Fender DR build.. Reverb BUZZ Gremlin.

[Chuck H]

Do not plug random, unknown tubes into any socket. I have no "good" description fro differentiating tube types within the 12**7 family and many other noval tubes look remarkably similar too.

Pull V1 and plug it into the V4 socket. And get a few know good preamp tubes. It's not as if you won't use them.

You can often see a number that looks like it's smoked into the glass by turning the tube in the light. This number is visible long after any colored printing is gone. This is most common of old Euro tubes.

[Sea Chief]

Ok I swapped what I thought a 12ax7 in for V4.. no different.

Taking the amp out again > onto workbench to do this, I noticed the hum about 1/2 as bad/ as the LOUD hum.

So, could it be the handle fat T-nut that although I set in rebated so nicely flush with cab top/ nothing poking down? as I just measured its -bang- above the reverb circuit area.. and also I havent foil-shielded inside the cab top (or could it be that?). No, and No..

I put it back in the cabinet, where amp was on workbench, expecting the hum to be as bad but maybe LOUD..

No. Its -better-. Repeat -better- and back to a normal-reverb-bit-annoying-hum level. Dial it to 11 and hum more prominent but not obtrusively so. wtf?

But there's -nothing- in the place where amp had LOUD hum to affect it. In fact, where it is now on workbench I have part of the houses 'leccy fusebox quietly humming to itself only 2 ft above it.. and still its behaving/ normal amp hum. And I tried it different place in room- same LOUD hum/ different plug socket. But it -did- behave itself for a while in the room, whereby the feedback f**ked off s'where thankfully (but inexplicably).

Perplexing.

[eschertron]

My experiences with odd hum/buzz coming and going when nothing but handling and observation was done to the amp all boiled down to lead dress. could wires be re-positioned by the act of taking the chassis in and out of the cab?

[Sea Chief]

I think I may be able to get slightly better escherton, but did do a pokin of these and made no difference- it is a pcb > tube socket affair, so little error to be made (1 reason I went the pcb route).

I may have solved it tho-(?) as I finally got normal hum, in my usual amp position in room.. whether it will remain is another thing (it didnt before right here/ same spot). But I pulled all else WAY away even PC in battery mode, and better. Then different combinations of plugs' in sockets nrby: some BAD, other combos ok. Trying now to narrow down exactly which-nr-what ok.

Christ- thats one afternoon Ill never get back!

Anyway slight increace from my 425 plate V when last measured is my only concern now (up hoverring @ 339V which is odd/ a bit high isnt it?). And minor ticking on trem/ a bit weak still too but enough- I'll do trem in a few weeks.

Enormous thanks chaps. Sea Chief.

[Sea Chief]

Argh... its back. I give up- Ive spent 9 hours on this today solid & Im spent.

I know for sure the PC, plugged in anywhere in the room (once a blue spark when laptop plug inserted, coincided with the hum doubling to LOUD again) doubles the hum to LOUD. It is though, bad at even half this ammount, so much I cannot bear turn rev past 2 as it drives me insane. Why I had it at normal level only 2 hours ago tho on the workbench?!!

gratefully- SC.

[eschertron]

I you haven't already glued tin foil on the roof of the cabinet, do so - if just to eliminate THAT possibility of RF interference. From your description, it looks like you might want to try the amp in a completely different setting. Take it to the library and see how loud it is (just half kidding!). You may need some kind of line filter at your place.

Do any of your other amps respond to the noise from the PC the same way? Even a little bit? There's something strange going on, but it may not be a single component that's responsible.

[Sea Chief]

Ive ruled out the need for foiling.. the amp worked fine in the cab 2 hrs ago, on my workbench, with a bloomin great humming fusebox just 2' above. Not even 1/2 loud hum, it was at last at normal reverb-amp hum. The workbench is far end of the very same room.

No other amp Ive had (same room, same place) have had any such issue, but none had reverb tho. My wem for eg, in another cnr of room is quiet as a mouse with a bloomin 70wpc hifi on permanently, whacked into the same double-wall-socket.

I just cannot find any correlation now between AB and C to entertain any logical course of action.

[eschertron]

Is there a change - experiment performed when the amp is humming to beat the band - if you pull the reverb's RCA cables out of their chassis jacks? (completely disconnecting the reverb pan and all wires)
Does it matter if the footswitch is in or out?

I know it's painful when there's an intermittent problem; that's the kind of stuff I chase all day. Develop a profile of the problem by performing A/B testing that cam be repeated ad nauseum (at least until the problem mysteriously disappears!). Only then can you definitely say "this is related" or "that is not related". Just because it wasn't humming on the bench that one time doesn't rule out shielding as a factor. You know Enzo's Razor: "don't think up reasons to not check something"

[Sea Chief]

Ok will do the foil, may as well. Is the idea to create a continuation of the chassis, IE is the idea for the foil to be touching around edges to the chassis edges, all around to 'seal' it as it were? or is the idea just a shield above the wiring to reflect xyz.

Ftsw in or out, no difference. Have not tried pulling both rca's.. but undid input from under amp, and noise remains.

[eschertron]

The more coverage (seal) the better. If perfection can only be attained by endless work, then try for mostly perfect and get it done.

[Chuck H]

Holy crap SC.!. I go to work for the day and you get all twisted up. Chill. Chiiillll.

It's a problem. It's fixable. We just need to keep focusing on the facts. Don't jump ahead with conclusions or speculations. All this tryin' stuff and frantic arm waving will get you nowhere. It's especially detrimental to the process if you start making changes or test on your own and tell us about changes that don't seem to correspond are only temporary changes. It doesn't help solve the problem to know that, for example, the hum is only there on Tuesdays because that would clearly be an arbitrary observation that can't be substantiated. So we stick to facts only. The facts don't mean to frustrate you. They are utterly ambivalent to your circumstances.

The problem IS NOT:

the tank
the microphonic nature of the tank
the location of the tank
BECAUSE IT ISN'T THE TANK
the reverb transformer
the reverb driver tube
the reverb cable
the footswitch

Absolutely ignore everything about those parts and their circuits.

The problem is in the recovery stage behind the reverb pot. There are very few parts there and it's a simple circuit. Only a couple of possibilities for causes of hum. We will find it if you're patient. Try not to jump ahead. Wait for responses and perform the tests requested carefully. Unless you are a more qualified tech than those on this forum or plan to take the amp to a repair shop, this is your best, and least frustrating course of action. You will learn plenty along the way. More than you will learn by any DIY attempts. At least for this problem. Ignore that there's a problem for the moment and just try to place yourself in a robotic position on the matter. We will find and fix the problem. You can now separate yourself from it and just perform the functions asked. And when it's over your amp will work. Just because your WEM isn't humming doesn't mean a thing. The DR has nothing against you and it didn't ask for a chance to piss you off. It's just broken. We'll fix it.

Do try some shielding as eschertron suggested. Be thorough but not fanatical. The shield must make clean contact with the chassis to work.

I'm still not 100% on the tubes you're trying. "I think this is a 12ax7? Not sure about it's condition.?. Eh?" doesn't qualify as a known good tube. Why not pull a preamp tube from your WEM? Clearly those are working fine! Do mark for location if you pull more than one so they may be returned to it accurately.

Do you have an alligator clip lead or can you make one in a jiffy? If you make one, be sure to get the rubber booted clips and use wire insulated to 1000V. You don't need these ratings for the testing here but you may wish to save the new tool and use it for something else later.

[Tom Phillips]

...It worked fine only a few days ago nothing moved or different at all.. so why suddenly its appeared is a mystery. It 'sounds' simply like a "pg 1 didnt plug the ground in dummy! error"...

...Taking the amp out again > onto workbench to do this, I noticed the hum about 1/2 as bad/ as the LOUD hum...I put it back in the cabinet, where amp was on workbench, expecting the hum to be as bad but maybe LOUD..
No. Its -better-. Repeat -better- and back to a normal-reverb-bit-annoying-hum level. Dial it to 11 and hum more prominent but not obtrusively so...

...I may have solved it tho-(?) as I finally got normal hum, in my usual amp position in room.. whether it will remain is another thing (it didnt before right here/ same spot)...

Argh... its back...

The described symptoms are indicative of a bad, intermittent ground connection. The fault could be a cold solder joint, a bad mechanical ground, a bad connection in a component where the lead is connected to the guts of the part or even inside a hookup wire under the insulation. It is wasted energy to rule things out just because they were checked once before and seemed to be good. There is no 100.00000000% “known good part” because many parts that are bad were once good. The transition between good and bad and then back from bad to good in the case of an intermittent is often instant and unpredictable. A very organized approach is required to keep track of the comings and goings of an intermittent fault plus a lot of patience. It also requires knowledge of the circuitry to troubleshoot. In a case like this troubleshooting may require bridging connections with an additional wire or grounding selected points in the circuit while the amp is on. Unfortunately, that’s hard to describe with a narrative in a way that can be followed by another person in a remote location.

The best I can advise at the moment is set it aside for a while, keep detailed notes and work on it only when you are not pissed off. And what’s up with turning the reverb up to “11?” The knobs are labeled “1” through “10.”

[patlaw]

And what’s up with turning the reverb up to “11?” The knobs are labeled “1” through “10.”

I wonder if some things are getting lost in the translation. All of the abbreviations SC makes up make it difficult to understand what he is trying to communicate. When it takes a lot of work to understand a post, I skip it.

[Sea Chief]

I wonder if some things are getting lost in the translation. All of the abbreviations SC makes up make it difficult to understand what he is trying to communicate. When it takes a lot of work to understand a post, I skip it.

Sorry- but I did think 'knob turned up to 11' would be abundantly clear to any guitarist! (whose seen spinal tap, IE all guitarists). I dont think I abbreviate much beyond gtr.. again thought that would be clear too.

Ok Ive no idea with the hum still: taking to another house- not there, but not here when i got back! so lord knows. Its back with a vengence tho & I cant do any more on it as nothing makes enough sense for a logical course of action/ frustrating. a mystery hum it is.

Very grateful tho chaps- SC

[Chuck H]

Solving for intermittent problems is a bitch (Sea Chief: "I know! Thank you Captain Obvious").

It may be a funny ground (solder joint, cable, jack) or it may be some terrible local EMF that comes and goes without a fixed schedule. It might help to know the actual Hz at this point, but without an oscope it's guess work.