myself way back in the day.

If the pot passes the tests, then i suspect either the wiring or the two resistors. I cant see how it would be anything else as the op has said swapping and etc had no discernible effect to which side faulty was on.

Also try swapping speaker wire around. If fault now on right hand then its the amp. If still on left then u have a dodgy speaker.....

Try powering up the amp and wiggling the ecf80 gently in its socket. Any chages to the sound could mean a loose pin in the valve socket.

[A Stuart]

It would certainly need something very strange to get DC to it, as you say there are no potentially leaky interstage capacitors involved.
I think that we need more detail on this comment about moving the pot quickly altering the amp's characteristics.
A simple bad connection would surely be very noisy, but I agree with you that isolating and confirming the pot's behaviour would be good.
Alastair

[Richard]

Hi Rob and welcome,


Is this a new kit you've built yourself? Is it a power amp (single input) or integrated amp (multiple inputs). If integrated, what input selectors are used; rotary switch only or with electronic relays?

[mudvin]

Hi Richard

It is a new kit, Integrated amp with the relay board

I have tried replacing the potentiometer today with an alpha 50k I had lying around.

The issue still remains, although it is even more obvious (The pot is a lot less linear, may not be logarithmic). Basicly Right channel works as you would expect. Left channel sounds like it is not playing but if you turn the volume way up, it is and sounds clean, however the volume is maybe 1/8th or a 1/4 of the right channel.

[snowman_al]

Just wondering out loud here, but if the feedback loop is missing or a poor connection on the Right Hand channel, it will play much louder than the Left channel. Perhaps the problem is not on the left side?

Either do a continuity / resistance check round the right side R32 and the RFB wiring. Or as an experiment lift the feedback wire off the Left side and then see if it plays as loudly as the right.

I assume you have swapped speakers around.

[bob orbell]

Hi Mudvin, if you can post pictures of your build ( internal ) this my help us to see any issues. Bob