Help needed for Noise and hum on my S.E.X. Kit

[LouisQ]

I am experiencing a fairly loud noise/hum while increasing the volume knob audible/starting at around 10 o’clock and escalating much louder at around 2 o’clock.

-The hum is persistent, but curiously increases while my hand approaches the input section on the back side of the amp, and disappears when I ground my other hand to the table.

-I also had an issue while/after changing the lamps. The sound drastically decreased drastically and sounded seriously out of phase and very ghostly.

The kit seems well built, and up to the manual's spec. Please keep in mind that I am a hobbyist with very limited experience....

Thank you in advance for any help and advice.

[Paul Birkeland]

Can you post some photos of the build?  The way you describe the issue, it could just be something simple like hardware that needs to be tightened.  If there's nothing plugged into the RCA jacks, you will get some noise as you turn up the volume pot because of the open input.

[LouisQ]

Thanks for your response Paul.

- There is an iPhone and a dac (dragonfly) plugged into the inputs. The noise is generated during playback.

- I will post you the pictures tomorrow after work.

Best,

[Doc B.]

The kit seems well built,

Is this your build, or a built kit that you bought?

The description of the hum changing depending on whether or not you touch it sounds to me like a bad ground connection. Could be in the amp, could be a cable issue.

[Paul Birkeland]

The noise is generated during playback.

So there is no noise otherwise?

[LouisQ]

Hello Paul, Hello Doc,

This a kit that I bought and built last spring.

The issue started at the end of summer while attempting to swap the tubes (to GE 6FJ7).

By reworking the tube sockets I manage to relatively attenuate the first hum (10 o'clock) but the second (2 o'clock) is still very well persistant.

Maybe I damaged my volume pot (custom ALPS)?

Here are the pictures :

[Doc B.]

It could very well be an issue with the pot. Do you have the original pot to try in place of the Alps?

[LouisQ]

Hi Doc,

Yes I do, I will change it over the weekend and keep you updated.

Thanks so much!

[Paul Birkeland]

If terminal 36 is allowed to touch the power transformer nut, that can cause some hum.

The pot you have must be mounted to a PC board to work properly and reliably.  Just soldering wires to the pins will cause problems. 

[LouisQ]

Hi Paul,

Indeed, I just changed the pot and I can hear a significant difference

But there is still a slight hum at around 2 o'clock, I am working on it...

I will keep you informed.

[LouisQ]

Hi Paul,

Could the it be that I rigged the output of the amp at 8ohms (for speakers) be part of the noise problem?

Thanks,

[Paul Joppa]

The difference in noise level is only 3dB, barely audible. So no, the impedance setting is not the issue.