New mk3 clone but Low humming (22mv) advice.

I just finished one of the mk3 clone and let me say the sound is exactly like I wanted. The VTA-octal is a must have.

Now I have unresolved grounding issues...

Some information on my build:
1. I didn't put the 0.002uf cap with the center tap of the heaters.
2. I grounded the output tubes to the capacitor bank
3. the case is wood and there is no earth attached from the power outlet to the capacitor bank ground.
4. there is a separate transformer for the bias on the VTA octal, one side of the secondary is connected to the bias and the other to the capacitor bank ground.

What I will do next:
The next step was to disconnect the ground from the speaker binding post and attach it directly to the capacitor bank (-) instead of the VTA board.

Any positive input is appreciated from people who knows tricks and amplifiers. Thanks!

add the filament winding capacitor.
make sure ALL grounds (PCB and speaker output and power supply caps are all connected together (ground)
try connecting ground to AC ground (3rd wire on power cord)

ok ill do that

1. I put the 0.002uf cap with the center tap of the heaters. = no change
3. I attached the earth to capacitor ground = more high frequency hum, no change

Hi Bob, thanks for taking the time to help. It means so much to me.

The capacitor(2x parallel 0.01 100v) is 0.02 actually.

The input is to the driver VTA board only, and the power supply ground is going with a twisted wire to the VTA with the power wire.

All voltages are within 1 % accuracy of the requirements...

Following-up I tried to isolate the issue by removing tubes...

the power tubes have 0 hum , the tube driving the power tubes has 0 hum. (tubes are 100 % new and functionning properly)

All the interference or ground loops happens only when all stages are powered up.

Something is happening at the input tube.

I moved the power supply wire for the VTA down and the input wire left. The hum decreased form 22 mv to 8 mv !!!!
Without feedback it is 13.5 mv, so feedback reduces by 5.5 mv the noise.

I will try to move even more the wires away and rewire the input wire that seems the most sentitive to catch interference from the VTA.

Moving the heater wires didn't change anything, everything else is dead quiet ( second tube and power tubes).

How much of this would be happening with a steel chassis instead of something nonferrous?

I'd suggest self-adhesive copper foil to line the interior and give a ground plane. I checked the prices at the one place I saw that sells 1.4 mil adhesive sheets in 18" widths by 5' length for $39.00 plus shipping. A pricy experiment for sure, but it could be the answer to this problem. Check out nimrodhall dot com.

Tony

GP49 wrote:How much of this would be happening with a steel chassis instead of something nonferrous?

+1.. I agree with GP49!

Sal

Sal,

I also agree with GP49 but figure instead of tearing everything out and replacing the chassis, this is a possibility to still use the wooden chassis and to have a ground plane to reduce the hum. That is what DIY is all about. You try something different. If it works the first time, great . If not , you try some tweaks until it works to your satisfaction .

Just my 2¢.

Tony

I moved the power supply to VTA even further and hum is now 5mv. Now it is acceptable.

To continue improving it I will use microphone shielded wire.

Just the shielding and connecting the ungrouded volume pot lowered volts by .0007. I will try grounding the shields of the wires and also the volume pot case.