Deluxe Reverb 'Volume Pot Noise'......Gremlin

[Pastortom]

I have a '71 Fender Deluxe Reverb that's been refit with a nice mallory cap job....smooth and clean.....all systems go.....I put very few hours on it since the job, but no issues until the other day when I switched it on............I heard what sounded like my volume pot was incredibly dirty all of a sudden.  Made no sense since it was smooth and quiet before this.   It persisted, and after letting the amp idle a little, I started hearing some backgound noise too (faint, a bit of crackle one in a while and such).


SOoooooo, I cleaned the pot & dried it out....Same noise.   I put in a brand new pot.  Same noise.  I replaced the original cap at the pot (47 ceramic).  Same noise.    I retouched every pre-amp tube socket connector, and cleaned the contacts.  Same noise.    I then checked the normal channel.....SAME NOISE there too, in THAT volume pot.      If I turn it up or down, I get noise....if I STOP turning it, the noise stops, BUT there's some residual background noise that shouldn't be there.  All tubes checked out fine.


I'm at a loss guys.........Anyone have an idea?   

[eleventeen]

Most often, noise when turning a pot (if the pot is/has been cleaned) indicates the presence of DC which is produced by leaky caps in the tone stack. Does the noise occur identically on both channels? With or without an instrument plugged in? Realize that with nothing plugged in, the input preamp stage should be shorted out. This is of course assuming the input jacks are clean and the little switch deals on them are making good contact. That assumption is not always valid. How clean are the insides? (corrosion, etc;) I would turn the amp on and check for DC on the tone controls. if so, leaky caps would be my guess. With the input shorted out, if you get scratchy tone controls it's almost certainly leaky caps. You should be able to check for DC on the various terminals of the vol & tone control pots. Compare, channel to channel.


Unfortunately, the tone control stack exists between halves of the first tube for each channel. Thus, there is nothing to be gained by testing the thing out with the first tube pulled.



Background intermittent/spurious rat-noise at idle is usually an indication of noisy plate resistors. You're talking about a 45 year old amp here. Most of the time on an amp this old I just shotgun replace all the 100K plate resistors arrayed in that characteristic "V" pattern with metal film 3W ones like Doug sells. For 6 * 30 cents = $1.80 the difference in noise floor can be just jaw-dropping. I had a '65 Pro Rev that was constantly making noises of several different types. It was pretty much original. Cap job, replaced those plate resistors, it went dead quiet.


Note: When you say "cap job" I am assuming you mean the electrolytic power supply caps. Yeah, those are certainly candidates for replacement at 45 years old. But what about the .1 and the .047 in the tone stack(s)?


You won't be sorry replacing those plate resistors. It *never* makes *no* difference but the diff it can make using the amp especially after it has been on for a few hours and has really cooked can be nothing short of dramatic.

[tubeswell]

Could be a leaky cap or a DC-short somewhere on the AC side of the coupling cap (like the cap lead on the back-side of an eyelet board prodding into contact with a B+ wire behind the board). Either way its most likely DC on the pot.

[Pastortom]

I haven't replaced the power caps, just the tone stacks........The kit was designed after blackface tones I believe.  The powers are original.  I'll replace those plate resistors on that good tip.......I've never bothered with them, but I'll do it from here on.


The noise from the pot going up and down is the same whether or not I have an instrument plugged in.  IF this might be a DC leakage problem, from WHERE, precisely (if it's a cap, such as has been suggested), would it be?  Can you be more specific please........


I'll check it over with a fine toothed comb for any other wire or connector problems where the voltage might be coming from.......

[Willabe]

The caps in red. To test for dcv leakage unsolder and lift up the end that's away from the plate of the tube right before it. (End that goes to the pots.)

Then clip your meters black lead to the chassis ground and the red lead to the caps lifted end. Make sure the hot end (lifted) CAN NOT short to anything. Set meter for DCV turn on the amp look for dcv. If more than, what(?) a volt it's leaking.

Unsolder and test 1 at a time then solder back in the cap that tests good before you test the next cap so nothing shorts by accident.

But you just changed the tone stack caps so......? It would be rare to get a bad new cap but it can happen. Or you could have "cooked' a new cap when you replaced it. (That would most likely be the 47pF treb cap.)

Or, what tubeswell said in reply #2. 


                     Brad     

[Willabe]

in THAT volume pot.      If I turn it up or down, I get noise....if I STOP turning it, the noise stops, BUT there's some residual background noise that shouldn't be there.

If the tone stack caps were leaky then the treble and/or bass pot should make the same type of noise.

So I'm thinking tubeswell is on to something.


                    Brad     

[eleventeen]

You *may* be able to test for DC leakage without disconnecting anything (certainly worth a try----same price)


Just poke the pot connections (I trust you understand I mean WITH A METER!) without disturbing anything. As Willabe said, if you get anything much over a volt or so, suspect leakage. Cut one lead of the suspect cap/part, and check further.


Please let me know if you get good results from the plate resistor change, since I am constantly raving about it!

[tubeswell]

Also be aware that Mallorys are notorious for being temperature-sensitive and prone to melting from over soldering. So its possible that a new mallory can be stuffed up from over-heating with the iron

[Ed_Chambley]

Check the center tap.  If artificial, check resistors if not check the grounds.  Had this happen 2 times on princetons.

[Pastortom]

Well, I've been on other important stuff (been restoring an old Kawai Grand Piano...I think my lungs are now brightly lacquered in antique white) and just got back to the Deluxe Reverb tonight....... After hearing the cautions about the Mallory's, I decided to just swap out ALL of them for Orange Drops..... I had a good supply of caps, so it was just a quick change.  While I was in there, I changed ALL of the electrolytics too with Spragues..... all of the 25uf's and the 5 big power caps (16's).  Rechecked my connections, and fired it up.......It obviously had more power than before, so something may have been up with the Mallory's, BUT the stinking noise is still there in both volume pots when you move them, and the treble pots too to a degree.    I had to cut it short tonight, so I'll check for DC at the pot in the morning, and try somehow to inspect the back of the eyelet board to see if somethings haywire back there.............  Thanks for all the advice thus far..................

[Greenmachine]

I would try replacing the plate resistors

[Pastortom]

I just checked the DC voltage at the normal channel volume pot, and got 3.65 volts (!)........I believe this is a wee bit out there, yes?  2 things are buggin' me .....First, the eyelet board has some exaggerated warping, so I'm going to lift it a little and check the insulator board for any problems....


and 2nd, I was going to pop in new plate resistors....one of you'se guys recommended  metal 3 watt 100K's like offered here in the store, but I have a bunch of those in the shop.......BUT, I just checked 'em and they're about 25mm long!  It's gonna be a magic act getting those in there with that size.......are the ones sold here at Hoffman's the correct overall physical size for a Fender Amp?....and are you sure the 3 watts are what's called for?

[SILVERGUN]

I just checked the DC voltage at the normal channel volume pot, and got 3.65 volts (!)........I believe this is a wee bit out there, yes?

Yes....you should not have DC voltage on that pot...
If you just changed out the preceeding coupling cap, and you still have voltage there, there is a chance that your board is conductive.

Unsolder the side of the cap that feeds the pot and the wire that feeds the pot and take a DC voltage reading from the turret that you unsoldered that pot/wire from (which should now be connected to nothing), and if you still see 3.65 VDC or any DC you will know that the board is conducting that voltage there...so you're just reading from the turret to ground, with nothing attached to the turret

First, the eyelet board has some exaggerated warping,

That's a good sign that it has absorbed moisture and may have become conductive

[John]

I just checked the DC voltage at the normal channel volume pot, and got 3.65 volts (!)........I believe this is a wee bit out there, yes?  2 things are buggin' me .....First, the eyelet board has some exaggerated warping, so I'm going to lift it a little and check the insulator board for any problems....


and 2nd, I was going to pop in new plate resistors....one of you'se guys recommended  metal 3 watt 100K's like offered here in the store, but I have a bunch of those in the shop.......BUT, I just checked 'em and they're about 25mm long!  It's gonna be a magic act getting those in there with that size.......are the ones sold here at Hoffman's the correct overall physical size for a Fender Amp?....and are you sure the 3 watts are what's called for?

On the plate resistors, you can get away with 1/2 watt, but using the larger wattage makes them quieter. I'm no Fender expert, but the 3-watters Doug sells should fit . I think they're 5/8 inch long at the most? Hope this helps!


*edit* Of course, if your board's conductive none of this matters :)

[Pastortom]

Let me toss this out before I go at the board......


Though I haven't had any problems with conductive eyelet boards (fiber) in the past, this particular amp has been in a facility for some months that has been pretty humid, probably between 58% (low) and 75% (high).....sitting idle, and mostly at a higher humidity.  I'm a little worried that trace absorbed moisture could be the culprit if the board has become conductive.  This is the only amp I have that's been stored in that particular area for any length of time...... We've had a long humid spell since last fall.


I've been told that problematic boards can be dried by baking the chassis at around 110 degrees for a couple hours (in an oven) with all components intact.......If any of you guys have any experience in this area, let me know.  I can't see where the temp. would be a problem for anything, and that super dry heat makes sense, and can dry out almost anything.......Just made sense.....But, I've never had a reason to try it.

[SILVERGUN]

I would try to confirm that is actually the problem before baking the amp.....if it's happening on more than one pot, then it's a bigger issue than one leaky path,,,,,and your course of action to fix it depends on how much you care about it being right.
If you believe that it's only a problem on those 2 pots, then I guess you 'could' just put a coupling cap between the board and the pot   

I've been told that problematic boards can be dried by baking the chassis at around 110 degrees for a couple hours (in an oven) with all components intact.......If any of you guys have any experience in this area, let me know.  I can't see where the temp. would be a problem for anything, and that super dry heat makes sense, and can dry out almost anything.......Just made sense.....But, I've never had a reason to try it.

I have heard that too, but have no experience trying it....
I was hoping you would get more advice from guys who have, and if you really want more opinions it might be a good idea to start a new thread called
"Fixing my conductive board?" or something like that so you will draw more attention about that specific question.


In my world, that board is scrap....I could never live with the idea that there are stray voltages floating around...
Even if I baked it out and it worked, it would always be in the back of my head that it could show up again.
Making boards, and fixing amps is what we enjoy doing,,,so for most guys here this is an opportunity and not a big problem.

[Pastortom]

Today I'm gonna do a recommended disconnect on that pot and see what, if any, voltage is present.........We shall see. 

[Pastortom]

Well, the humidity down here in Central FL just ain't quittin', so I brought the chassis in after pondering whether or not the eyelet board has become conductive.   I was gonna desolder and check the turret, but decided to try a "drying" of the board first and check the voltages since it was already in the house.


My oven wouldn't go down to 110, so I had to put a thermometer in there and turn the warm cycle on and off for short bursts of 30 seconds or so to get it up to temp......checked it every so often to keep it at appx. that temp, and let it ride in the oven for about an hour and a half.   My DC voltage across the normal channel volume pot BEFORE the bake measured 5.64 volts, and that was consistent with every test.   AFTER the bake, it just measured .54 at the pot.    Had I left it in for what I've heard is a "normal" time period (2 hours or better), it would probably be taken care of.


I reset the tubes and fired it up to check the noise at the pot...... I could still hear it when the pot moved either up or down......below 5, it's not anything I would ever have been alarmed about....but as you go from 5 up, the white noise is noticeable, but when you stop moving, the noise stops also.   I plugged in a Strat, checked the adjustments, and it sounds great (with the new cap job.....remember, I removed the Mallory's and put in O-Drops).....at this point, the only thing remaining that we've discussed is changing out the plate resistors, which I'll do as soon as the new order arrives.


SO, baking a fiber eyelet board (complete chassis, all wiring and circuits intact, tubes removed) at around 110 degrees for some time (I would say 2 hour minimum if the board has become conductive) definitely DOES dry out the board and reduce conductivity.   In my case, I believe it's purely the humidity over time that caused the problem.   Makes common sense, since I can do the same thing with a thinly sliced London Broil to make jerky......although the jerky would taste much better than the fiber board, in my humble opinion.


When I get those plate resistors changed, I'll post the results.  Thanks guys, for the help.  I believe I'll put up another post specifically for the drying of the board, as previously suggested. 

[PRR]

> My oven wouldn't go down to 110

Anyway you don't want your cookies to taste like Fender.

For such low temps, find a box and put a 60W incandescent lamp in it. You can use your meat thermometer to check the temp. Switch to 45W or 100W as needed.

The incy lamp in box is an old-old-old trick. My pal from the backwoods of Tennessee says they had lamps in the closets and left them on all humid-season. My friend the piano-fixer kept his stock of piano-wire in a footlocker size box with a lamp in it.

I'd think a Fender could stand a LOT more than 110 deg F. Hotter than you can hold, around 160 F. Power off, it should stand 200 F for a day, though I think lower temp for a week would be better for driving-out deep moisture.

[shooter]

fwiw I fixed a '70s era twin for a customer, the board was warped n amp noisy like you have.  He wanted it done cheap, so I put a fiberglass insulator under, drew the fiber board flat, waved a heat gun (hairdryer on low) over the board till everyone was nice n warm, repeated 2 or 3 times n that fixed it, not sure if it lasted cuz he was getting it ready to sell.

[Leevi]

I have met this problem some times as well.
In my case there was not a question about measurable DC on VOL pot and for instance an extra cap
before the VOL pot didn't help. A working workaround is to add an extra cap + 1M grid leak resistor
after the VOL pot and before the 2nd stage grid.

If I remember correctly it helped in one case when I separated the plate voltage supplies for the 1st and 2nd stage
with an resistor and filter cap. Those of you who know the theory better please explain if that makes sense.

/Leevi