How to reduce hum MojoTone Fender Deluxe Reverb Kit guitar amp proper Grounding build technique
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- Опубликовано: 9 окт 2021
- Hum in kit amps? Always comes back to grounding techniques. This time, a MojoTone Fender Deluxe Reverb. The owner did a great job constructing this machine. However, grounding became an issue. The amp had such a high noise floor, that it was un-useable. So, he sent it to me for evaluation. Initially, I was also taken back by the high noise/Hum level. However, employing the proven Fender grounding system, the amp now plays beautifully. So, everyone, Fender was in business for a long time. They were not after money (initially) They were after the quest of creating the ultimate tube amp! Many years of research went into the design of their product. The ground plane (Brass plate) was one of their masterpieces! I am a firm believer in this method. Listen for yourself. Every Fender amp that comes thru D-lab lacking this technology will have it when it leaves. Enjoy the show!
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As soon as I saw the missing brass strip I said to myself, Self, this thing needs that brass strip to fix that problem. Seems I missed a couple of other things, but boy does it ever sound better!
Great work as usual!
I'm guessing that Fender may have a patent on it?
Yay, hum is gone, and Tony's Magical Mojo is back! Thanks, Terry; another well done repair!
It's always so fun watching an expert explain something. Thank you for a wonderfully informative video.
I once took an apprenticeshio with Mojotone's Andy Turner. It has led me to your channel. Thank you for the great videos and how-to on amps
Great video Terry, as usual and of course grounding is super important...Great fix and tech info. Tony's shake down is fab also..! Nice sounds..Ed..UK..😁
Terry…. Congratulations on your Mojo deal, and I always learn something from you. Thank you!!! Also I want to give a shout-out to Tony…. He is the man from one guitarist to another. I really like his style. Thanks for your time guys. Lee. PA
Another quality D-Lab fix. I’d like to give a shout-out to his ever present lab assistant Bob Mondavi.
Top job and thanks for the "Brass plate" info. Here in England we call it an "Earth noise". Some times (cos we didnt know better) we would simply disconnect the last pre amp ground and it usually worked but you do it the right way . We learned about amps by trial and shock! Then off to college to learn the trade properly.
Always a pleasure watching Tony's face expresion when he plays!. The best QC available!
Back in the day Tony got more ass than a toilet seat at Taco Bell.
Nice clean-up! Enjoyed watching and listening test. Thanks.
Another great job. Thanks Terry. Merry Christmas 🎄
Great amp and episode. Mojotone has sent some other channels kits. It'd be great to see you do a kit from start to finish..
The consensus I've always heard about grid stoppers is that they work best (at filtering out RF) when attached directly to the tube socket so I'm with the original builder on that one.
Stoppers, absolutely as close to the grid as possible. ... wire the resistor to the socket, with the 'body' of the resistor as close to the grid socket terminal as possible.
Yeah, I bought one of your kits and it was humming like you wouldn't believe, after going through it multiple times unsoldering all kinds of things for a Week. I found you sent me THREE bad preamp tubes THANKS ALOT
The marks on the Orange Drop caps tell me the builder tested them to find the outside foil for each cap. That was one step in the correct direction. I am sure that MojoTone is aware that star grounding does not work well with the AB763 and I suspect Leo may have known that as well.
Well done, Terry!
Terry- always a great job. I wonder why Mojotone does not sell that brass plate ? it only make sense.. perhaps your videos will influence that :)
Just a guess, cost...
From what I have seen there are some other improvements they can make, like putting the fuse and switch on the hot lead and providing a proper ac safety earth
Well done Terry. Star grounding, when done properly works very well, I have used it many times in mic preamps. But the brass strip works so well in guitar amps. I have retro-fitted Marshalls with it, and it lowers the general hum and noise level.
Where to you get your brass shim stock from?
Nice work Terry!
great stealth mojo commercial ! good job ! I learned a lot.
Great job, Terry. Looks and sounds like it should now.
Nice work! It sounds great!👍😎🎸🎶
Thanks for the video :-) Yeah that missing brass piece was the key when i saw that and the star ground, uh oh lol. Thanks for pointing out separating the preamp and power tube grounds.
Yes to separating preamp grounds and power supply grounds. Cleaning up the wiring was great! Lead dress makes a real difference in all Fender amps. I don’t like the plates. Fender used them to facilitate cheaper assembly. Unless you solder the plate to the chassis (Fender often did), you are still relying on a mechanical connection between the plate and the chassis (dissimilar metals), through pressure provided by the pots.
Snozzeramus can provide a rock solid ground point near the input jacks without the plate. Then the ground buss works great. System will stay completely grounded, even if one of the pots manages to get loose.
Amp sounds great.
Beautiful work by the way!
You may have fixed it but if I plugged a guitar into it and tried to play it would be an entirely different kind of noise issue. 🤣
Nice work Terry. I’ve used a brass plate in a 5E3. I now use separate chassis grounds for inputs, outputs, power supply and filter caps. Drill the chassis by before assembly and use stainless steel screws and keps nuts - tight !
I’d like to know more about that white Tele Tony is playing !
Its amazing to see this. How often people throw out the instructions and go about it how they please. There is a reason they are laid out the way they are. If you follow the layout given, you most likely will have a quiet amp that performs as it should. Follow instructions people, they're there for a reason.
Seriously, mojo does not have instructions for the dr. They have a schematic. The only place to ground preamp is to pots, which doesn’t work well. They don’t give a brass plate.
Yeah, I don't understand why someone would make a huge ground modification and then complain about hum. Seems like it's time to at least check if the original design works!
Very helpful video. Thank you!
The brass plate was as much for assembly line efficiency as it was for grounding. It actually is not needed, the chassis does the exact same thing if you solder to it correctly. I have built some Deluxe Reverbs from Reissue chassis, and I use a copper buss flown from the pot grounds (but NOT touching the shells) over to a chassis connection next to the input jacks. This makes the amp so quiet you would think it was not on. And of course the HV grounds at the power transformer side.
Agree--you can replace that entire plate with a single screw, nut, and locking solder lug washer connected to a bus wire--that's why I usually build with a turret board, but running it off the pot grounds'll get you there too!
your work and know how is fantastic. fab.
Thanks for the tips!!!
Nice repair.
I just finished a build on the Stewmac version. Sounds great but I got diddly on the reverb side. No hiss, no tank noise, nothing. swapped tubes, checked the wiring, all looks correct.
My Mojotone kit DR hums just like that if the reverb is turned up. (Pretty quiet otherwise.) Did not use a star ground though--did follow MT instructions. Brass plate will be installed ASAP.
@John McCane I was wondering the same. I just ordered the DR kit.
Grounding tube amps is a science.
nice work...thanks.
Another great video! I will use the brass plate on my next Fender build. Serious question what makes the brass plate better than the connection to the metal chassis. Does anybody have any theory's on this? Im not arguing against using it, just try to wrap my head around why its better. I have been grounding to solder tabs and using PT and Tube screws to anchor. I need a snozeramous to get enough heat to solder directly to the chassis.
Very nice fix Terry.
I don't have any problem with star grounding and have used it successfully in many amp builds. I think that this was just a poor implementation of that technique as it seems there were several potential ground loops. I would also likely use it in an amp build that is designed around that ground scheme as a whole, whereas this build came to you as a Fender style kit trying to graft in a star ground scheme. For example a point to point build it can work nicely.
In the end it seems like the builder got out over their skis a little trying to redesign a kit that simply doesn't need to be redesigned.
There are those who think they know better than Leo. LOL
Did Tony get a new guitar? Great video!
Terry, did you notice the outer shields on the reverb and preamp channels coupling caps were backwards? Nice video,
Mike
15:13 - It IS the kit! It should INCLUDE that brass strip that you made!
True. That’s the whole issue. The pots don’t provide a good ground.
@@NT-ze5nx where can we get the brass strip? any details? Does it have to be .010" thick? im having trouble locating just were to get it ...
@@dparham D-lab made the strip I think.
@@NT-ze5nx i found some 10 thousandths 6" x 60" on ebay! i may make a few of these! since you cannot buy just enough" but u can buy enough to make 10 lol
Fender twisted the wires going to the tone controls and reverb/trem controls- you think that will help to reduce noise at higher volume?
Instead of all those wires going to a common solder point on the chassis I see that on Mojotone's website
they actually show a brass rod parallel to the pots but this brass stock idea Fender did is far better.
I did notice the 5 Watt wire wound ceramic resistors mounted under the 6V6's . Originally they were 5 Watt
metal oxide film type resistors. Would this cause any problems because of inductance inherit in a coil of wire
inside the ceramic resistors ?
I built a DR and made a brass plate but still when I plugged in the reverb tank I got hum. The solution there was simply re-orientating the reverb tank 180 degrees.
Hi Terry, I quite envy your ability to look at this amp(s), and determine what has gone wrong and how to remedy it. Really nice work! I suspected a ground loop or fault as soon as you turned it on. I would love to build my own amp one day, my way, a bit like how Mojave circuit boards look. So linear and visually pleasing. Likely the OCD in me that prefers artistic symetrical order on a circuit board, to point to point spaghetti chaos that a lot of amps exhibit. Wish I had your workshop too! Thanks from all of us for the time you take to put these videos together. Much appreciated. Time for a glass of wine now :) Best regards from the west coast of Canada ss - VE7-GNZ. di di dah di dah
If the brass strip is at the same potential as the chassis, how is it better?
I could only find a 1x12 strip of brass. It is big enough to reach all the inputs and control pots on my build. Would that work or is a larger piece like you have here necessary? Thanks for the vids.
Hi D-Lab, (Haven's learned your name yet) I came across this video after building my first amp, a Mojotone 5E3 kit. It has the same hum as shown in this video. It is not good for playing. As such, I want to install the brass ground plate as shown, but can't find any stock narrow enough. I find 6" width. Where do you get the shim stock that is appropriate for making the grounding plate? Thanks for your great teaching videos and helpful info.
Dan Cash
Lol, tell Tony it's got the same caps as the one he thought sounded bad! The emperor's last outfit was way snazzier.
My question is sir, why didn’t mojo tone sell you the brass plate for the potentiometers, etc, etc for proper grounding purposes?
Does the kit come with screen or foil to use in the casework above the big open top of the chassis? My 1966 Vibrolux Reverb has a copper screen in the inner surface of the enclosure that completes the full shielding when the amp chassis is installed in the cab.
How did you cut the hole for the AC outlet? With a punch? I wish the kit chassis came with these.
Terry what was wrong with the reverb ❓
I just built this same kit. I powered the amp up properly using a 150w bulb current limiter in steps checking my voltages. The current limiter bulb seems to be dim with all tubes in and all the tubes are lit and appear normal. However, I can hear and feel the amp running through the speaker but I have no output from either channel. Adjusting the bias pot I finally started to hear crackling and then the amp had output that starts then fades out. This process repeats with hearing the guitar then it fades out completely repeating every few seconds, but only at a certain range in the bias pot. From what I understand the thick yellow wires going to the recto tube should be 5v dc however I have very high voltage on those pins? Is this correct? I am going to check output noise on the grid and plate of each tube down the line until I find one that is dead. Any suggestions anyone? Thanks in advance.
The Preamp Section Ground and Power Amplifier Section Ground are NOT Tied together connect to chassis ground, which is technically not a good ground scheme because you want all your grounds to be tied to chassis ground which is earth ground zero volts. If all of your front panel pots are connected to the Copper strip for ground which is NOT tied to chassis ground or earth ground, then it will be Floating above chassis ground and earth ground which is NOT a good grounding scheme. The Signal to Noise floor increases when you TIE together the preamp section ground and power amplifier section ground both to chassis ground/earth ground is because there is a potential difference from the preamp section ground compared to the power amplifier section ground which needs to get fixed first before moving forward. My main point is that You don't ever want your preamp section ground "isolated" or "floating above" earth ground/chassis ground.
what is the advantage of the brass plate?
The amplifier hums because it doesn't know the words....... :)
I'll see myself out the door now........
Thanks for stating the amp wiring was NOT done as specified by MojoTone.
Is that added brass plate electrically isolated from chassis ground?
Beautiful work but I bet there is a ground loop. Like lyle sez,, there is perfect, a star ground, good enough, fender and not so good, marshall and vox. In a guitar amp unwanted frequency's are filtered out the high's and lows getting back into the whole system causes the noise issues. Also, it's just me but, I don't like the orange drops!! I think the mollory 150's just sound Fender.
Question: how is the RG174 coax grounded on the pre-amp end? (anyone)...Thanks!
Christ! All of that 120Hz AC from the power supply rail is dumped right into the rest of the signal path grounds. That is a great recipe for lotsa hum. 36 gauge copper craft foil from Hobby Lobby works in place of the brass plate. It is only about .007" so be careful not to let those star lock washer spin into it. Being it's copper it will conduct better than the .010" brass plate. If it's too short it solders together very easily. Clean it to a shine with some Scotchbrite.
But wait, were the two 100 Ohm Resistors installed originally??? I didn't see them on the Pilot light. That would have contributed mightily to the hum.
How thick is the brass plate you used Terry? I want to get some to do what you did. Great repair and demo! Love from the Rocky Mountains. Thanxz
.010" shim stock as stated in video at the 5:00 mark.
@@billsoule5824 Thanks Bill! I must have missed Terry explaining the thickness. I went back and heard him. Love from NW Colorado. Thanxz
I sell the brass grounding plates pre drilled for the deluxe!
How would I go about getting one for a Deluxe Reverb?
Where I can get a brass plate?
It is normal for a tube amp to have a slight hum when there is no guitar plugged in and the volume knobs are turned down? This is what happens with a 5e3 clone that I built. The hum is quieter than TV volume.
So do you think Mojo well including brass grounding plate?
It would seem that the kit builder did not follow the instructions properly, as I would think that Mojo would get away with selling all of their kids with hum.
Points to you Terry for going above and beyond, perhaps the kit manufacturer will come out with a modification to their kit's, and a D-lab upgrade.
Kit was initially built with grounds to pots per mojo instruction, then changed to star bc of hum, which didn’t work. Terry used brass plate, which is not part of kit
Shematic diagram please fender de luxe reverb mojotone..
So... those ground issues kept the reverb from working? Or did you forget to add the repair of the reverb?
The hum was bad with Reverb turned up. After the fix, we didn't hear you test it with the Reverb turned up... the reason I mention it is because my Mojo Deluxe has a little hum with the reverb turned up.
Was this an example of ground loop hum?
Could this also be called circulating currents?
Groundbreaking
What's the song at 13:28
where is the best place to buy the brass shim stock?
ebay
Just looked on eBay under d-lab can’t find the brass plate
I SO don't get this -- the grounds are NOT separated, they all run to chassis -- so what is really going on? The wire runs and the EMF they are picking up?
You put Tony out of work with that looper.😢 First Fink now Tony, who next on the chopping block?
10:12
As an engineer, I'm all about over engineering things lol. But, there's so little gain in those amps why even bother or take the risk of improperly implementing that grounding scheme?
3:50 wow that is not good.
Or.....you could have just fixed the quite obvious reverb section problem.
Ok let me say this...I am assuming Terry that you want to teach us, therefore why not 1. Troubleshoot the actual issue and demonstrate repair. 2. "Hey guys, here is how to do this better" Obviously you are a great tech but those of us who are "lesser" techs but still know a bit can recognize that that issue was confined to the reverb tank circuit and somewhere in there you saw the actual issue which likely was not that crazy star ground crap.
Could I use the continuity test on my multi meter to ensure my leads are going to the correct place on the board for the leads that travel under the board?