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Well I built th mojotone 18w Marshall clone.
I was hoping that some one might point me in the right direction in the issues I am having with 60 cycle hum.
With the volume at zero on channel 1 with nothing plugged into the input there is a very annoying hum coming from the speaker.
The only time the hum leaves is when I turn the volume to 8, that is the only time the hum is affected, the hum doesn’t get louder when I increase the volume

I have rebuilt , re soldered , regrounded , re tubed and looked over the build countless times .
Any help would be greatly appreciated

Thanks

Welcome to 18 Watt.com.

It sounds like you have a very localised section of the build to review to find the issue - channel 1 preamp section which is presumably the "Normal" channel not the TMB.

Since you have done so much already, can I ask did you try cleaning then if required changing the two pots, esp the vol pot? It sounds like something is grounding when it shouldn't and when you turn the vol to a certain point it is bypassed. A spray with switch cleaner is certainly worth a try.

If that does not work we can think more deeply about it.

Dd

Thank you for your response, I cleaned the volume and tone pot and all that changed is that now the volume pot makes no diffrence the hum is constant. So I moved the chassis to my workbench plugged the amp when the tubes warmed up it blew the power fuse. Now every time the amp starts to warm the tubes it blows the power fuse . Could I possibly have a bad PT or OT?
Thank you

hello,
are your 6.3 volt heater wires okay?
also if you could add some pics someone might spot something you've overlooked.
is it the TMB or trem amp?

It s the tremolo buil, heater wires are not touching anywhere[img]

the white wire to pin5 of the rectifier looks to have overheated a bit,it's not grounding on the nut underneath is it?

Good eye , but that’s where I got the iron a little close.
I don’t now why the first fuse blew,
I believe the reason the others were blowing is because they were only rated for 225 ma. I Installed a 2amp fuse and the amps powers up now.
However I still have the 60 cycle hum issue. I’m going to order all new control pots , change them and see what the out come is.

Thanks

Wnively321 wrote: ↑

Wed 12/05/18 7:46 pm

Good eye , but that’s where I got the iron a little close.
I don’t now why the first fuse blew,
I believe the reason the others were blowing is because they were only rated for 225 ma. I Installed a 2amp fuse and the amps powers up now.
However I still have the 60 cycle hum issue. I’m going to order all new control pots , change them and see what the out come is.

Thanks

Your heater wires coming off the PT should be against the chassis, not floating in the air.

What's your grounding scheme?

Does the amp still hum with a guitar plugged in?

Why is there a 2-conductor non-insulated cable being used inside the amp?

Hum is often caused by a combination of lead dress and grounding scheme assuming no other issues...which is not a valid assumption yet. You should do a highlighter test with a fresh schematic. Make sure nothing is grounding out that shouldn't be.

Get rid of that bare wire, and make sure it isn't touching your output jack. You can throw some heat shrink tubing on it if you need to keep it for some reason. Insulated shielded wire is what you should be using if you need shielded wire.

The 2 conductor wire is Belkin shielded cable , I will surely move the heater wires, the bare wire under the output jack is shielded cable that came with the build kit for the foot switch

The origins grounds all mounted on the PT mounts , I’m still using the terminals but all grounds are grounded to the chassis

Thank you so much

i would swap that bipolar 250uf cathode cap for a polarised one as well first and get good quality pots like CTS or alpha.
i tried a bipolar on my preamp cathodes and it sounded awful!
good luck

Your build is not very clean which makes it difficult to see exactly what could cause issues. You should definitely get a fresh print of the schematic and the layout and check off each piece as you go along and trace out your connections. There is almost always at least one small detail you will miss as a beginner.

You should also go online and look at other peoples build to get an idea of what the amp should look like when you're done. You can't just start flying leads wherever you want, you have to tuck certain things away and objectively you want any wire that does cross another to do so at a 90* angle. You can fly the heater wires the way you did and technically that is the preferred method ( from a textbook standpoint ), but it has to be done very cleanly and everything else under them also has to be done very cleanly. Most will line the heater wires along the chassis which is also very acceptable. The later is simply cleaner looking and easier to deal with in the long run.

Your pots are fine is my bet. I wouldn't spend more money until you get the things working correctly, or actually find the fault. The pots can handle some voltage if wired incorrectly and most of the preamp circuit is fairly devoid of super scary voltages. Go back and be sure every connection is correct an try to clean things up a little. Once you can get the thing running then tackle the hum more proactively by moving wires around to see if noise changes ( with a chopstick!!!!! ). Good luck and have fun. Once you get her figured out you will love it.

Just to clarify, my understanding from your first post is that only the normal channel is affected by the hum. Is that how it is? If it is, then your issue is in the preamp for that channel.

If not then all the other aspects mentioned here are valid considerations, and especially I would endorse the advice of chopsticking first then marking up your schematic as you re-examine every component and connection.

From bitter experience I know how frustrating this sort of problem is. But you will find the cause and feel great about it.

Dd

Wnively321 wrote: ↑

Wed 12/05/18 8:42 pm

The 2 conductor wire is Belkin shielded cable , I will surely move the heater wires, the bare wire under the output jack is shielded cable that came with the build kit for the foot switch

The origins grounds all mounted on the PT mounts , I’m still using the terminals but all grounds are grounded to the chassis

Thank you so much

Get some heat shrink tubing to insulate that 2 conductor wire. That's mildly insane that they provide that for internal use in a kit. We can discuss your actual ground scheme if that becomes necessary after you answer Dd.

First,
I want to thank everyone for all of your help.
I have made changes to the suggested areas and not much changed . What did find is the .022uf cap between the center lug on volume control 1 and pin 2 on V2 is microphonic and the .01uf cap between pin 7 on V2 and the center lug of volume control microphonic also. I have posted a voltage chart and was wondering if the plate voltages on V2 might be a little out of range?

your plate voltages of 258 & 254 shows the tube has good balance which is good for the phase inverter, however your cathode voltages are uneven at 60 & 87 and should be the same voltage.check the jumper wire between pin 3 & 8 and make sure it's not touching the heater wires. then try swapping the tube,did you replace the bipolar 250uf cap?
have you got the pin voltages mixed up on the chart!

Yes I did , I wrote down the wrong values for V2
I have reposted the voltage chart
I haven’t replaced the bipolar cap yet.
I live in a rural part of Florida and had to order the parts which should be here next week.
Thanks again for all of the help

Guess it’s just not my day
This is the correct voltage chart

that's more like it!
yeah i bet swapping that bipolar will make a big difference,i agree with ewizard about the heater wires,fender amps looped over the tubes for donkey years with no problems so it's up to you to decide if you want to rewire it.
i would use a better shielded cable for the inputs like mentioned earlier.
keep us posted with your progress

Some progress, new tube sockets came today.
I found a bad tube socket on V5, cleaned up channel 1 a bunch, also reworked the heater wires. Still have a low level hum and some faint sputtering every 30 seconds for about 1 second . Channel 2 cleaned up quite a bit as well .
Filter caps and the rest of my parts hopefully will be hear next week.

Thank you to everyone for your help and your patience

While waiting on parts I’ve been trying to trace down the low level hum, so I think it’s got to be in the pre amp stage.
So I pulled the first pre amp tube, pulled all 3 preamp tubes and the hum is still there. So now it’s got to be in the output stage, right?
Does any one have any ideas?
I’ve probably spent enough time and money to have had mojo tone build it and ship it