Sounds like someone dragging a cloth over a mic

[rhodco]

My Hoffman Princeton Reverb build sounds great, and idles dead quiet most of the time... but sometimes it makes this strange, soft, rustling sound like someone slowly pulling a cloth across a microphone.

Since it is intermittent, it has been very difficult to find. Especially since I can't reproduce it when I have it on the work bench. This much I know so far –

1). It happens even with nothing plugged into the input.
2). Volume and tone controls have no effect on it.
3). Reverb and tremolo controls also do not affect it.
4). I replaced all of the preamp tubes (12AX7 and 12AT7) and it still happened.
5). I replaced both 6V6 power tubes and re-biased and it still happened.
6). It usually happens after the amp has been warmed up for a while.

Short of calling Ghostbusters, can anyone please make a suggestion?

Thanks,
Cliff

[shooter]

does it stop on it's own?
does it "oscillate", (build up-fade, build up-fail)
got a scope?

[jjasilli]

Always suspect tubes first.  Then: 


Sounds similar to "frying bacon" noise which indicated bad plate resistor(s).  Maybe you overheated and burnt a resistor???


I would also check for DC on pots, and preamp tube input pins. 

[dunner84]

That is a good description of a sound I was hearing on one of my 5e3s... It had me troubleshooting for months, until I noticed it only happens when I play with one of my hands.... It turned out to be another guitarists cell phone. It didn't have the typical cellphone induced noise that mine will sometime cause. It was somewhere between frying bacon, and cloth being dragged over a microphone.

[davidwpack]

I had an intermittent sound like that. I found that I missed soldering the buss wire to a couple lugs in the tone stack.

[rhodco]

Thank you guys for all the good ideas. To answer some of the questions:

It does not oscillate or escalate. It just fades in and out randomly. If I turn the amp off and back on, it usually goes away for a while.

The sound is not like a sizzle or frying sound. It's softer, really just like someone holding a mic against their clothing and moving it around slowly.

I have inspected every solder connection and replaced every tube except the 5U4G rectifier, could that cause this?

I do have a scope, but when I pull the chassis and put it on my work bench it works fine so far.

So, if I put it on the work bench again and get my scope ready... assuming it will happen if I wait long enough... I should be able to trace it back step by step until I find a spot where it exists on one side of a component, but not on the other side. Does that necessarily mean that component is the culprit? Or is there more to it than that?

[jjasilli]

Yes, if the noise is on both ends of a component, then it must have arisen earlier in the signal path.  (Note also that the B+ rail can put noise in the signal path.)


As dunner88 points-out, the noise might be induced form an outside source, and not be component related. 

[rhodco]

If the noise is being induced from an outside source like cellphone or radio, should I be able to stop it by covering the amp with aluminum foil for a moment? Or would the foil have to be grounded first?

[shooter]

on my work bench it works fine so far.

that's a start, but takes awhile to prove it

covering the amp with aluminum

better is move the amp, outside, in the bathtub.......
but I have wrapped my builds in "chicken wire", grounded at one end, used a steel plate, foil, you can be creative here 

does it have a bottom shield in the chassis, plate, or foil tape?

I do have a scope

start at the speaker, since it does it without anything plugged in, you can also make a cheat cord, for in and clip scope to it as an "input", but probably will inject more junk than its worth

[rhodco]

The amp cabinet does have a foil shield across the top inside that makes contact with the chassis so shielding is normal.

I hate the intermittent problems most of all. It's like hunting for ghosts. I would rather have a constant buzz that I can track down on the bench with a scope.

Thanks for all the suggestions.

[sluckey]

You could also try this to narrow down the source of the noise... Starting at the input and working your way toward the power amp... Pull one tube at a time and listen for the noise. If it is still present, leave that tube out and pull the next tube, again listening for the noise. Continue pulling tubes and listening until you finally kill the noise. If you're lucky you have just pinned down the noise to that tube circuit, or something feeding it.

[VMS]

I had one OT that made sometimes a sound like wind was blowing to a microphone. Sluckey has a good advice on pulling the tubes one at a time.

[rhodco]

Update: Problem resolved! I finally got the idea to take a closer look at the shielding in the cabinet. It was comprised of 3 aluminum strips running across the inside of the top. When checking continuity with my multimeter I discovered that each of the two outside strips had connection to ground through the mounting hardware, but the center strip was completely isolated. No ground. Even though all three strips appeared to be touching, there was no electrical contact with the one in the middle. After some seam soldering with a hot iron I had a good ground connection across all 3 strips and put the amp back together. That was about a month ago and no problems since.

Also, just BTW, I later decided to swap the Tung Sol 12AX7 in position V3 for a Ruby 12AX7AC5. Wow! what a big difference. Higher gain for a more ballsy punch, but still nice and clean. Very happy with my Hoffman-Princeton amp now! Thanks for all the help you guys.
 

[sluckey]

How did you solder those aluminum strips?

[tubenit]

THANK you for posting the resolution to the problem!  I sure wished more people did that. 

With respect, Tubenit

[punkykatt]

How did you solder those aluminum strips?

[st]

How did you solder those aluminum strips?

[Ed_Chambley]

How did you solder those aluminum strips?

I have used tin/silver and aluminum soldering paste. If you prep very well it can be done with propane or torches. TIG is the preferred method for sheet aluminum. If he did not remove them I have no idea. The wood in the cab will absorb all the heat.


The problem is you cannot see it get hot, it gets whiter and stays whiter until so it will melt away. Test the heated surface with a wooden handle and when it begins to burn on contact the aluminum is ready to solder. This is also contingent on working fast. Got about 8 minutes before contamination from flux breakdown due to heat. And then there I'd 5000 grade aluminum which I wouldn't attempt since it has a higher level of magneisum. Also a solder 91 zinc 9 tin is easy and of course the proper flux.


And stay under 600 degrees f or 316 c or the heat will destroy the flux.