Matching the KLPP1 pre amp to K5881 MK2

[barnesdavid20]

Hi Richard, Changing the cathode returns, didn't make any difference so you are right there is no noise getting across between the wires even though they are running close together. The connection to chassis by the switch in the centre rear, yellow and green wires is the mains earth and mains TX screening, direct to chassis ( safety ).
I have all cathode returns running around the outside of the chassis under the lip, front and right side up to the input sockets where they are all earthed at the socket, also earth return from the PS and the two earths from the smoothing Caps C3/C7 via one lead to the star at the input socket.I did have the centre tap of the Heater winding going to earth but have now attached it to one of the power valve cathode resistors to elevate the heaters. screened input signal wires also run around the outside of chassis under lip and are earthed at one end only, at the star/input socket. The only one that i have changed is the earth return from the secondary of the output TX's and that is now through the screening directly onto the cathode earth of the first valve,(did read that this was a good place to connect it)
I have connected the elevated earth at the input sockets to chassis via a resistor/cap close to the star point near the signal input sockets.
Does all this seem ok?
With respect to the feedback, I only had two large 8ohm resistors, the big oblong white ceramic looking ones, but I did try with them making 19 ohms, as you said, mv across the speakers went down to 0.6mv and I had to turn the volume up a bit more, but I think that voices sounded more raspy, so better as it was. the funny thing is even with just 0.6mv you could still here a little hum on the right.

[Richard]

Originally Posted by barnesdavid20

I did have the centre tap of the Heater winding going to earth but have now attached it to one of the power valve cathode resistors to elevate the heaters.

I would try this back on signal ground point.

Originally Posted by barnesdavid20

screened input signal wires also run around the outside of chassis under lip and are earthed at one end only, at the star/input socket.

Do you mean the screen is earthed at one end only and that there is a signal and a signal ground wire attached at both ends? (just checking it's not coax with one end of the braid not attached)

Originally Posted by barnesdavid20

The only one that i have changed is the earth return from the secondary of the output TX's and that is now through the screening directly onto the cathode earth of the first valve,(did read that this was a good place to connect it)

Not sure if I follow this but if you mean the black speaker terminals each should connect directly to it's OPTX then they should be very solidly connected directly to signal ground not through any other ground route. If they are connected through the grounds of the first valve it may well hum.

[barnesdavid20]

Oh!! I may have made a mistake then, I am using screened phono cable from the input to the first valve and the screen is only connected at the input socket/star point, is this wrong then?, thought I read that you would create a hum loop if you had it connected at both ends.
Also I thought that the earthing of the black speaker terminal was only for the return of the negative feedback and as I was using screened wire, thought I would just use the screen for the return, and I thought that I read that you were to connect it direct to the feedback point on the first valve, ie where the 11ohm resistor connects to earth.
If I change the centre connection of the heater winding, do I have to have a separate lead all the way to the star point or can i connect it directly onto the earth rail of the power supply?
Cheers
DAvid

[barnesdavid20]

HI, JUst been reading about earthing of the output TX, I see what you mean, there is a safety issue incase of a short inside the TX, high voltage would be seen on the speaker terminals!!!. What it says is you should ground the negative lead to mains chassis with a nice thick wire, and if you have negative feedback you can take a smaller wire back to where the feedback is used ie the cathode earth of the first valve, must have been late when I read it last night.
Am I understanding it correctly now?
David

[Richard]

Originally Posted by barnesdavid20

Oh!! I may have made a mistake then, I am using screened phono cable from the input to the first valve and the screen is only connected at the input socket/star point, is this wrong then?, thought I read that you would create a hum loop if you had it connected at both ends.

When you say screened phono cable is it single core with an outer screening braid or two core with a screen? You need signal connecting at phono socket and to first valve grid resistor. You need a ground connection from phono socket to first valve ground point or have you made it directly to ground another route and are you just using the braid as shielding? What does your meter say? With no external connections to the amp and a probe on the socket outer shell and the other on chassis mains earth do you get, short circuit, open circuit, or 10R?

Originally Posted by barnesdavid20

Also I thought that the earthing of the black speaker terminal was only for the return of the negative feedback and as I was using screened wire, thought I would just use the screen for the return, and I thought that I read that you were to connect it direct to the feedback point on the first valve, ie where the 11ohm resistor connects to earth.

I would connect the black speaker posts directly to the main signal ground point as otherwise anything picked up on the speaker cables or voice coils etc will ground through the first valve earth path. The feedback signal wire is then taken from the red speaker post to the first valve cathode using single core screened coax. Wire the signal core to the red post and to the cathode. Isolate the braid at the red post end and connect only the braid at the first valve cathode ground point ie the grounded end of the 1000uF cap which will then just provide screening for the feedback signal.

Originally Posted by barnesdavid20

If I change the centre connection of the heater winding, do I have to have a separate lead all the way to the star point or can i connect it directly onto the earth rail of the power supply?
Cheers
DAvid

The cleanest way is its own wire back to ground.

[Richard]

Originally Posted by barnesdavid20

HI, JUst been reading about earthing of the output TX, I see what you mean, there is a safety issue incase of a short inside the TX, high voltage would be seen on the speaker terminals!!!. What it says is you should ground the negative lead to mains chassis with a nice thick wire, and if you have negative feedback you can take a smaller wire back to where the feedback is used ie the cathode earth of the first valve, must have been late when I read it last night.
Am I understanding it correctly now?
David

You can't do that with signal ground lifted by 10R or you'll lose the lift (the black post will be connected to chassis and signal grounds). Ground the black posts directly to signal ground star point/psu caps neg. You don't need a separate ground back to the first valve as it will already be grounded then. Then just take the signal wire from red post to first valve cathode, screened as described in the last post.

[barnesdavid20]

First point, I get 10ohms. Only using a coax type lead with a single conductor, just using the outer braid as a screen connected at one end only, the return for the signal would be through the first valve cathode earth, is this wrong?

[Richard]

Originally Posted by barnesdavid20

First point, I get 10ohms. Only using a coax type lead with a single conductor, just using the outer braid as a screen connected at one end only, the return for the signal would be through the first valve cathode earth, is this wrong?

10R is good, are the phono socket grounds connected to the first valve with a separate ground wire?

[barnesdavid20]

No, I think that may be the problem, it can only get back via the first valve cathode earth return wire, which I have not separated from the signal return.
I have got some twin balanced screen lead should i use that so that the signal return is separated from the cathode return.

[pre65]

Valve earth ?

I thought you were talking about a star earth configuration.