Eliminate transformer hum in reverb tank?

[spiral]

I built a Mojotone Deluxe Reverb kit and the build went fine (it's my 3rd amp). The amp is near silent, one of the quietest amps at idle I've owned. The problem is when the reverb is turned up there is a hum. It's not crazy loud but it's there and sucks for recording. My '71 Deluxe Reverb had the same issue. The hum is coming from the power transformer, I am 99% certain. If I move the tank very far away from the amp, it is very quiet. What can I do to completely shield the tank or transformer? If I plug in the reverb footswitch and turn the reverb off, the hum goes away. Below are some things I've tried, and some techniques I've read about.

I've tried with no love:

• Different RCA cables
• Wrapping the tank in aluminum foil
• Isolating the RCA jacks from the chassis and grounding them on the signal ground near input
• Rotating the tank every which way
• Tried a different reverb tank (from Allen) in Tuki cover (maybe slight hum reduction)

Suggestions I've read:

• Create a steel box to put around the PT
• Screw a steel plate on the bottom of the tank
• Place mu-metal pieces on top of the tank, adjusting for best elimination
• Build a mu-metal box that is totally enclosed
• Get some lead roof flashing and cover the tank in it (just the top? around the whole thing?)

The lead flashing seemed easiest to try and hold the most promise but I don't know if hardware stores carry it anymore. Your experience is really appreciated on this extremely annoying issue.

[nevetslab]

Hum in this case is magnetically induced...power line frequency (50Hz or 60Hz). Your hum source is the power transformer in your amp. Inside your reverb tank are two small transformers, one on the input side, one on the output side. The Input side is most susceptible to picking up the stray magnetic field from your amp.

I assume you've already tried the two orientations of your reverb tank, mounted to the floor of your case. If not, turn the tank around the other way. You do have some latitude in mounting it....move to point of lowest hum pickup (obviously).

Flashing material for roofing won't help, though it DOES help with regards to electrostatic noise pickup. What you find is Aluminum flashing, typically 0.010 to 0.015" thick. I use it to replace the torn adhesive-backed ground foil on inside tops of Fender and other amps all the time.

In the Grain-0riented Steel domain, M6 is available. It's a Grain-oriented Silicon Steel material, typically 0.007 to 0.010" thick......better power & output transformers have their laminations punched out of this material. It does work very good in knocking down stray fields. It's an industrial metal product, so you'll have to find a source for it that will sell to you in small quantities. Best to have it cut to width, as working with tin-snips to cut to size is prone to cutting you up. mu-metal works better, but is considerably more expensive. I went on ebay a couple years ago to find shielding material, found mu metal and one other type, somewhat lower in magnetic shielding strength. You could try cutting a band of M6 the same height of the transformer laminations stack, and fold the M6 band so it tightly fits all the way around the xfmr, and tape it into position. You normally find the field strength the highest when you're perpendicular to the transformer stack, as opposed to off the ends where the end-bells are.

Depending upon the severity, a combination of furthest distance/best orientation, M6 band around the pwr xfmr's lamination stack, and then a shield between the sensitive end of the tank and the power xfmr.

As for screwing a steel plate on the bottom of the tank....unless that open end is facing the power transformer it won't help, but could help from any strong magnetic field sitting underneath your amp's tank.

[Enzo]

I have to disagree. Fields are all around, and putting a steel plate over the open side of the pan usually helps, in my experience. I think shielding the pan is a lot easier than shielding the transformer. One source of sheet steel is "tin" can sides. Make sure they are ferric by testing with a magnet. Cut a piece of sheet steel to cover the opening, and run screws through the flange to secure it and ground it.

[J M Fahey]

It's a complex problem, hard to impossible to solve inside the distance limits of a combo.
FWIW Paul Young is known for putting his reverb tank (without electronics) inside a padded case ... in the backstage or under the stage.
In his case, not because of hum but for incredible intense yet feedback less reverb.
I bet that helps with hum too

FWIW 2: a friend of mine handwires balanced microphone input transformers ... and was plagued by small but annoying hum.

Desperate, he cut a 1" section of common 3/4" galvanized gas/water pipe and put the tiny transformer inside .... he got some 16 dB reduction in hum ... measured, not estimated.

[trobbins]

Have you been able to look at the hum waveform and confirm it is the mains frequency, and not twice mains ?

Does the power transformer have bell ends and a copper belly band/hum strap ? It's always good to start attenuating at the source first.

[spiral]

Thanks for the replies so far.

Depending upon the severity, a combination of furthest distance/best orientation, M6 band around the pwr xfmr's lamination stack, and then a shield between the sensitive end of the tank and the power xfmr.

Thanks for the suggestions. You may be interested in Billm's experiments with transformers. The angled strip seemed intriguing.
Reducing Blues Junior Hum (transformer shielding)

I think shielding the pan is a lot easier than shielding the transformer. [...] and run screws through the flange to secure it and ground it.

Thanks Enzo. This is an easy thing to try and I found an amp maker that has premade plates + screws the he says work wonders. Shouldn't be an expensive experiment.

It's a complex problem, hard to impossible to solve inside the distance limits of a combo.

No doubt: I'm demanding a lot from a 50 year old low-tech design.

Have you been able to look at the hum waveform and confirm it is the mains frequency, and not twice mains ?
Does the power transformer have bell ends and a copper belly band/hum strap ? It's always good to start attenuating at the source first.

Yes to the bell end () and copper band. It's a Mojotone transformer: http://www.mojotone.com/transformer-mojo761.jpg
I don't have a oscilloscope to test the hum. Is there a way I could record the frequency and test it in an audio app?


Another thought: I have some frying bacon in my amp (it's not even 6 months old), so I need to replace some resistors, and it occurred to me that a bad resistor could be creating the hum. Can I jumper the send/return of the reverb to remove the tank from the equation and see if the hum is still there? I didn't know if I would blow anything up if it was expecting a certain load from the return.

[Tom Phillips]

I built a Mojotone Deluxe Reverb kit... The problem is when the reverb is turned up there is a hum...The hum is coming from the power transformer, I am 99% certain. If I move the tank very far away from the amp, it is very quiet. What can I do to completely shield the tank or transformer?...

You seem to have covered most possibilities and presented good evidence that the PT magnetic field is causing the problem. How far away do you need to move the reverb tank to eliminate the hum?

In addition to the advice already given I offer the following comments.

Trying to shield the Magnetic field from the PT is really tough.
Mu metal is not only very expensive but an effective shield must be continuous with no gaps and the bends at the edges should be rounded. In addition, working mu metal negatively affects the shielding performance so the finished shield needs to be annealed. Given all that, a mu metal shield is only practical for the most demanding situations with larger budgets than we have for our guitar amps.

Since there are many Deluxe Reverb amps that do not have the hum problem you describe there must be something different about your particular amp. Have other people reported similar issues with the Mojo DR?

One idea I can offer that has not been mentioned yet is the possibility that the PT in your amp is generating a stronger magnetic field than it should. All transformers are not equal and if yours has a fault or a design / manufacturing flaw it could put out a stronger magnetic field. Since it is not practical to troubleshoot by jumping into changing the PT you could try some tests using your electric guitar as a field sensor as follows. Plug your guitar into one of your other amps which is placed some distance away. Turn on the DR and move the guitar around the DR noting how the hum picked up by the guitar and amplified by the other amp is affected by the guitar pickup's proximity to the DR. Now substitute another amp for the DR and do the test again. (Ideally you would use another DR, borrowed from a friend, that doesn't have the reverb hum problem. However, any similar sized amp will give you an initial data point.) The goal is to determine if your DR PT is putting out a significantly higher intensity magnetic field than normal. If it is then a different / better / non-faulty PT may be the solution. Note that you will be looking for significant differences at similar distances. We don't want to be splitting hairs between 12" vs 14" or hum levels that are barely perceptible. You will need to insure that nothing else in you test area is affecting the results. You can do that by turning the DR completely off and then moving the guitar around the area to verify it's not picking up high hum levels. It will be easiest if the amp under test is placed on a wooden stool or chair to raise it up to a convenient working level..

That's my best idea given that you and the others have covered most everything else.

Tom

[spiral]

Thanks for the "guitar sensor" idea Tom. That's a good one. Thanks so much for the suggestions.

I'm surprised you haven't heard of Fender reverb tanks having noise issues. The transducer essentially act like single coil pickups in the tank so they are prone to pick up radiation. I read here that Dick Dale suspends his with a rope but that may be more for vibration / feedback. I've found lots of threads about people having the same problem as I am describing and a steel plate seems to be the most successful. One guy went overboard but also got the noise to be zero: OffsetGuitars.com ? View topic - Strange reverb feedback problem ... even stranger solution

Can I remove the tank from the equation by plugging an RCA cable from the send to return on the amp without damage? I don't know if the return is expecting a load or not. If I can skip the tank I can determine if it's the tank picking up the noise, or maybe a component in the driver causing issues. Thanks.

[loudthud]

Can I remove the tank from the equation by plugging an RCA cable from the send to return on the amp without damage? I don't know if the return is expecting a load or not. If I can skip the tank I can determine if it's the tank picking up the noise, or maybe a component in the driver causing issues. Thanks.

That won't tell you much because the tank attenuates the signal by an unknown amount. You could connect a speaker to the Send line the goes to the tank input and listen for hum. Then replace the tank output with a short across the cable to make sure there is not some hum being picked up in the ground. Final check, touch the shorted cable to ground at the tank.

[Tom Phillips]

...I'm surprised you haven't heard of Fender reverb tanks having noise issues...

I've certainly heard of it and personally heard it. However, I was under the impression that your Mojo kit build was worse than the norm. The hum is not usually a problem if you set the reverb below about #4. It's difficult to quantify problems via verbal descriptions. If your problem is within the normal range then the metal plate cover for the bottom of the tank should produce the improvement that you need. Seems like you have already isolated the problem to the tank picking up magnetic fields since you said that moving the tank away from the PT eliminates the hum. I'm still interested in how much farther you moved the tank away from the PT to eliminate the hum.
Cheers,
Tom

[spiral]

@loudthud.
Thank you. The speaker is a great idea.
As for "replace the tank output with a short across the cable", are you saying short the output RCA ground and hot? Does the reverb pedal do the same thing when clicked off? If yes, I get absolutely 0 noise, even when the reverb is @ 10, when the footswitch is connected and clicked to "off".

I've certainly heard of it and personally heard it. However, I was under the impression that your Mojo kit build was worse than the norm. The hum is not usually a problem if you set the reverb below about #4.

It's in the normal range of vintage Fenders, and between 4 & 5 is where I use it. I wanted to try and tamp it down as much as possible because it's not far off from perfect. I know there will always be some noise. I've heard lots of suggestions but wasn't sure which worked. Sorry if my description was bad. English is my first language. Steel plate seems to be the most popular so I'll give that a go.

I only have 1.5-2 feet of play in the RCA cables. If I can find some longer ones, or can extend them with female/female connectors I'll let you know if that helps at all.

Thanks again for the assistance.

[loudthud]

@loudthud.
Thank you. The speaker is a great idea.
As for "replace the tank output with a short across the cable", are you saying short the output RCA ground and hot? Does the reverb pedal do the same thing when clicked off? If yes, I get absolutely 0 noise, even when the reverb is @ 10, when the footswitch is connected and clicked to "off".

I forgot about the footswitch. That should confirm that the noise isn't comming through the ground, but... what is the ground configuration of the tank? Some ground only one of the RCA jacks, some ground both.

[spiral]

I forgot about the footswitch. That should confirm that the noise isn't comming through the ground, but... what is the ground configuration of the tank? Some ground only one of the RCA jacks, some ground both.

Thanks. I isolated both jacks from the chassis and grounded them where all the input / preamp grounds are near the input.

Taking Tom's suggestion one step further, I extended the RCA cables to about 5ft and was able to pull the tank pretty far away. What I found was:

• it was most quiet when the tank was at the normal height, pulled just out of the cabinet, and the jacks facing away from the speaker. Also, touching the shield of the send/return to the foil wrapped around the tank made the noise go away.
• with jacks facing away from the speaker, and the tank in the normal mounting spot, there is a lower hum
• with the jacks facing toward the speaker, and the tank in the normal mounting spot, there is a higher buzz

I put a mic in front of the amp (LDC) and shot a video. It was eye opening. You can also hear the noise I am trying to eliminate. Now I just need to see if the bottom plate will work. Maybe it will be interesting to someone else too.

[nevetslab]

Power Supply Choke?

One other thought. A couple years ago, I had a Fender Twin Reverb on the bench with a Reverb Hum problem. I had exhausted all but one obvious source: the power supply choke. When I swapped it out with another Fender amp from the bone yard, the problem went away. Bad Choke. And, radiating 120Hz-based harmonics like crazy.

For lack of a spare one to try, you could temporarily unplug the choke and jumper those two power supply points together. The Power Supply ripple will increase considerably, but, if it is a choke radiating field problem, that should tell you pretty quick. You do need the choke in the P/S circuit, of course. Make sure you've completely discharged the supply caps & have disconnected the AC Mains before going anywhere near that supply wiring.

My earlier post....I meant to say the Output Transformer that's picking up the return signal from the reverb springs is the one that is most susceptible to hum pickup, NOT the driver transformer.

Traditionally, most reverb tanks ground the Output RCA connector, as that's the most sensitive circuit...the return signal from the springs. When you mentioned having isolated both grounds from the chassis..were you referring to isolating both tank connections (input and output), or the connections at the amp (Send/Return jacks)? If you've isolated the tank chassis from the connections, you do have to ground the tanks' chassis.

After listening to your hum pickup, when you rotated the tank, RCA's now facing in rather than out, the hum pickup was lower (I'd guess the tanks' Output jack is the 'left side', facing the connection side of the tank). And, of course, moving the tank out of the combo case, problem gone.

I have run into power transformers where the heater winding, loaded with all the amps tubes heater coils was causing major hum issues (Vox AC30-6/TB). I had resorted to installing a separate 6.3VAC/10A Heater Xfmr, removing the heater load from the main PT. Stray hum field went way down.

Reverb tanks are sensitive to mounting orientation. Some, you can get away with mounting them on the lower vertical panel instead of open-end face down in the bottom of the case. Others won't work or are unreliable in that orientation. You could try moving the tank with the RCA's face down, mounted to the inside rear bottom panel. That rotates the tanks' transformers 90 deg from your PT. If that proved to be effective, it would probably be wise to stiffen that panel, and also see how much havoc the speaker creates.

[spiral]

For lack of a spare one to try, you could temporarily unplug the choke and jumper those two power supply points together. The Power Supply ripple will increase considerably, but, if it is a choke radiating field problem, that should tell you pretty quick. You do need the choke in the P/S circuit, of course. Make sure you've completely discharged the supply caps & have disconnected the AC Mains before going anywhere near that supply wiring.

Thanks for all of your brainstorming! Rewiring, or removing the choke and knowing what to change in its absence is beyond my technical knowledge. My electrical knowledge is very limited which is why I ask all you smart people.

I wanted to follow up with everyone else as well. I bought some steel from the hardware store. Luckily we just got a new furnace and I sweet-talked one of the workers into cutting the steel to the perfect size for the bottom of the reverb tank. I then wrapped the whole thing with 2 layers of aluminum foil. It has nearly eliminated all the buzz I was getting before when turning up the reverb. It definitely killed the lower hum. It was likely all three things that helped: rotating the tank so the jacks face the front, the steel plate, and wrapping the tank in foil.

Thanks to all your time and suggestions.