Hum problem

[mongiak]

Hello,

I´ve built a tube amp and it works great, I just have a problem with the hum and its not because the filament ac current, because i´ve tried with dc and the hum it´s the same.

I´ve notice that when I plug the low canel there is not almost any hum or noice, it just starts when I use the hi channel, as soon as the ground connection of the jack umpluges the hum startes. I´ve look at the diagram an it´s ok, Can any one help me, I´m just getting crazi with it.

Thanks a lot

Mongiak

[SY]

You probably have a ground loop. Some explanation of how the grounding is done plus a few pictures will be helpful in diagnosis.

[poobah]

The hum is coming from the source... your guitar. Why all the difference between pickups

[mongiak]

I just conected each tube ground in star and them all toogether with only one ground point, that´s what I was told to do. So any one knows how can I finish with this hum or any advice??

Thanks a lot

[SY]

Where is the input socket grounded? Is it isolated from the chassis and returned to the star? Is the star at the input? Where is the earth safety ground? Are signal and earth safety ground connected together and if so, where?

[mongiak]

the input jack is grounded at the first tube section, it´s not chasis isolated, (I think that I´m getting radio frecuency interference). the start is not at the input, it´s close to the first filter, the earth safety ground it´s at the star point, and it´s conected by a 100k cap and a 10 k resistor.

I only notice the noice at hight volumenes, and like i said beafore it´s more noicy when I puge the hi channel.

Thanks a lot.

I´ve been triying other ground configurations that I found in the aiken amp page, and the noice it´s almost the same.

[Miles Prower]

the input jack is grounded at the first tube section, it´s not chasis isolated, (I think that I´m getting radio frecuency interference). the start is not at the input, it´s close to the first filter, the earth safety ground it´s at the star point, and it´s conected by a 100k cap and a 10 k resistor.

I only notice the noice at hight volumenes, and like i said beafore it´s more noicy when I puge the hi channel.

My current project is very much like this. I also had a hum problem that was directly caused by an RCA-type input jack that wasn't insulated from the chassis. Main PS noise was getting in that way. Even though the measured DC resistance across the steel chassis was but a couple of ohms, and the noise barely showed up on the o'scope attached to the first preamp grid, that was enough given the open loop gain to cause some 50mVp-p at the speeks.

Insulating the input jack and connecting the return to the DC grounding point of the first stage cathode fixed the problem. It doesn't take very much, given any sort of half-way decent speeks, and sufficient gain, for the PS noise generated across a few ohms to become problematic.

[mongiak]

I´ll try that,

first I have to get the isolated jack, because this days here are the shops are closed but as soon as i can i´ll isolte the input, I hope that this works.


Thank you

Mongiak