KEL84 hum when source plugged in?

[Audiophill]

Hi Evereone, new member here, II have a diy kel84 already made that i purchased that works well and is silent untol a source is plugged in then i get a low hum that gets louder as volume incrrases, dosent matter if its a cd, phono stage or rca to jack to plug my phone in to stream? ook a chance on a tube amp of ebay that was very cheap as it looked like a kel84 but in a granite plinth? When it arrived ( it weighed a ton becawhen i got it I use the top was granite on a thick welded steel chassis) I flipped her over and it was a kel84! but they had used the kel84 transformers but not the circuit board! There made it with 2 separate point to point boards, one for each channel, quite messy but quality components used. Can anyone offer any help please?
Many thanks Phill

[bikerhifinut]

Photos are always useful.
A quick test, plug in a couple of shorting plugs into the inputs and turn the wick up.
If you get no background hum then its possible that the build was ok to begin with.
If you get Hum then you've got an internal earth routing issue, this is not uncommon even amongst experienced builders at times.

Theres a possibility theres a hum loop and it's worth checking if the signal ground is "Lifted" above the chassis safety ground which on a power amp should be connected to mains safety earth.
This is usually in the form of a resistor from chassis to the amplifiers main circuit earth and usually around 10 ohms. Other methods exist such as a pair of back to back parallel wired diodes which effectively insulate signal ground from mains earth until a PD of 0.6V or more occurs across them, such as if a fault condition happens. theres often a high voltage capacitor of around 100nF wired in parallel too.

Anyway some photos would help to begin with.

andy

[Audiophill]

Many thanks Andy, hopefully I have uploaded photos correctly, looks like a 10ohm resistor with a .1uf safety cap has already been fitted to the ground for the boards. I tried with shorting plugs and no hum,plug anything else in and still hum? Many thanks in advance for any help

[John Caswell]

The fact that the hum gets louder when the vol control is turned up usually means the the screening of the input cables or the layout is at fault, ie heater cable near input grids and the like.
Disconnect the input to the volume control and see if the hum is still there when turned up, that may help isolate the area.

John

[Audiophill]

Originally Posted by John Caswell

The fact that the hum gets louder when the vol control is turned up usually means the the screening of the input cables or the layout is at fault, ie heater cable near input grids and the like.
Disconnect the input to the volume control and see if the hum is still there when turned up, that may help isolate the area.

John

Hi John, I don't get any hum when nothing is plugged in, ots totally silent, so I'm not getting any interference in the wiring from the rca sockets, it's only when I plug something in.!

[Richard]

Hi Phill and welcome,


Check the signal ground is actually 10R away from chassis earth using a meter as often signal ground is unintentionally being shorted out by RCA socket shells etc.

[snowman_al]

It's me with dumb questions...
Is the mains earth actually connected from the plug to the chassis?
Are the feedback components fitted and connected?
What is the black wire doing in the picture?
How difficult it is to follow red and black wires...

[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

[Audiophill]

Originally Posted by Richard

Hi Phill and welcome,


Check the signal ground is actually 10R away from chassis earth using a meter as often signal ground is unintentionally being shorted out by RCA socket shells etc.

Measured both signal ground to earth and 10.1 ohms !!many thanks

[Audiophill]

Originally Posted by snowman_al

It's me with dumb questions...
Is the mains earth actually connected from the plug to the chassis?
Are the feedback components fitted and connected?
What is the black wire doing in the picture?
How difficult it is to follow red and black wires...

All wires traced and all connected as per circuit diagram, that black wire comes from the anode of one tube and is twisted around the other red wire from the other anode off the other tube. I have gone through the entire circuit and all components are connected correctly, It sounds awesome with whatever source is played but its just the hum you can hear between tracks.