Hum in mojotone 18w build

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JMPGuitars
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Re: Hum in mojotone 18w build

Post by JMPGuitars »

With all the tubes in place:

Does the hum stay the same at different volume levels, or get louder as the volume is raised?

Does plugging in a guitar affect the hum?

Does guitar sound come through the speaker as you would expect?

Do you know what "chopsticking" and "lead dress" are?

The time you spend is your education, and that's valuable.
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Wnively321
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Re: Hum in mojotone 18w build

Post by Wnively321 »

On channel 1 the hum disappears when the volume reaches 4.
Plugging in a guitar does not change the hum. The tremolo channel noise gets louder as the volume is turned up

I am familiar with chop sticking and have tapped or moved everything I can find. I did find a defective tube socket on v5 which has been replaced, which helped . Even if I pull all three preamp tubes the hum returns when the power tubes and rectifier warm up . The hum is constant ,
I grounded the amp according to the directions . I feel like I need a grounding scheme, I just don't know where to find a diagram of one

Thanks
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Wnively321
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Re: Hum in mojotone 18w build

Post by Wnively321 »

On channel 1 the hum disappears when the volume reaches 4.
Plugging in a guitar does not change the hum. The tremolo channel noise gets louder as the volume is turned up

I am familiar with chop sticking and have tapped or moved everything I can find. I did find a defective tube socket on v5 which has been replaced, which helped . Even if I pull all three preamp tubes the hum returns when the power tubes and rectifier warm up . The hum is constant ,
I grounded the amp according to the directions . I feel like I need a grounding scheme, I just don't know where to find a diagram of one

Thanks
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Re: Hum in mojotone 18w build

Post by Daviedawg »

Grounding schemes are a constant bugbear. There are as many schemes as amp builders I think.

What I have used for almost all my builds is a simple one with three grounding points. The incoming earth is grounded on a dedicated tag right beside the incoming socket. The preamp stages are all grounded at one point on a tag beside the input jacks. That includes the pots but not the phase inverter stage. The phase inverter, power amp and power supply are all grounded on a tag close to the power tranny but not on a fixing bolt. It would probably be ok to include the PI with the preamps if it helps the layout. It can take some thought to get the routing as efficient as possible on a long chassis like the 18 watt original. Some people use a ground buss or a "star" ground tab to gather the many connections but I prefer to use the board I am building on locating on a few adjacent tags or turrets then running that to another group close to the ground tab. This keeps connections short and neat.

I expect that there are many examples of schemes on line. Unfortunately some use bad practice such as grounding the incoming supply earth at the power transformer bolts. So if you adopt a different scheme at the least make sure it is safe.

Dd
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Re: Hum in mojotone 18w build

Post by JMPGuitars »

Daviedawg and I follow the same principles, but I also use a partial star ground. You can see an example of this ground scheme in the drawing I did for the lite 2b: files/JMPGuitars_18_Watt_Lite_2b_Layout.pdf

You can apply the same principles to your amp:

1 ground near the Input jack covers input, pots, preamp.

Star ground covers center taps, power amp, filter caps, output transformer (if you earth it).

IEC socket / power plug ALWAYS gets its own connection to the chassis.

It's always better to avoid using transformer bolts to attach grounds to the chassis.

Thanks,
Josh
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Re: Hum in mojotone 18w build

Post by geoff 1965 »

hello,
good advice from the guys,also go onto the forums downloads and then "classic 18 watt" and there are a few layouts showing grounding points for this amp and compare to yours.i still think the bipolar cap and dual lead input wires are suspect!
i know this can be frustrating and you say you could have got mojotone to build the amp but stick it out and what you learn will benefit you in future for mods or repairs.
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Janslyn
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Re: Hum in mojotone 18w build

Post by Janslyn »

Did you try with new output tubes?
I have a set of output tubes with annoing hum, and some that is almost quiet 8O
... And I use shielded wire to all my grids, only grounded at the output end :wink:
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Re: Hum in mojotone 18w build

Post by Daviedawg »

Hi Janslyn. Welcome to 18 Watt. Nice to hear from you.

Dd
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Wnively321
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Re: Hum in mojotone 18w build

Post by Wnively321 »

First, thank you for all of the help and advice I have received,
What a great community of people!

So here is an up date, changed the grounding scheme, replaced 250uf bipolar cap , replaced 16uf cap both with Sprague atom caps. Cleaned up channel 2 a bunch. A huge improvement!

There is still audiable hum at 0 volume and increasing volume doesn’t affect the hum.
If I pull all 3 preamp tubes it does not effect the hum.
I have tried 2 different pairs of power tubes, Aligator clip jumpered , the backboard jumpers and grounds still nothing I do
changes or affects the hum in any way.

Any advice would be greatly appreciated

Thank you so much
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Re: Hum in mojotone 18w build

Post by JMPGuitars »

Do you have a cover plate for the amp chassis?

Have you tried plugging the amp into a different (confirmed grounded) outlet?
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Re: Hum in mojotone 18w build

Post by Wnively321 »

I've tried the chassis in another part of the shop with other cabs.
It will be Wednesday before I have a chance to look at the amp again. Scratching my head for sure. Reworked heaters again, replaced all tube sockets. My shop is outlets are grounded.

Thanks again for all of the help
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Re: Hum in mojotone 18w build

Post by JMPGuitars »

Post new photos of the build when you can. Calm down on reworking everything. ;)
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Re: Hum in mojotone 18w build

Post by Wnively321 »

Merry Christmas to all
Sorry for the delay, my trip was longer than expected.
Here is the latest update , changed the grounding scheme major improvement! No noise in channel 1 or channel 2 .
I changed the power scheme to the one in the drawing , but that didn’t change any thing.

I still have a audible hum that is there even with the preamp tubes pulled and you can feel it if you place your hand on the speaker cone.
If I intall the preamp tubes the hum leaves if channel I volume is turned up to 4
I have supplied my latest pics, am I wrong in thinking if the preamp tubes are pulled that the noise must be in the PT or OT ?

Thank you
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Re: Hum in mojotone 18w build

Post by JMPGuitars »

Can you get some heat shrink tubing on that braided cable? Try and cover the whole thing if possible.
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Re: Hum in mojotone 18w build

Post by Wnively321 »

Yes, I got the whole cable covered and this is as quite as the amp has been as of yet . Cut the hum by 1/2 Looks like the cable must have been grounding to chassis

Thank you so much
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Re: Hum in mojotone 18w build

Post by Wnively321 »

Putting the heat shrink on the braided cable helped so much, along with the grounding they have been the 2 biggest improvement and changed a lot of the issues I was having.
Thank you so much
Tomorrow I’m going to change the power side of the amp to the diagram you provided.
The amp is actually usable
Thanks again
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Re: Hum in mojotone 18w build

Post by JMPGuitars »

Glad to hear it! That's why I mentioned that wire before, and why I think it's ridiculous that it's part of the kit. Besides potentially grounding out other things, running a shielding wire against ground is wonky, and then there's other potential (hint) problems too.

Hopefully the other change will get you where you want to be. Keep us posted!

Thanks,
Josh
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Re: Hum in mojotone 18w build

Post by Wnively321 »

Today I have good news, changed the power scheme and to my surprise a hole new hum issue , crazy right
. After hours of trying to figure out what I did wrong
Would you believe that the only power tube socket I hadn’t replaced was bad.
Replaced the socket ,hum gone , could I say that 1 more time hum gone !!!
The amp sounds like it should. Amazing

I would have never figured this out with out your help.
All of the advice you gave me was 100% correct


Thank you so much

One thing for sure it has been an education
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Re: Hum in mojotone 18w build

Post by geoff 1965 »

great news! glad you have found the gremlin and got rid of the hum,one thing i've noticed zooming in on your pics is there are a few tiny bits of loose solder on the board and in the chassis corners. i would get the vacuum in or air blower and get them out so they don't cause any problems.
enjoy playing the amp and happy christmas & new year.
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