Ground loop/defective filter cap ? Noise anyway.

[archaos]

Hi all,

          I have a problem with one of my AC30s :
            A noise rather weak, but a noise though, that I would describe as rattling (drrrrrrrrrrrr) like a ground loop occurs often, not always. Its intensity can be very low, or, less low...& that bugs me a lot. I have checked a lot of things : continuity, bas solder joints, star grounding... : nothing is suspect.
Nevertheless, when pulling the phase inverter tube, it becomes dead quiet. So I swapped that 12AX7 with another one, & it's noisy again. Thus I was wondering if it could be due to the filter cap of the stage ?
Any idea/suggestions ? Thanks.
 

[archaos]

The cap used is a multi-section 8/8µF, 450V, F & T. The ground is common with the 1st preamp stage.

[jojokeo]

There's many opinions on proper grounding and the use of multi section can caps and their associated potential problems that they can cause. They can't be isolated like using seperate caps therefore grounding issues (and other things) can arrise. The pi is considered part of the output stage therefore shouldn't be connected w/ the preamp section - especially the first stage. I say this as in "proper amp building & design" the way it's taught in many books. Not as an absolute that it just cannot be done under any circumstance. It's also suggested that the filter cap should be fitted as close as physically possible to it's associated stages or tube socket(s). It doesn't mean you can't have them elsewhere either but that it's just good design and practice to do so. Ground loops, oscillations, wierd buzzes, parasitics, etc. are attributed to these kinds of things as well as lead dress too.
Are you sure that it's an "electrical" noise you're hearing and not a physical noise rattle of a tube, speaker, wire, socket, loose part, or cabinet?

[archaos]

There's many opinions on proper grounding and the use of multi section can caps and their associated potential problems that they can cause. They can't be isolated like using seperate caps therefore grounding issues (and other things) can arrise. The pi is considered part of the output stage therefore shouldn't be connected w/ the preamp section - especially the first stage. I say this as in "proper amp building & design" the way it's taught in many books.

Thanx for the tips jojo. Actually, I initially wanted to build a AC30 à la JMI, but I've discovered after it was finished that multi-section caps were not recommended at all due to all kind of problems they can't cause.

Are you sure that it's an "electrical" noise you're hearing and not a physical noise rattle of a tube, speaker, wire, socket, loose part, or cabinet?

Yes, I am 99.99 % sure of that.  Rattling tubes, speakers, or others mechanical rattles don't use to emit that kind of noise. To my ears it sounds like nothing but "electrical" noise, & therefore I will try with 2 separate caps to check if I can definitely kill that noise.

[rafe]

Have you tried it in a different location? My computer monitor reeks havoc on some of my amps. Fluorescent lamps also can affect them. If you haven't tried it elsewhere, I'd do that first.....

[jojokeo]

I was thinking of RFI or = too. But, w/out hearing exactly for ourselves we have to go by descriptions which can be subjective making it even more difficult. You would think that if it was RFI, the noise would dissappear if v1 were removed not the pi tube? The description being a "rattling" type noise is why my prior question of it being mechanical because usually electrical types are described w/ buzzes or hisses.

[archaos]

Have you tried it in a different location? My computer monitor reeks havoc on some of my amps. Fluorescent lamps also can affect them. If you haven't tried it elsewhere, I'd do that first.....

My PC is off when I listen to my amp, & I don't have fluo lamps @ home. 

I was thinking of RFI or = too. But, w/out hearing exactly for ourselves we have to go by descriptions which can be subjective making it even more difficult. You would think that if it was RFI, the noise would dissappear if v1 were removed not the pi tube?

I don't think it could be due to RFI/EMI ; no noise only when pulling the pi tube, it remains when another tube is pulled.

The description being a "rattling" type noise is why my prior question of it being mechanical because usually electrical types are described w/ buzzes or hisses.

Actually I wouldn't say it hisses, that's rather a kind of buzz, but I should record it. One record would be better than 1,000 words.

never say 'nothing is suspect', because something definately 'is' suspect.

Yes, I'm looking for the guilty.

never say never.

Totally agree. Anyway I have swapped the PI section cap, & the noise still remains ; to be continued...

[archaos]

Hey guys I'm both sorry & happy now :

  - resoldering the pi tube's filter cap/resistance has definitely eradicated the noise
  - nonetheless I won't be helpful for others as far as I didn't have time to record the noise 

Anyway thanks for all the input.