BFDR "paper rustling"

Discussion in 'Amp Tech Center' started by schmee, Jun 18, 2019.

  1. schmee

    schmee Poster Extraordinaire

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    Well, I'm about a year into trying to fix my old DR. It started with occasional "popping/crackling" but more the Paper Rustling sound. I don't get back to it often.
    There was another thread on it maybe a year ago which I cant find right now. Here's the latest:
    -I was working on another project so I left the DR on for at least 3 hours. Every once in a while it would start making noise but not much at all and not for long. When it started I noticed that If I turn the normal channel volume off it quit. But occasionally the reverb channel would do it to a lesser extent.
    -Last time I tried it was at rehearsal and it just got to crackling and hissing too much to use. Maybe sitting just "on" without playing isn't a good test?
    -I noticed that both V1 and V2 are slightly microphonic when tapped with a finger. Even when the amp is quiet. It's NOT the tubes I have changed those out heavily in the past.
    -Voltages are all within reason and were posted in my old thread.
    -I'm not going into all the things that have been done but plate resistors, Elytics etc are all fairly new. Solder has all been reflowed and flowed again. many resistors have been changed out on a whim.

    So at this point I have two possibilities:
    -What can make V1 and V2 microphonic? I'm thinking the old Blue Mallory tone caps may have a bad one or two. ? I hate to start changing those out one at a time, as testing is inconclusive at best. I guess I could replace them all and see, then start filtering them back in.. ugh!
    -Someone in the past mentioned measuring for DC on the volume pots. I'm not sure how this is done? The pot case is grounded already, so what would I connect to measure this?
    -The other thing is some of the pots in this old amp have like zero resistance to turning, very loose. Could a pot be the culprit? I noticed the other day when it was "rustling" that when I turned the Bass pot back and forth fast, it stopped making noise. But these things are all often related, microphonic pre tubes, pots effected by that... etc.

    Any thoughts?
     
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  2. ArcticWhite

    ArcticWhite Tele-Meister

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    Start by swapping in known good tubes. But if you have old blue Mallory capacitors, I think that's your problem.
    Crackling paper rustling sounds like bad capacitors to me.
     
  3. 8barlouie

    8barlouie Friend of Leo's Gold Supporter

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    My Deluxe made the paper rustling sound and it turned out to be a noisy preamp tube. Just get one and try it out one at a time in the preamp section. They aren’t expensive.
     
  4. schmee

    schmee Poster Extraordinaire

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    I have about 75 12AX7's. Been there done that with replacing the tubes. I mostly use new Mullard RI's now days. I'm gonna have to try the blue cap replacement I guess, I hate to do it...
     
  5. getbent

    getbent Telefied Ad Free Member

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    before you do that... you might just check cleanliness of pins and retension if needed... that can fix it sometimes.
     
  6. Wally

    Wally Telefied Ad Free Member

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    First things first.... since you did not mention it, how old are the electrolytics?
    Those must be fresh or all bets are off for me.
    If those are fresh; then one might suspect tubes, sockets, solder joints, dirty pots/jacks/bright switches, power supply resistors, tone caps....especially the treble caps. If it comes to pulling tone caps, I would start with the treble cap. Those blue caps need to stay where they are unless they are known to be bad, imho.
     
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  7. getbent

    getbent Telefied Ad Free Member

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    "-I'm not going into all the things that have been done but plate resistors, Elytics etc are all fairly new. Solder has all been reflowed and flowed again. many resistors have been changed out on a whim."
     
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  8. schmee

    schmee Poster Extraordinaire

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    In fact, there isn't much in the old DR that's original other than the blue caps, sockets and pots (as far as small stuff goes) The filter caps may be 10 years and are the Blue Ajax. But this doesn't sound like a filter cap issue to me.
    I think I will try the treble caps as Wally suggests, and just do a temporary solder at each tone cap station one at a time. Then listen for noise change. maybe I'll get lucky.
    Fortunately I haven't been using the amp as I prefer my BFD or modded Princeton Rev.
     
  9. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Pot cases don't have to be (and I would argue they should not be) part of the circuit. Instead, you could think of them as little bulbous outgrowths of the chassis. They have continuity to the chassis unless you go out of your way to isolate them, but the chassis need not be part of the circuit and so the pot cases need not either. Held at ground potential by the safety ground, the entire chassis-pot-cases construct is like one big shield.

    To measure VDC on pots, like when a capacitor fails at its job of blocking DC, measure voltage from some known spot (like ground) to the insides of the pot; the easiest way is to probe the pot terminals. Or measure from one outside terminal to the other outside terminal. Scratchiness is a result of (the wiper moving across) the difference.

    That's how I think of it anyway. Looking forward to seeing what you discover. Good luck!
     
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  10. Southboundsuarez

    Southboundsuarez TDPRI Member

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    Unfortunately with these aging amps it can be an accumulative type of an effect of many things.
    Shotgunning in a bunch of parts is not the solution. Systematic parts substitution is not troubleshooting and can further create problems that previously were not present before.
    Rob Ronnette has published an excellent tube amp troubleshooting resource that you should read about divide and conquer.
    Fact is that fizz, snap, crackles and pops come from a multitude of sources within these old classics.
    Old Carbon comp resistors are often a source of fizzling types of noise. Dirty pins and loose dirty contacts on the tubes sockets will produce sudden snaps and scratchy sounds especially as the speaker causes vibration. These poor tube socket connections can cause a tube behave microphonic. All tubes will display some microphonic properties especially as the gain goes up. The tubes themselves will sometimes make fizzy noises or exhibit sudden increases of white noise for no apparent reason. I dont care how new or well a given tube is understood to be, it does not hurt to swap with another tube and make an observation. Many times simply removing and reinstalling a tube may temporarily resolve the issues, as it is usually a poor contact.
    The fiber board or card that the the main circuits are built upon are known to be a problem for snap crackle and popping. They orig. impregnated with a waxy oily like coating to make them somewhat impervious to moisture. over time this coating may be washed away by cleaning solvents, heat or just old age. The boards can be susceptible to moisture and humidity as they can absorb moisture and also collect dust to the underside. They expand contract, can cause stray capacitance and create a multitude of gremlins.
    The mechanical condition of your pots does not sound too good by your indications. perhaps the tracks have been damaged by the presence of DC or perhaps excessive and improper type of cleaning solvents. Once again subtitution is not the way to troubleshoot. Check for DC on the pots and also test the pots by viewing the signal through the pot on a scope while working the pot back and forth or by reading the resistance while turning the pot. Check for DC on the grids of your preamp tubes this would indicate a leaky cap. I doubt that it is a leaky cap causing the type of noise as you have described, such failures would be of greater magnitude. Those blue mallorys seldom problematic, besides some will argue that they and their associated shortcommings are a large contributor to that vintage mojo.
    Check out Rob Ronettes website as he does a much more superb way of describing these things than I can ever wish to explain myself!
     
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  11. schmee

    schmee Poster Extraordinaire

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    ? Wont there always be voltage from ground to the inside of a pot? Unless it's not in a circuit?
     
  12. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Well, let's start with a simple circuit. A volume pot after a gain stage. The AC voltage amplification happens at the plate load resistor which will have some DC voltage on it and direct current through it. The whole point of a tube is to develop and control the voltage across that resistor. Next comes a coupling capacitor that blocks DC and passes AC, so the signal may pass through. That feeds one leg of the volume pot. The opposite leg of the volume pot is held at AC ground potential/signal ground and for simplicity's sake that is often also DC ground -- like attached to a ground buss that is connected to the chassis. The middle leg/wiper is our signal send to the next stage, after however much signal has been attenuated by the volume pot acting as a voltage divider, what is left ends up there.

    Inside the pot is a resistive element, and one leg is at ground potential (at least in our simple example). So if any current were flowing through the resistive track, then there must be a voltage from the grounded leg to the ungrounded leg. One important job of the coupling cap is to isolate this part of the circuit from direct current. So zero direct current should be flowing through any part of the pot's resistive track. DC voltages at all three legs of the pot should be zero, because the potentials are the same everywhere.

    AC current is flowing whenever the signal the pot is passing is nonzero, but 1) the AC current in the preamp is tiny and 2) AC voltages don't seem to make pots scratchy the way DC does.
     
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  13. schmee

    schmee Poster Extraordinaire

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    Well, maybe some progress today. I have to say, at this point the amp isn't making a lot of noise, just a little "paper rustling" and no loud pops or crackles. It had been on for hours yesterday with little noise also. If it had been only "paper rustling" and not too loud, I probably wouldn't have worried in the first place , starting a year ago.

    First pot DCV check: all the pots have some DCV on them. Most of them turn with little resistance. You'd have to take some out of the circuit to get a good reading. (I was getting wacky numbers, measuring resistance) Not sure if this DCV is typical or not:
    (DCV, amp active) Normal channel: Vol=.028, T=.060, B=.011, Reverb Channel: V=.024, T=.047, B=.009, R=.099, S=.062, I=.064 I decided to ignore this pot thing for now.

    Chopsticking inside the amp:
    -the Normal Channel end, the end most .1 UF cap, is pretty microphonic (only when tapped) I replaced it temporarily with a Mallory 150 and there was no improvement, still microphonic.
    -The wire off of V1, pin 7 is very sensitive/noisy when tapped. (Incidentally the Reverb channel wire off of Pin 7, V2 is dead quiet.)
    -I messed with a few things including cleaning and tightening the V1 tube socket, and tried other known good tube (again!) Resoldered the wire off of pin 7 , and reflowed the solder on V1 socket tabs. I thought about just replacing V1 socket but I'm out of new 9 pin sockets. No change.

    Possible fix?:
    -Just before I quit today I tried replacing the wire from pin 7 to the volume pot... on a whim. Fitted in there loosely it was still a bit noisy tapping it, but it was better. So I properly replaced that wire with the Fender wire routing. It seems to have resulted in the microphonic issue GONE.

    Whether I'm side tracked on the microphonic end of the amp, and whether that has nothing to do with Pop or Crackle... I don't know. I'll just have to try the amp for 3 hours or so at a rehearsal I think. Last time it wasn't too bad then after 2.5 hours of playing it start making loud pops and crackles.
     
  14. Wally

    Wally Telefied Ad Free Member

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    Apologies for missing this.
     
  15. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Cranking and playing through a tube amp for 2.5 hrs = things can get hot. (You probably crank the mofo, as one should)

    Frying on-the-fence resistor maybe? Or possibly a fragile solder joint?
     
  16. Dacious

    Dacious Poster Extraordinaire

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    Perhaps the pins of the noval sockets are loose or dirty? You may have cleaned up the contact on that wire. You can reclinch them carefully with jewellers screwdrivers.
     
  17. Wally

    Wally Telefied Ad Free Member

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    As more amps pass through my bench, I find more ways that symptoms can be caused. I have a 1964 Dual Showman that I have rebuilt and installed in a Pro Sonic 2 x 10 cab, hitch is a perfect fit. I noticed a bit of crackling that occurred randomly. I touched a couple of power tubes and caused the crackled. I thought that was odd because retensioning and cleaning of those sockets was done on the bench as part of the regimen. I had to pull the chassis for new tubes, so I cleaned and retensioned the sockets again just to make sure. I installed the new power tubes and set the bias.....and checked the crackle.....still there. So, a chopstick came to use...and the eyes. While poking and prodding around on the power tube sockets, I thought I saw a strange movement of a wire connecting to pin 6.....sure enough, that wire was sitting there making and breaking contact. It appears that this poor contact has been this way perhaps since the amp was on the bench in the factory. The pin soldering looked to be original. The end of the wire was there...it did no break. At any rate, I resolved that connection and all was good.
    Factory errors???? They occur. In this same amp, I found two bypass resistors that had been soldered in in the wrong positions. I pulled the electrolytic bypass caps for replacement and noticed that the 820 ohm resistor and the 2.7ohm resistors in those adjacent bypass circuits had been incorrectly installed. That would change the gain in those two triodes....and not for the better.
     
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