KEL84 hum when source plugged in?

[Audiophill]

Originally Posted by davebms

hello all,, just a thought, there are different types of RCA connectors, the older
ones had fibre washers each side and could/did contact the chassis
later types, nylon with shoulders I HAVE HAD problems with the older type

dave

Hi Dave, all my leads are quite new and the rca sockets look quite recent too.

[Richard]

What do the valves look like in a darkened room Phill, any glowing red anodes?

Have you changed the valves around to see if they may pair up better? Have you checked voltages around the valves; anode, cathode and grids?

Old or mis-matched power valves can cause hum. Leaking coupling caps (those large gold coloured caps) are paper in oil and can leak electically letting voltage on the EL84 grids which turns them on. Replace them with plastic films if you find more than a couple of hundred mV on the grids.

[Audiophill]

Originally Posted by Richard

What do the valves look like in a darkened room Phill, any glowing red anodes?

Have you changed the valves around to see if they may pair up better? Have you checked voltages around the valves; anode, cathode and grids?

Old or mis-matched power valves can cause hum. Leaking coupling caps (those large gold coloured caps) are paper in oil and can leak electically letting voltage on the EL84 grids which turns them on. Replace them with plastic films if you find more than a couple of hundred mV on the grids.

The el84 valves are new jj,s and I also tested with some nos 6P14 reflectors, also tried some mullard and ael ecf80, will check the coupling caps.
Many thanks

,

[John Caswell]

Hi all,
This is definitely not a valve imbalance problem.
It has something to do with the input and I suspect wiring.
Are you sure your interconnects are screened and the screen is connected at both ends? Are you sure that the screens are connected at the preamp end?

John

[Audiophill]

Originally Posted by John Caswell

Hi all,
This is definitely not a valve imbalance problem.
It has something to do with the input and I suspect wiring.
Are you sure your interconnects are screened and the screen is connected at both ends? Are you sure that the screens are connected at the preamp end?

John

Hi John, yes checked and I even replaced them just to be sure.
Many thanks

[Richard]

Hi Phill, hopefully easy yes/no to most of this,

1) is the hum on both channels?
2) does it hum with just 1 channel RCA input plugged in, if so, on one or both channels?
3) does it hum with no input device connected?
4) if it has multiple inputs do all positions cause similar hum?
5) does it hum with a source connected to an input but the selector turned to another position which doesn't have a source connected?
6) does it hum with a source connected but switched off?
7) have you tried different mains sockets for amp and source?
8) does a source that causes hum have a 3 wire mains lead?

[Audiophill]

Originally Posted by Richard

Hi Phill, hopefully easy yes/no to most of this,

1) is the hum on both channels?
2) does it hum with just 1 channel RCA input plugged in, if so, on one or both channels?
3) does it hum with no input device connected?
4) if it has multiple inputs do all positions cause similar hum?
5) does it hum with a source connected to an input but the selector turned to another position which doesn't have a source connected?
6) does it hum with a source connected but switched off?
7) have you tried different mains sockets for amp and source?
8) does a source that causes hum have a 3 wire mains lead?

Hi Richard, been working shifts and only just got the chance this afternoon to have another look, will answer each question no or yes with comments if needed
1=yes, hums on both channels
2=no, dosent hum with one channel plugged in.
3=no him with no source plugged in.
4= yes, multiple inputs and all hum with source plugged in
5=no hum if another source connected to a different position with no source connected on selected position.
6=yes still hums with source turned off.
7=Yes, tried different sockets but no change.
8=yes but cd player has 2 pin and cause hum and also phone when plugged in.

Many thanks for continuing to help, I'm going to fit better shielded cables from sockets to selector, selector to volume pot , volume pot into boards and the feedback and speaker to opt to see if this helps.
Cheers Phill

[Richard]

Hi Phill,

It sounds like the output stage (or another stage) of one channel may not have a good ground connection to the signal ground point (which should be the earthiest point of the circuit - usually the negative of the large PSU caps). One stage may be earthing instead via the RCA input ground connections causing a hum loop. Such a fault may be difficult to spot using just a meter as the hum may be from small different relative earth impedances then presenting on the inputs.

Your amp construction is on tags rather than a PCB layout so check all the stage signal ground wires are fitted and go back to the star main signal ground point. In particular check the feedback wires are connected to their own channel black speaker posts and both the black speaker posts are well connected back to the star signal ground point.

[Audiophill]

Originally Posted by Richard

Hi Phill,

It sounds like the output stage (or another stage) of one channel may not have a good ground connection to the signal ground point (which should be the earthiest point of the circuit - usually the negative of the large PSU caps). One stage may be earthing instead via the RCA input ground connections causing a hum loop. Such a fault may be difficult to spot using just a meter as the hum may be from small different relative earth impedances then presenting on the inputs.

Your amp construction is on tags rather than a PCB layout so check all the stage signal ground wires are fitted and go back to the star main signal ground point. In particular check the feedback wires are connected to their own channel black speaker posts and both the black speaker posts are well connected back to the star signal ground point.

Hi Richard
The earth is quite a mess on the tag boards with wires going back and forth in all directions but I have checked them all already, just rerun new better shielded feedback wires direct from each board to the speaker post ( the shield connected one end only to ground) and a separate wire from the opt to the speaker post, have noticed the the negative speaker posts are connected and with one ground wire coming from the star point, should they have separate ground wires?
Also was wondering if anyone had a schematic which I could check component sizes are correct ie resistor and capacitor sizes? I did manage to find one but it’s a bit too blurry too read correctly?
Many thanks again.
Cheers
Phill

[Audiophill]

Also I noticed that the signal wire going to pin 2 on the ecf80 is not shielded so I’m going to replace that also with a shielded wire attached to ground at one end only!