Hum problem....

[Chris]

Putting back the earth wires to the Earth post I found that there was a 10R resistor on earth wire going back to star earthing on the centre tag board.
Checking the parts list there is 10R resistors only on speaker binding post and on the valve bases.

Chris.

[bob orbell]

The yellow/green from mains transformer soldered to earth post, star earth tag cL9 to R47 (10R 2Watt) soldered to earth post, R43/44 (10R 2Watt) in series with c19/20 (0.1uF 100v) across the L/S posts. But this will not solve your loud hum . Bob Bob

[Chris]

Thanks for that Bob, have asked Greg for the 6550 Power Amp files. The files I have omitted this..(6550i)

- solder the star earth,tag cL9 to the earth post, using 1/1.2 wire.

No R47 in parts list.

Chris

[pre65]

That 10R is the earth lift resistor.

[Richard]

Yes, I've checked my copy of the 6550 build sheets and they don't seem to cover the fitting of the 10R earth lift resistor. Maybe Greg has a later copy.

Anyway, think of the "earth" system in the amp as being 2 parts joined by a 10R 2W wirewound resistor (often in parallel with a 0.1uF capacitor);

1) the Mains Earth is the mains cable yellow/green which goes solidly to the amp chassis along with any earth wires on the mains TX.

2) the Signal Ground 0V is all the circuit signal ground paths including the RCA socket shells.


The Mains Earth wires are usually directly connected to the amp chassis at the earth post. The Signal Ground wires are usually grouped together at a point near the chassis earth post then connected across to the earth post with a 10R resistor.


What you should end up with is the Signal Ground 0V "floated" inside the Mains Earthed amp case by the 10R earth lift resistor.

Way to check is with your meter on low ohms setting clip one probe to the amp chassis post and use the other probe on all 0V parts of the circuit you can probe easily, looking for 10R between, say, the RCA input socket shells and the amp chassis earth post etc. Check things that should be solidly bonded to mains earth read 0V and things that should be lifted read 10R.

What can sometimes happen is the "lift" is lost by a short from, say, the RCA socket shells, they must be isolated from the chassis, or from the inputs being earthed to chassis etc.

[bob orbell]

Richard, I quoted from my paper KAT6550 sheets and the parts list states R43/44/47-10R,2W and this is from around the year 2003, but I doubt none of this is relevant to the hum issue. Bob

[Chris]

Thanks Bob for always being on the ball and Richard for a very concise and easily to follow instructions.

I have 0v on mains Earth, 10R on both + and - speaker connectors, and 10R on RCA socket earth.

I will get to the hum part Bob just putting down the "Ground" work first.

Chris.

[bob orbell]

Chris, do you mean 0 ohms, you wrote 0v ? if you are measuring with your meter at ohms you should not have 10 ohms at the speaker posts, i think it should read infinity . correct me someone if I am wrong. Bob

[Chris]

Sorry Bob, yes 0 ohms. no voltage..

Chris

[Richard]

Originally Posted by bob orbell

Richard, I quoted from my paper KAT6550 sheets and the parts list states R43/44/47-10R,2W and this is from around the year 2003, but I doubt none of this is relevant to the hum issue. Bob

Yes that would be correct but no mention of R47 in the parts section, nor the fitting text before it, nor the drawings only sheets - nor my Kit34 (sister amp) instructions. Strange, I'm unsure about Chris' problem and think it could be an earthing issue as hum on both channels is more likely a PSU or hum loop, whereas a signal problem would likely be one channel, but let's hope Chris finds it and puts us out of our misery soon