Debugging a nice 60/120 hum in a reverb circuit

[mikepukmel]

Down to the last 'bug' in this Deluxe Reverb AB763. With the reverb knob at minimum, amp is quiet even at high vol except for normal hiss when the vol is way up. As I turn up the reverb, hum increases with the knob setting.

Hum is a very clean 60/120 (I can't tell), not a squeal, so I think its got to be something leaking in from the PS somehow. will grab some photos.

Wishful thinking that it was a tank placement issue, I turned everything down, and moved the tank outside the cabinet, pointing in various directions far side from the transformers, no reduction in hum.

The wiring is fairly close to stock, as close as I could get it with my skills. I do have an extra node off the main power supply: one dropping resistor and a 16uf cap, that feeds the reverb driver. Will remove that and see if the hum goes away.

What should I try next, chopstick test? Ive enjoyed playing this amp so much, I hate to take it apart for a few days, but the hum is at an unacceptable level, since I play mostly very low volume, and with reverb.

I also have an old juker 'scope that does provide a trace. Not confident enough to go probing yet. (no isolation xformer yet, will buy one this week).

Thanks, All.

[pdf64]

If the hum doesn't reduce with the reverb 'send' cable removed from the back of the amp, I wouldn't bother to bother messing with removing that extra HT node (I suspect it won't).

The hum is most likely a 0V loop, due to a minuscule difference in Vac between the reverb return's HT 0V and signal 0V; have you got your head around Merlin's document on this? http://www.valvewizard.co.uk/Grounding.html
An isolated RCA socket for the return may be helpful, even if real Fenders managed without it; then you can play with exactly where to reference the return 0V connection.

[Chuck H]

THAT^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

IMHE all vintage Fender reverbs hum. The cure is usually to isolate one of the tank jacks from the tank chassis so the cable shield is only connected to 0V at the amp chassis. Usually done at the tank input jack.

[Enzo]

Think about it, if the power supply was hummy, it would still hum with the reverb all the way down.

[mikepukmel]

Thanks everyone. I did read (many times) Merlin's grounding doc. I think I have a pretty good understanding, even if not exactly how to 'see' the issues in an actual amp.

Its more than a little hum, its really annoying. Hard to remember whether I put both socks on this morning, but I don't remember our old, beat up, war torn tube amps back in the 70's having that lound a hum. I think this is out side normal.

I didn't measure it, but one of the blog threads here talked about the Accutronics tanks big plastic isolators on the jacks, and someone said that they did not 'ground' both jacks, only one side has the shield part of the jack connected to the chassis of the tank. I will verify that.

I will try isolating one side, easy enough to do.

As for removing cabling, the reverb driver is an amplifier, can I damage something if I pull the tank cables off with the amp running, do I need some kind of dummy load?

Will try first to isolate one side, and hope it reduces.

As an aside, if I scope the signal, should I see 60/120? Would this be in the several volt range p-to-p (3v 5v?) its after the preamp? I haven't scoped inside anything with high voltage before, heard that its a must to use an isolation xformer on the scope.

[mikepukmel]

GOing to go in to work a few minutes late anyway, so: popped the tank out. Accutronics 9AB2C1B. put the ohm meter on the shield portion of the input RCA jacks. Red jack shield to case: 0.3ohms. Red jack shield to white jack shield 0 ohms. white jack shield to case 0 ohms. So, the output jack shield on the tank is connected to the case of the tank, but not the input.

So, any chance that instead of a loop, there is no ground and that is the problem? I will check the amp, shield side of both jacks to the amp chassis case when I get home.

[mikepukmel]

maybe my 'extra node on the power supply' wiring is no good. Will jumper that, run a wire the same way the original Fenders's did, and cross my fingers.

What was that saying again .... "If you don't have any idea what you're doing, then ... build it stock..."

[mikepukmel]

Think about it, if the power supply was hummy, it would still hum with the reverb all the way down.

I think I don't know enough to ask the right questions. So, if a power supply is hummy, the noise goes everywhere. But you could have a quiet power supply and still get 120 leaking in someplace, right? Maybe a badly run heater wire?

And, a related question: if you have a ground loop, is there a specific frequency to look for, or could it vary?

[Tom Phillips]

GOing to go in to work a few minutes late anyway, so: popped the tank out. Accutronics 9AB2C1B. I put the ohm meter on the shield portion of the input RCA jacks. Red jack shield to case: 0.3ohms. Red jack shield to white jack shield 0 ohms. white jack shield to case 0 ohms. So, the output jack shield on the tank is connected to the case of the tank, but not the input...

Hi Mike,
You are misinterpreting the results and misunderstanding the design goal. In the stock Fender design the reverb tank input (Drive) connector ground is isolated from the metal case. Only the the output transducer connector ground should be connected to the metal case. THis is done to prevent a ground loop.

You measured "Red jack shield to case: 0.3ohms" That is essentially a connection. Mayne not a perfect one but the resistance is so low the 0.3 Ohm reading could be just a imperfect probe connection.
You measured "Red jack shield to white jack shield 0 ohms" This indicates that both shields are connected together. In this case that path will be through the metal tank case.
You measured "white jack shield to case 0 ohms" More evidence that both connector shields/grounds are connected to the case.

...So, the output jack shield on the tank is connected to the case of the tank, but not the input...

That's an incorrect conclusion as explained above.

Therefore, disconnect the reverb tank input connector from the case. That will break the ground loop and, I suspect, solve the hum problem.

Edit: You will notice that I refer to the connections as "Input/Drive" and "output" rather than "red" or "white." I have seen the colored connectors incorrectly installed by the manufacturer so I don't trust them unless I have the tank in front of me to verify. There are also lots of reverb tanks in use that don't use the colored connectors at all.

Cheers,
Tom

[mikepukmel]

Hi Tom, Sorry, Oh BOY do I feel stoopid (and I looked in the mirror and look stupid too). I should have written OL not 0 Ohms, in 3 places.

Output jack shield to case: 0.3 ohm.
Output jack shield to input jack shield: OL (no connection)
Input jack shield to case: OL (no connection)

Output pin to shield: 258 ohms
Input pin to shield: 2 ohms

So, anyway, I think its not a ground loop through the tank, maybe in my internal wiring.

[Enzo]

Mike, if we report the reading as "open" it eliminates such confusion. OL is the way your particular meter indicates open, it is not an industry term.

[Chuck H]

I can't remember which brand I ended up with in my last reverb build, but I had to do something pretty unconventional to mitigate power transformer induced hum. And it's not as if I was in tight quarters either. This was a full size 2X12 combo cab. Anyway, I tried positioning the pan, jacks facing front AND back, all over the floor of the amp with no joy. The only orientation that would silence the hum was with the tank bottom facing the inside of the lower rear baffle, jacks up. So that's how I mounted it. I had to take some additional measures against acoustic vibration because that panel is prone to do that. The amp is a little more tender than average to reverb crash when moving it while it's on. NBD though because at least it doesn't hum.

I don't get what's happening to reverb tanks anymore at all. I have to reinvent the game every time I use one and one occasion I had to buy three tanks to get one that sounded good and wasn't horribly sensitive acoustically. Then on the last one I found that transducer orientation in the standard mounting plane hums so I can't mount the tank as it's intended. How is this crap getting to the parts vendors without someone at the design level picking up on it?

[Leo_Gnardo]

I don't get what's happening to reverb tanks anymore at all. I have to reinvent the game every time I use one and one occasion I had to buy three tanks to get one that sounded good and wasn't horribly sensitive acoustically. Then on the last one I found that transducer orientation in the standard mounting plane hums so I can't mount the tank as it's intended. How is this crap getting to the parts vendors without someone at the design level picking up on it?

The last few years of USA made Accutronics tanks, they were no picnic either. Failing left and right, never mind the hum follies. I bought a dozen at a time from Magic Parts and there were typically two maybe three that sounded above average, most of the rest OK, one or two that were awful. But even the awful ones found amps they miraculously sounded good in. It was about the same time they started using the two pin slip on connectors to the transducers when things started to suck big time. Beware "improvements," sometimes they are the opposite of improvements. I'm having better luck with the current tanks labeled Ruby (thru Magic) and CE (thru Antique/CE/tubesandmore.) I think they're the same tanks from the same production line, labeled for the outfit that ordered them.

[mikepukmel]

The last few years of USA made Accutronics tanks, they were no picnic either. Failing left and right, never mind the hum follies. I bought a dozen at a time from Magic Parts and there were typically two maybe three that sounded above average, most of the rest OK, one or two that were awful. But even the awful ones found amps they miraculously sounded good in. It was about the same time they started using the two pin slip on connectors to the transducers when things started to suck big time. Beware "improvements," sometimes they are the opposite of improvements. I'm having better luck with the current tanks labeled Ruby (thru Magic) and CE (thru Antique/CE/tubesandmore.) I think they're the same tanks from the same production line, labeled for the outfit that ordered them.

Thanks Leo. This one says "Accutronics & Belton ... manufactured in Seol Korea". I did get it from Antique Electronic Supply. it doesn't sound very good. That is being generous. I might break down and try a different tank, one of the ones you list. Thanks

[mikepukmel]

I can't remember which brand I ended up with in my last reverb build, but I had to do something pretty unconventional to mitigate power transformer induced hum. And it's not as if I was in tight quarters either. This was a full size 2X12 combo cab. Anyway, I tried positioning the pan, jacks facing front AND back, all over the floor of the amp with no joy. The only orientation that would silence the hum was with the tank bottom facing the inside of the lower rear baffle, jacks up. So that's how I mounted it. I had to take some additional measures against acoustic vibration because that panel is prone to do that. The amp is a little more tender than average to reverb crash when moving it while it's on. NBD though because at least it doesn't hum.

I don't get what's happening to reverb tanks anymore at all. I have to reinvent the game every time I use one and one occasion I had to buy three tanks to get one that sounded good and wasn't horribly sensitive acoustically. Then on the last one I found that transducer orientation in the standard mounting plane hums so I can't mount the tank as it's intended. How is this crap getting to the parts vendors without someone at the design level picking up on it?

Thanks Chuck. I tried taking the tank out of the combo cabinet, and sliding it around on the floor. The last trial, it was outside the cabinet, pointing away from the left side of the amp, and as far from the amp as possible. BUT I did not orient the tank as you suggested, jacks up. THe jacks were facing front or back. Anyway, no matter how far I got the tank, the hum character didn't change, so I think I screwed something else up inside the amp.

If the tank has a problem, like Leo mentioned, transducer mounts, etc, can this cause hum?