100hz noise problem

[grtamp]

Hi all , in my new HRM amp , I take a 100hz noise when I have the volume and the master to zero!
if I raise the volume or increase the master pot, the problem disappears.
I have isolated the point where it seems to create the noise that seems to be on V1b.
Initially approaching the finger to the coupling capacitor(0,01uF cap clean out) until touching it, the noise disappeared!

I then started to replace the components:

the capacitor 0,01uF.
plate resistor on V1.
replace tube
the shielded cable from 220k to Volume pot.
changed the relay
disconnect the relay od/clean board from all wires.
disconnect supply relay.


Finally , I removed the 500pF capacitor in parallel with the 220K resistor(V1b grid) and the problem is solved!

I tried to re-solder the cap and the noise is come back!

what happen When that cap is on place? capacitive interference?
Now I have to run without that 500pF cap?

[Structo]

Is the 500pf cap defective?

Try another one.

[grtamp]

no , I changed the 500Pf cap with another one , but it's the same!

I changed the ground point from PS board also , but nothing!

[VacuumVoodoo]

Sounds like a grounding error anyway. One or both pots (vol & MV) are probably grounded at wrong node.

[grtamp]

both Vol & MV are grounding to the Bus bar!

[VacuumVoodoo]

both Vol & MV are grounding to the Bus bar!

Each pot should be grounded at the same point on the bus where the triode it feeds with signal has it's cathode resistor & decoupling cap grounded. It's also preferable if it also is the point where corresponding supply filter cap is grounded.
Pot grounded at the point where preceding triode is grounded is an invitation to hum.
A ground bus has a finite resistance and it is not zero ohms.

See below where pots are grounded. Points C'-C''-C need not be local star grounds, they can just as well be on a common bus but as single ground points.

[img:800:600]http://i26.photobucket.com/albums/c149/ ... round1.gif[/img]

[glasman]

Sounds like a grounding error anyway. One or both pots (vol & MV) are probably grounded at wrong node.

I agree, have seen the same thing a few times.